Using Nectar Cover Cropping in Vineyards for Sustainable Pest Management

Final Report for SW07-022

Project Type: Research and Education
Funds awarded in 2007: $178,300.00
Projected End Date: 12/31/2010
Region: Western
State: California
Principal Investigator:
Mark Hoddle
University of California
Co-Investigators:
Dr. Nic Irvin
University of California
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Project Information

Abstract:

Research investigating the use of cover crops in southern California vineyards for pest control has demonstrated that access to floral resources greatly increases natural enemy fitness, and that cover crops may positively affect natural enemy numbers. However, additional irrigation required to keep cover crops alive over summer may lead to increased pest populations, reduced berry quality and increased vine vigor. Cover crops may harbor pathogens (Xylella) which could be transmitted to vines by sharpshooters. Summer cover cropping may not prove to be a viable option for grape growers in southern California due to cost of water and difficulty in establishing.

Project Objectives:
  1. Determine if buckwheat flowers and cahaba vetch extrafloral nectaries increase longevity and fecundity of key natural enemies.
  2. Determine when to sow cover crops to maximize nectar availability for natural enemies.
  3. Determine if buckwheat and cahaba vetch, sown in alternate rows of grapes, enhances natural enemy populations and reduces pest populations below economic thresholds at study sites over a two year period.
  4. Determine if buckwheat and cahaba vetch influence grape yield and quality.
  5. Determine if buckwheat and cahaba vetch affect vine vigor.
  6. Verify that buckwheat and cahaba vetch do not provide refuge for grape pathogens (e.g., Xylella) or pathogen vectors (e.g., sharpshooters).
  7. Determine if buckwheat and vetch out compete and suppress unwanted weed species.
  8. Determine the rate of dispersal of natural enemies from buckwheat and cahaba vetch plots.
  9. Extend the information gained from this research to the Californian grape community through outreach and education.
  10. Promote increased adoption of nectar cover cropping practices in Temecula, Lodi and Coachella Valley if research results merit application.
Introduction:

The Californian wine industry promotes sustainable practices through the Code of Sustainable Winegrowing Workbook (CSWW) (Dlott et al. 2002), however, sustainability is severely affected by the glassy-winged sharpshooter problem in Temecula, which triggers growers to apply prophylactic Admire applications to prevent spread of Pierce’s Disease (Xylella fastidiosa). This costs approximately $225/acre (Ben Drake, pers. comm.). Furthermore, insecticide applications in the Coachella Valley are often ineffective against vine mealybugs since these insects are often protected underneath bark of trunks and cordons (Daane 2000). The Californian grape industry requires investigation of new cultural control options for insect control to help reduce pesticide reliance and promote sustainable pest management.

Although the CSWW outlines the use of cover crops for maintaining soil quality and preventing erosion, cover crops are also beneficial for enhancing natural enemies of vineyard pests and controlling spider mite and leafhopper populations in vineyards (Altieri & Schmidt 1985, Settle et al. 1986, Hanna et al. 1996, Costello & Daane 1998a,b, Klonsky et al. 1998, Nagarkatti et al. 2003). Maintaining cover crops in vineyards can enhance natural enemy populations by providing habitat, shelter, nectar and alternative food for predators and parasitoids. Costello & Daane (1998a) demonstrated that third generation leafhopper nymphal densities were significantly lower in cover cropped plots compared to controls over a four year period in juice and table grape vineyards, with pest densities in cover crop plots in some years being maintained below economic thresholds exclusively by natural enemies.

The Lodi-Woodbridge Winegrape Commission has been encouraging implementation of environmentally benign vineyard management practices since 1991, including the use of cover crops. Currently 46% of Lodi-grape growers have adopted cover cropping practices (Ohmart, pers. comm.). Of these, 60% of growers disk under the cover crop in spring, while summer heat causes permanent cover crops, grown by 20% of growers, to die over the spring/summer period (Ohmart, pers. comm.). Therefore, the cover crop becomes a limiting resource for natural enemies at this critical time for pest control. We proposed that maintaining a ‘nectar cover crop’ throughout the spring and summer through additional minimal irrigation may further enhance natural enemy populations, supply natural enemies with nectar to increase survival and fecundity, resulting in pest numbers being consistently maintained below economic thresholds.

It is important to select cover crops that will benefit natural enemies that can reduce pest populations, while simultaneously having no detrimental effects on vine growth, yield or grape quality. Studies show that buckwheat increased longevity and fecundity of parasitoids in the laboratory that attack sharpshooters, a key pest of grapes (Irvin & Hoddle 2007), and led to lower abundance and higher parasitism of leafhoppers in vineyards (Nicholls et al. 2000, English-Loeb et al. 2003). Buckwheat shows promising potential as a cover crop in vineyards, as it germinates easily, has a short sowing-flowering time and its seed is inexpensive and readily available (Bowie et al. 1995).

Previous studies have demonstrated that cahaba vetch suppresses populations of damaging nematode species in Californian vineyards (McKendry 1992) and is suggested in CSWW as a cover crop option to improve soil nutrition, fertility and structure, and reduce erosion and dust. Research needs to be conducted to determine whether extrafloral nectaries of cahaba vetch are of benefit to natural enemies of grape pests and to investigate the use of cahaba vetch for management of arthropod grape pests in California. Work on buckwheat and cahaba vetch includes determination of how these cover crops can be practically incorporated into CSWW (by investigating the most optimal time to sow and how many rows require cover crops), and whether these cover crops have any detrimental effects of on vine growth, yield or grape quality. We sought to investigate nectar cover crops (buckwheat and cahaba vetch) for grape pest management in southern California, including an evaluation of how these cover crops can be practically incorporated into the Code of Sustainable Winegrape Workbook.

Cooperators

Click linked name(s) to expand
  • Imre Cziraki
  • Carmen Gispert
  • Paul Jepson
  • Stuart Musashi
  • Cliff Ohmart
  • Thomas Perring
  • Nick Toscano

Research

Materials and methods:

Objective 1: Parasitoid survival and fecundity in the laboratory on nectar resources

Laboratory trials were proposed to investigate if buckwheat flowers and cahaba vetch extrafloral nectaries increase longevity and fecundity of three natural enemies, G. ashmeadi (glassy-winged sharpshooter parasitoid [GWSS]), Anagrus epos (grape leafhopper parasitoid) and Anagyrus pseudococci (vine mealybug parasitoid). We have completed these studies for A. pseudococci and G. ashmeadi and methodologies are outlined below. Trials with the leafhopper parasitoid, A. epos, could not be conducted. Efforts to establish A. epos and A. erythroneurae (variegated leafhopper parasitoid) colonies in 2008 and 2010 were unsuccessful. In 2009, we were unable to attempt to establish Anagrus sp. colonies since labor was concentrated on insect monitoring and maintenance of the 2009 field trial and processing 2008 sticky traps.

Maintenance of insect colonies and buckwheat plants

Colonies of G. ashmeadi and GWSS were maintained at the University of California, at Riverside, CA (UCR). G. ashmeadi colonies were reared on GWSS eggs laid on ‘Eureka’ lemon leaves, a preferred lemon variety for GWSS oviposition and parasitoid foraging (Irvin and Hoddle 2004). Maintenance of Citrus limon cv. ‘Eureka’ trees used for GWSS colonies and GWSS and G. ashmeadi colony maintenance is described in Irvin and Hoddle (2005). Petri dishes containing leaves with GWSS eggs previously exposed to G. ashmeadi were held at 26o ± 2oC and 30-40% RH under a L16:8D photoperiod and checked daily for parasitoid emergence.

Mummies containing unemerged A. pseudococci were provided by University of California, Berkeley, CA (UCB). Laboratory colonies of A. pseudococci at UCB were held at 24oC ± 2oC under a L16:8D and reared on the vine mealybug, Planococcus ficus, feeding on organic butternut squash, Cucurbita moschata L.. Maintenance of A. pseudococci and host colonies is described in Daane et al. (2004). On arrival, mummies were placed in Petri dishes (10 x 1.5 cm) and held at 26oC ± 2oC and 30-40% RH under a L14:10D photoperiod. Petri dishes were checked daily for parasitoid emergence.

P. ficus colonies were held at 26o ± 2oC and 30-40% RH under a L14:10D photoperiod with fluorescent lighting and maintained at UCR. One organic butternut squash heavily infested with P. ficus (provided by Kearney Agricultural Research and Extension Center, Palier, CA) was placed into a wooden cage (32 x 34 x 37 cm) painted white, with a glass top, mesh back for ventilation and hinged front door containing a material sleeve for access. Squash was stabilized by resting on a simple wooden stand constructed of two pieces of wood held parallel by two pieces of wood. One additional P. ficus colony was set up each week from this initial colony by brushing ovisacs laid by P. ficus in the initial colony gently with a paint brush and transferring them to a new cage containing one squash.

Plants of buckwheat (F. esculentum; obtained from Outsidepride, Salem, OR) and vetch (Vicia sativa L. cv. ‘Cahaba White’; obtained from Bailey Seed Company, Salem, OR) were grown from seed in a greenhouse at 26o ±5oC under natural L14: 10D light. Seeds were sown in 1 gal pots, placing 4-5 seeds per pot. Synchronous nectar production was ensured by performing staggered sowings at 7-10 day intervals. Plants were fertilized every three weeks with Miracle-Gro (20 ml/3.5 liters of water, Scotts Miracle-Gro Products Inc., Marysville, OH). Prophylactic applications of pyrethrin + canola oil (Garden Safe Brand Fruit & Vegetable Insect Spray, Schultz Company, Bridgeton, MO) were applied to vetch plants every 7-10 days to control greenhouse insect pests. Plants used for experiments were free of Pyrethroid applications for at least 14 days and were sprayed down with water and dried before use in experiments.

Experimental set up

For each parasitoid species, 10-16 replicates of three treatments (water only, buckwheat and vetch) were set up in the laboratory at 26o ± 2oC and 30-40% RH under a L14:10D photoperiod. Wooden cages (32 x 34 x 37 cm), as previously described, were used for treatment replicates and a white piece of cardboard was placed on the bottom of each cage. Water was provided in each cage via a 7.4 ml glass vial (2 dram Fisherbrand Glass Vial, Fisher Scientific, Pittsburgh, PA) with a 5 cm cotton wick which was placed on the bottom of each cage and topped up daily. Plant treatments consisted of one 1 gal potted buckwheat or vetch plant with the bottom and top wrapped in Parafilm (Parafilm‘M’ Laboratory Film, Pechiney Plastic Packaging, Chicago IL). Plants in cages were watered as needed with an 8 oz wash bottle (Nalagene, Thermo Fisher Scientific, Rochester, NY) inserted through a hole in the Parafilm. Tape was placed over the hole after each watering. Plants were removed and replaced every 4-5 days to ensure constant supply of nectar. One newly emerged (? 12 h old) naive male and female G. ashmeadi or A. pseudococci was released inside the cage. Parasitoid longevity was recorded daily until death for each sex, and males were not replaced once dead. Female longevity is reported here.

Hosts were provided to G. ashmeadi by placing the petioles of ‘Eureka’ lemon leaves (a preferred lemon variety for GWSS oviposition and parasitoid foraging; Irvin and Hoddle 2004) containing GWSS eggs (<24 h old) through holes drilled in the lid of a 130 ml plastic vial (40 dram plastic vial, Thornton Plastics, Salt Lake City, UT) filled with water. Leaves bearing hosts exposed to parasitoids were removed and replaced every three days until death of the female parasitoid. On the 1st, 4th, and 7th day, 80, 80 and 60 hosts were provided to G. ashmeadi, respectively. On subsequent host changing days, 40 GWSS eggs were provided. Host numbers and age were selected based on previous studies for G. ashmeadi (Irvin and Hoddle 2005, Pilkington and Hoddle 2006, Irvin and Hoddle 2007). Exposed leaves bearing host egg masses were placed into Petri dishes (9 x 1 cm, Becton Dickinson Labware, Becton Dickinson and Co., Franklin Lakes, NJ) lined with moist filter paper (9cm Whatman Ltd. International, Maidstone, England) and left at 26oC for three weeks to allow offspring to emerge. Leaves sometimes decayed, which often prevented successful insect emergence, therefore unemerged eggs were dissected and the numbers of unemerged parasitoids were recorded and included in progeny calculations.

Hosts were provided to A. pseudococci by placing one butternut squash infested with at least 50 3rd and 4th instar P. ficus on a wooden stand inside the cage. This was removed and replaced after six days to ensure that A. pseudococci were provided with life stages suitable for parasitism during their entire lifetime. Host numbers and age were selected based on previous studies for A. pseudococci (Daane et al. 2004). Small pieces of tissue paper were placed over large areas of honeydew excreta daily to prevent parasitoids from getting stuck and dying unnaturally. Exposed squash bearing P. ficus and plants removed from cages were placed into label wooden cages (previously described) for three weeks to allow offspring to emerge. The number of male and female A. pseudococci offspring was recorded for each cage replicate.

Statistical Analyses

Only parasitoids that died of natural causes were included in statistical analyses. Those females that did not mate (producing only male progeny) were excluded from the male progeny totals and sex comparison analyses. For each species, the effect of treatment on the total number of offspring and longevity was determined using ANOVA in SAS (1990). For A. pseudococci, total offspring and longevity data were square-root transformed prior to analyses. Tukey’s Studentized range test at the 0.05 level of significance was used to separate significant means. Logistic regression was used to determine the effect of treatment on logit offspring sex ratio (percentage female). Pair-wise contrast tests at the 0.05 level of significance were used to separate means. Means (± SEM) presented here were calculated from untransformed data.

Objective 2: Cover crop phenology

A tractor and cultivator were used to create a furrow surrounding each of 156 plots (twelve rows of thirteen plots) at the University of California, Agricultural Operations. Plots were 1 m squared and separated by 1.8 m. In the middle of each month from August 2007 until August 2008, five replicates of buckwheat and vetch were sown in 10 randomly selected plots following recommended agricultural sowing rates (buckwheat: 50 lb/acre = 5.62 g seed/m2; vetch: 60 lb/acre = 6.71 g seed/m2). Seed was sown in each plot, covered with approximately 2.5 cm of soil using a rake and watered with 2.5 gal of water from a watering can. Plots were irrigated via a 0.89 mm orange O-Jet 6000 Series Micro-Spray sprinkler head (Olson Irrigation Systems, Santee, CA) installed in the middle of each plot for 2 h. An adjustable pressure regulator was installed to deliver 15 PSI under which sprinklers emitted 7.2 GPH. Plots were irrigated every 2-4 days from August 2007 until July 2009 depending on time of year.

Irrigation was turned off during periods of rain. Weeding of plots was conducted as necessary. Vetch plots were susceptible to mites, aphids, thrips, the three-cornered alfalfa hopper (Spissistilus festinus [Say]) and the false chinch bug. Therefore, prophylactic applications of Ortho Systemic Insect Spray (8% acephate and 0.5% fenbutatin-oxide; The Ortho Group, Marysville, OH) were applied once a month in March 2008 and April 2008 following label directions, and then every 7-10 days from May 2008 until September 2008. From October 2008 until July 2009, vetch plots were sprayed with Ortho Systemic Insect Spray once a month. Plots were checked weekly and then every 2 days when plants were nearing nectar production. The number of days until at least one plant produced nectar (i.e., floral nectar from flowers of F. esculentum or extrafloral nectar from the stipples of vetch) was recorded per plot. At six weeks, the height of ten randomly selected plants per plot was measured with a ruler and average six week height calculated for each plot. Plants were monitored until nectar production ceased (i.e., when all flowers died for buckwheat or when vetch plants stopped producing extrafloral nectar). The length of the nectar producing period was recorded per plot.

Statistical analyses

Multiple regression was used to determine the effect of sowing date, plant species and sowing date * plant species interaction on average six week height (data logged transformed), days until nectar production (raw data) and length of nectar production (raw data). Tukey’s Studentized range test at the 0.05 level of significance was used to separate significant means. Means (± SEM) presented here were calculated from untransformed data.

Objective 3: Natural enemy enhancement and pest population suppression

In 2008, thirteen plots (28.7m x 6m [2 rows] and separated by at least 36 m) were selected in four blocks of Cabernet Sauvignon grapes at Bella Vista vineyard, Temecula, CA (GPS coordinates: 33o 33’26.18’’N x 117o 00’52.12’’W; elevation: 1,637 feet). One or two cover crop plots and control plots were randomly allocated per block, to total seven cover crop plots and six control plots for the entire study. In both years, six plots maintained under current vineyard practices, which included machine and hand cultivation between rows to remove unwanted weed vegetation, were used as controls. For the 2008 field trial, vetch was randomly allocated the north or south side of each cover crop plot and seeds were sown on February 19. On May 1, 2008, buckwheat was sown on the opposite side to vetch in each cover crop plot. Vetch did not establish, therefore, this side of the row was cultivated on June 11, 2008 and re-sown with buckwheat. In 2009, treatments were re-randomized using the same thirteen plots outlined above. Only buckwheat was sown in the 2009 field trial since vetch previously performed poorly in phenology trials, and there was no cahaba vetch seed available in 2009 because the only seed supplier in the U.S. lost their entire crop to mildew fungal disease in 2009. Buckwheat seed was initially sown for the 2009 field trial on April 1, 2009.

Seed was sown in cover crop plots at recommended agricultural sowing rates, which translated to 336 g of buckwheat seed or 404 g of vetch seed per 60 m2 plot. Seed was evenly sprinkled over the soil and covered with approximately 2.5 cm of soil using a rake. Sprinkler irrigation was installed on the existing grower’s grape irrigation in the cover crop plots. In 2008, irrigation consisted of five sprinklers (blue Micro Bird Spinner sprinkler heads per plot, 12 GPH, 360o x 12 ft diameter coverage; Temecula Valley Piping and Supply, Temecula, CA) each installed into 7 mm tubing attached to a 18 cm bamboo stick each side of the 60 m2 plot. Plots encompassed two rows so a total of 10 sprinklers per plot were installed. In 2009, irrigation consisted of 9 sprinklers (red 7.9 mm Micro sprinkler jet sprayer, 12-14 GPH, 360o x 12 ft diameter coverage; DIG Irrigation Products, Vista, CA) each installed into 7 mm tubing on a 32.5 cm plastic spike (DIG Irrigation Products) each side of the 60m2 plot (total of 18 sprinklers). Irrigation sprinkler number and type was changed in the second year of the trial to increase spray coverage and theoretically increase likelihood of buckwheat establishment. Buckwheat seed was re-sown in cover crop plots 2-3 times throughout the trial, and each cover crop plot was irrigated for 2 h the day after each sowing to ensure good germination, then approximately every 7-10 days for approximately 6 h. Additionally, irrigation was supplemented with 16 gal of water per plot, applied via a 16 gal NorthStar ATV Tree Sprayer (Northern Tool + Equipment, Burnsville, MN) and 4WD motorbike, approximately three times a week. Information on number of irrigation days and supplemental watering were recorded in order to estimate the amount of additional water required by the cover crop plots. Susceptible cover crop plots were treated with Rabbit Scram (Enviro Protection Industries Co., Kirkwood, NY) following label directions starting on July 25, 2008 and April 29, 2009.

Despite all these efforts, we encountered a number of problems with establishing and maintaining a cover crop during these trials. Four out of seven allocated cover crop plots were established in the 2008 field trial, and no replicated plots of buckwheat were established in the 2009 field trial. Poor establishment of cover crops in 2009 was due to a batch of poor quality seed with a low germination rate (just 10-33% in greenhouse studies) that was brought from suppliers, irrigation issues (including sprinkler head blockages and flooding), birds eating seeds before they germinated, rabbits eating large patches of germinated seedlings, extreme summer temperatures killing seeds and seedlings, and severe damage to cover crop plants from tractor and vineyard workers during routine vineyard maintenance. The 2009 field trial was discontinued in July 2009 after it was apparent that establishment of buckwheat replicates was unattainable. This report outlines additional methods and results from the 2008 field trial.

Experimental design

During the 2008 trial, four replicates of the cover crop treatment were established with buckwheat growing on one side of the cover crop plot. The other three allocated ‘cover crop plots’ had irrigation installed, but since buckwheat did not establish, these plots were reassigned as an ‘irrigated treatment’. Consequently, the three treatments for this study were: (1) buckwheat cover crop with irrigation; (2) irrigation with no buckwheat cover crop; and (3) a cultivated control with no buckwheat cover crop or irrigation. Including an ‘irrigated treatment’ will help determine whether effects of buckwheat cover crop on insect fauna, grape yield, fruit quality and vine vigor was due to the buckwheat cover crop or the irrigation required by the cover crop plants.

Details on insect monitoring

Sticky traps. Two transparent sticky traps (16.7cm x 13.2cm) mounted at a height of 1.45m and orientated parallel to the vines were placed on the north and south side of the middle row of each plot, 3.7 m apart. Traps were made from clear Perspex (Plaskolite Inc, Columbus OH) blotted on both sides with hot Tanglefoot glue (The Tanglefoot Company, Grand Rapids MI) using a 7.62cm wide paintbrush. Traps were collected and replaced weekly from June 10 through August 9, 2008. On collection, individual traps were placed between two labeled acetate sheets (21.5cm x 28 cm, C-line Products, Inc. Mount Prospect IL) indicating date trap was deployed, treatment, replicate, direction (north or south) and side of trap (‘open side’ facing out into the plot row or ‘foliage side’ positioned towards the grape foliage). Traps were stored in a -20oC freezer until counting of insects. Traps were viewed under a dissecting microscope, and each insect was identified to family or genus level. Counts of each insect were kept using a five-slot multiple-tally denominator (The Denominator Company Inc., S Woodbury CT). The number of pests and natural enemies were recorded separately for each side of the sticky trap to provide information on whether insects were flying towards or away from the grape canopy and buckwheat cover crop. Groups of beneficial insects that were counted on sticky traps and that are reported here were parasitic and predatory wasps, predatory thrips, and ‘other beneficials’ which included pirate bugs, ladybugs, lacewings, big eyed bugs, predatory mites, spiders, carabid beetles and scarab beetles. Groups of pest species that were counted on sticky traps were thrips, leafhoppers and ‘other pests’ which included miridae, sharpshooters, false chinch bugs, mites and aphids.

Visual counts. To obtain visual counts of leafhoppers and predators, a total of five leaves were visually examined per plot every two weeks between June 5 and August 2, 2008. Five vines on each of the north and south side were chosen at random in each cover crop and control plot. One first generation leaf (a large, mature leaf located three to four nodes up from the basal node of a cane) was examined with an OptiVisor (Donegan Optical Co., Lenexa KS) per vine and numbers of variegated leafhoppers, grape leafhoppers, lacewing eggs and predators were recorded.

Funnel beat sampling. Funnel beat sampling was carried out every two weeks between June 7 and July 9, 2008. The funnel was constructed using PVC pipes (JM Eagle, Livingston NJ) and consisted of four 60.96cm-long leg pieces and four 85.09cm-long frame pieces, secured with four 90? PVC joints (Dura Plastic Products Inc., Beaumont CA). The overall opening dimension of the frame was 0.86m by 0.86m. The frame was fitted with a static-resistant polyester material (Jo-Ann Fabric and Craft Stores, Hudson OH) which tapered to 0.1m by 0.1m at the bottom, creating an attachment point for collection bottles. The attachment point was constructed from 5 cm of the top of a 16 oz wash bottle (Fisher Scientific, Pittsburgh PA) glued into a hollowed out lid which had been glued to a second hollowed out lid. To conduct a funnel beat sample, the funnel was placed under a randomly selected vine near the middle of each plot, and a 16oz wash bottle was screwed into the attachment point. Using a small mallet, the main branch of the vine was struck forcefully and repeatedly, and foliage was agitated for 30 seconds. The sample was swept down from the funnel into the collection bottle, capped, labeled and placed into a large cooler for transport to the laboratory. Vines used for funnel trap sampling were labeled to ensure each vine was not sampled twice within a 28 day period. It was noted whether vines were wet from irrigation at time of sampling, and later these data were removed from statistical analyses. Samples were stored in 95% ethanol and the number of pests and natural enemies were counted as previously described for the sticky traps. Groups of beneficial insects that were counted in funnel beat samples and that are reported here were parasitic and predatory wasps, predatory thrips and ‘other beneficials’ which included pirate bugs, ladybugs, lacewings, big eyed bugs, predatory mites, spiders, nabilae and staphylinid beetles. Groups of pest species that were counted in funnel beat samples were thrips, leafhoppers and ‘other pests’ which included miridae, sharpshooters, false chinch bugs, mites, aphids, Hemiptera, ants and weevils.

Sweep net sampling. A sweep net was used to sample buckwheat and grape foliage in buckwheat and control plots, respectively, every two weeks between June 19, 2008 and August 14, 2008. This was conducted by vigorously shaking foliage into a sweep net (Bioquip, Rancho Dominguez, CA) for 1 min and placing contents in a labeled 1 gal Ziplock bag. Sweep netting was conducted between 7am and 12 pm. The time plots sampled were randomized, and the time the sweep net sample was conducted was recorded for each sample. Bags containing samples were placed into a large cooler for transport to the laboratory. Samples were stored in -20oC freezer until counting of insect groups. Groups of beneficial insects that were counted in sweep net samples and that are reported here were parasitic and predatory wasps, predatory thrips and ‘other beneficials’ which included pirate bugs, ladybugs, lacewings, big eyed bugs, spiders, nabilae and earwigs. Groups of pest species that were counted in sweep net samples were thrips, sharpshooter, leafhoppers, ants and ‘other pests’ which included miridae, false chinch bugs, mites, aphids, psyllidae, Hemiptera and grasshoppers.

Statistical analyses

Sticky trap data. Buckwheat only grew on one side of the cover crop plot, while sticky traps were deployed on both the north and south side of the row within the cover crop plot. Therefore, the sticky trap data in the cover crop plots was further reclassified as ‘buckwheat present in the plot, but not in the row’ and ‘buckwheat present in both the plot and row’. Consequently, for this data set there were four post-hoc treatments: (1) control; (2) irrigation treatment; (3) irrigation and buckwheat present in the plot, but not in the row; (4) irrigation and buckwheat present in both the plot and row.

The experimental design for the sticky trap data was a Double-Split Plot design, and since there is no obvious model for simultaneously analyzing these data across time, the data was analyzed on a per time point basis. The ten sampling dates associated with sticky trap data were averaged into five distinct bi-weekly time periods. Insect counts were log transformed using ln (x+1) prior to performing statistical analyses and presented as means estimated by the model. For each time period, the effect of treatment, row (north versus south side of the row within the plot) and trap side (open side versus foliage side of trap) on Total Pests, Total Beneficials, Thrips, Leafhoppers, Other Pests, Parasitic and Predatory Wasps, Predatory Thrips and Other Beneficials was determined using a linear mixed model. Tukey-Kramer at the 0.05 level of significance was used to separate means (Kramer, 1956). Means (± SEM) presented here were calculated from untransformed data.

Funnel beat data. Funnel beat samples were collected over four dates from the middle of each buckwheat plot. Sample positioning was not row specific, therefore, for this data set there were three treatments: (1) control; (2) irrigation treatment; and (3) irrigation and buckwheat treatment. As previously indicated, all data associated with vines that were wet from irrigation during the time of sampling were removed from the analyses to avoid confounding a wet vine effect with the treatment effects. Insect counts were log transformed using ln (x+1) prior to performing statistical analyses. The effect of treatment, date and date x treatment interaction on Total Pests, Total Beneficials, Thrips, Leafhoppers, Other Pests, Parasitic and Predatory Wasps, Predatory Thrips and Other Beneficials was determined using a linear mixed model. Where an interaction term was not significant, this term was removed and the model re-run. Where the date x treatment interaction term was significant, data was analyzed by date and treatment. Tukey-Kramer at the 0.05 level of significance was used to separate means. Means (± SEM) presented here were calculated from untransformed data.

Visual counts data. Visual counts were conducted on both the north and south side of the row within the cover crop plot. Therefore, for this data set there were four post-hoc treatments: (1) control; (2) irrigation treatment; (3) irrigation and buckwheat present in the plot, but not in the row; (4) irrigation and buckwheat present in both the plot and row. The five pseudo-replications (leaves) within each row were averaged into a single sample before performing statistical analyses. Additionally, the six sampling dates associated with the visual counts data were averaged into three distinct monthly dates (June, July and August). Total Leafhopper counts (variegated leafhoppers + grape leafhoppers), Predatory Insect counts and Lacewing Egg counts were log transformed using ln (x+1) prior to analyses. A linear mixed model was used to determine the effect of month, row, treatment and month x treatment interaction on Leafhopper, Predatory Insects and Lacewing Egg counts. Where an interaction term was not significant, this term was removed and the model re-run. Where the month x treatment interaction term was significant, data was analyzed by month and treatment. Tukey-Kramer at the 0.05 level of significance was used to separate means. Means (± SEM) presented here were calculated from untransformed data.

Sweep net data. Sweep net samples were conducted on the buckwheat foliage in the cover crop plots and grape foliage in the controls. Therefore, there were only two treatments: (1) buckwheat and (2) control. The time the sweep net sample was conducted was broken into two groups: before 9am and after 9am. The effect of treatment, time, treatment*time interaction and sample date on Total Pests (data log transformed), Total Beneficials (data square-root transformed), Total Leafhoppers (square-root transformed), Ants (log transformed), Other Beneficials (log transformed) and Other Pests (raw data) was determined using a linear mixed model. Where an interaction term was not significant, this term was removed and the model re-run. Where the date x treatment interaction term was significant, data was analyzed by date and treatment. Similarly, this was conducted when a significant time x treatment interaction term existed. For Thrips, Sharpshooters, Parasitic and Predatory Wasps and Predatory Thrips data (which did not fit a normal distribution), Friedman’s Chi-square was used on raw data to determine the effect of treatment and time. Means (± SEM) presented here were calculated from untransformed data.

Objective 4. Grape yield and quality

On September 18, 2008, the number of grape clusters present within a 3 m section of vine in the center of each plot was counted. Ten randomly selected clusters were harvested from each section (five from each of the north and south side of the row), placed into labeled Ziplock bags and transported to the laboratory in a cooler for grape yield and quality measurements. The weight of each cluster was recorded to within 0.01g and the number of berries per cluster counted. Each berry was inspected and categorized as ‘normal’, shriveled (berry shriveled due to dehydration), having broken skin (skin pierced and broken open by large insects such as wasps) or being split/squashed (as a possible result of handling).

Additionally, 25 berries were randomly selected from the ‘normal’ berry category and berry size (diameter) was measured for each berry using digital calipers (150 mm Absolute Mode Digital Caliper, Tresna, Guilin Guanglu Measuring Instrument Company, Guangxi Province, China) to within 0.01 mm. Each of the 25 berries was inspected for damage from thrips (scarring) and presence of black sooty mold. Finally, all 25 berries were placed into a 1 gal Ziplock bag and squashed with the palm of the hand to extract juice. A refractometer (Pocket Refractometer Pal-1, Atago, Itabashi-ku, Tokyo, Japan) was used to measure sugar content of the juice.

Statistical analyses

For overall cluster counts, sample positioning was not row specific. Therefore, for these data there were three treatments: (1) control; (2) irrigation treatment; and (3) irrigation and buckwheat treatment. The remaining grape yield and quality parameters were measured from both the north and south side of the row within the cover crop plot. Therefore, for these data there were four post-hoc treatments: (1) control; (2) irrigation treatment; (3) irrigation and buckwheat present in the plot, but not in the row; (4) irrigation and buckwheat present in both the plot and row. The effect of treatment on the overall number of clusters per 3 m row (data logged transformed) was determined using simple linear regression. The effect of treatment, direction and treatment*direction interaction on weight of clusters (square-root transformed), total number of berries per cluster (raw data), number of ‘normal’ berries (square-root transformed), Brix content (raw data) and berry size (raw data) was determine using linear mixed model. To separate means, pair-wise t-tests were performed and p-values were adjusted using Tukey’s method. A generalized linear mixed model was used to determine effect of treatment, direction and treatment*direction interaction on the number of scarred berries since these data were not normally distributed. To separate means, pair-wise t-tests were performed and p-values were adjusted using Tukey’s method. A Chi-square was used to determine the effect of treatment and direction on the number of broken and shriveled berries. A logistic model was used to conduct pair-wise comparisons for treatment. Means (± SEM) presented here were calculated from untransformed data.

Objective 5. Vine vigor

The influence of cover crops on vine vigor was investigated in October 2008 by measuring the weight of winter prunings from three randomly selected vines in the center of each treatment plot. For each vine, the number of canes growing from each arm was recorded. All canes were removed from the vine by cutting just above the basal node, and any remaining leaves and secondary shoots growing from the primary cane were removed. Canes from each vine were placed into 1 gal Ziplock bags and labeled with treatment and replicate. The contents of each bag were weighed to within 0.01 g. The average weight per cane was calculated for each vine by weighing the contents of each bag and dividing the weight by the number of canes.

Statistical analyses

Average cane weight was calculated for each plot, therefore, sample positioning was not row specific. Consequently, for these data there were three treatments: (1) control; (2) irrigation treatment; and (3) irrigation and buckwheat treatment. The effect of treatment on average cane weight (data logged transformed) was determined using one-way ANOVA. Tukey’s Studentized range test at the 0.05 level of significance was used to separate significant means. Means (± SEM) presented here were calculated from untransformed data.

Soil analyses

It was observed during the trial that soil type may have differed between plots used in the cover crop field trial. Soil type can have a marked effect on moisture retention which could have significantly effected the establishment and performance of cover crop and vine vigor, consequently effecting pest and natural enemy abundance. To aid interpretation of results from this field study, ten core soil samples were removed from each plot (four buckwheat plots, three irrigated plots and seven control plots) using a 0-20 cm soil auger. This was conducted by removing five sample from each side of the plot at 6 m intervals. All ten subsamples for each plot were placed into a labeled 1 gal ziplock bag transported to the laboratory. In the laboratory, soil within each bag was mixed and 500 g per was removed and sent to Analytical Laboratory, University of California, Davis, CA for soil testing. Tests included particle size analysis (sand, silt and clay content), organic matter content and moisture retention at both 0.3 and 15 ATM. At The Analytical Laboratory each of 13 soil samples was placed into individual paper bags and dried at 40°C for 12 h. Soil was crushed in a Bico-Braun soil pulverizer to pass through a 2 mm sieve.

A detailed description of methods used for particle size analysis is described in Sheldrick and Wang (1993). Particle size analysis was determine via the hydrometer method which is based on the dispersion of soil aggregates using a sodium hexametaphosphate solution and subsequent measurement based on changes in suspension density. The method has a detection limit of 1% sand, silt and clay (dry soil basis). A detailed description of methods used for moisture retention is described in Klute (1986). Moisture retention determines the soil moisture content under constant preset pressure potential. Soil was brought to near saturation and then was allowed to equilibrate under both 0.3 and 15 ATM. The method detection limit is 0.5%. Organic matter content was determined using the Walkley-Black method. This method quantifies the amount of oxidizable organic matter in which OM is oxidized with a known amount of chromate in the presence of sulfuric acid. The remaining chromate is determined spectrophotometrically at 600nm wavelength. The calculation of organic carbon is based on organic matter containing 58% carbon. The method has a detection limit of approximately 0.10%. A detailed description of the Walkley-Black method is described in Nelson and Sommers (1982).

Statistical analyses

A one-way ANOVA was used to determine the effect of treatment on the percentage of sand, silt and clay, organic matter content and water retention at 0.3 and 15 ATM. Tukey’s Studentized range test at the 0.05 level of significance was used to separate significant means.

Objective 6. Grape pathogens, pathogen vectors and grape pests

Approximately twenty five buckwheat and vetch plants were needle inoculated in the laboratory with X. fastidiosa and tested with ELISA kits after four weeks to determine whether these plants could act as a host for X. fastidiosa, the causative agent of Pierce’s Disease (PD) in grapes. Since both plants tested positive to X. fastidiosa, further testing was conducted to determine whether GWSS could acquire X. fastidiosa from buckwheat or vetch and successfully transmit the pathogen to grape vines. If GWSS can transfer the pathogen from the cover crop plants to grapes, then cover crop plants may act as a potential reservoir of X. fastidiosa and be detrimental to grape growers. This is an important question that needs addressing. Consequently, the ability of GWSS to transmit X. fastidiosa from the cover crop to grapes, and then from grapes to the cover crop was investigated. For this work, forty GWSS (to allow for mortality) were released into cages containing cover crop plants infected with X. fastidiosa. Insects were left for a 48-hour feeding and acquisition period, then insects were collected and five GWSS were placed into a sleeve cage on each of five grape plants. The insects were left to feed for 48 hours, after which the insects were collected into individually labeled 1.5mL microcentrifuge tubes and frozen at -80?C for processing. Following the 48 hr feeding period, the grape test plants were grown in a greenhouse and tested for X. fastidiosa infection 8, 12 and 16 weeks post-feeding using ELISA and plate culturing techniques. Using the same protocols, GWSS transmission from cover crop to cover crop, and grapevine to grapevine (controls) were tested.

Greenhouse transmission results for vetch were inconclusive. Vetch plants grown in the greenhouse were susceptible to pest problems and were extremely difficult to keep alive long enough to allow adequate testing, and the grape to grape controls resulted in no transmission, which may indicate there was a problem with the GWSS used for this cohort. Given these issues with greenhouse testing, field transmission tests were conducted. Trials that investigated natural inoculation of buckwheat and vetch under field conditions were conducted at Agricultural Operations, UCR where X. fastidiosa is known to occur. Buckwheat and vetch was sown in the field in August 2008 and after three weeks, 10 buckwheat and 10 vetch plants were randomly selected and individually covered with acetate cages. Acetate cages were 12” tall and 4” in diameter, with 2 x 4” ‘windows’ on opposites covered with nylon mesh organdy. The top was also covered with nylon mesh organdy. The seam of the acetate and the fabric were glued using a hot glue gun. Cages were secured in place over plants with a 3 foot long length of 1” diameter PVC pipe positioned in the ground directly east of the plant, and the cage was placed over the plant and fastened to the PVC using a size 32 rubberband (114g or 0.25lb) to prevent the cage from being blown over by afternoon winds.

One hundred and twenty-five adult GWSS were collected from the field and placed in a bug dorm with a potted grapevine (variety Redglobe). The grapevine had been needle-inoculated and infected with Xylella fastidious subspecies fastidiosa (Temecula strain of PD). Insects were left to feed for a 48-hour acquisition access period (AAP), then live GWSS were aspirated into plastic 40-dram vials (5 per vial). One vial was placed into each cage for each plant. Four potted grapevine controls (non-infected) were placed beside the buckwheat and vetch plants and fitted with nylon organdy sleeve cages. One vial of GWSS was released onto each potted grapevine. All GWSS were given a 96-hour inoculation access period (IAP), after which, insects were collected and plants labeled. Grapevine controls were returned to the greenhouse. Buckwheat plants started dying at 2-3 weeks post-IAP, so all plants were collected at three weeks post-IAP and tested for PD. Four plants were too dry for culture testing, so these were tested with ELISA only in case dead cells could be detected. The remaining six buckwheat plants were tested with ELISA and culture. The vetch plants were sampled at four-weeks post-IAP by collecting a small branch from the base of the plant. Lowest leaves from each branch of the grapevine controls were collected and the lowest 2cm of petiole tissue from each leaf was used.

Objective 7: Weed competitiveness

The ability of buckwheat and vetch to outcompete weeds was not investigated in the 2008 field trial because vetch was such a poor competitor that plots needed to be weeded by hand to ensure establishment of vetch for the trial. Vetch is a winter cover crop and did not perform well in hot weather. This objective could also not be examined in the 2009 field trial because replicated buckwheat plots could not be established and vetch was not investigated in 2009 field trial (see Objective 3).

Objective 8: Dispersal of natural enemies

We intended to investigate the dispersal of natural enemies from cover crop plots into the vineyard, spraying the plants that natural enemies were visiting and living on. This dispersal information will help determine how many rows of cover crops growers would require for adequate dispersal of biological control agents from resource-providing plants. It has been shown in the laboratory that a topical application of triple mark containing yellow fluorescent SARDI dye, casein from milk and albumin from chicken egg white marked up to 88% of the minute parasitoid G. ashmeadi (Irvin and Hoddle, unpublished data). On July 22 2008, insects in the four replicates of buckwheat plots were triple marked with a solution consisting of 20% chicken egg white (strained All Whites, Papetti Foods, Elizabeth, NJ) (containing ~5% albumin [Anon 1, 2011]), 78% milk (Ralphs 2% Reduced Fat Milk, Inter-American Products, Inc., Cincinnati, OH) (containing ~80% casein [Anon 2, 2011]) and 2% yellow SARDI fluorescent pigment (liquid dye) (Topline Paint, Pty Ltd, Aldelaide, SA, Australia). This was conducted by spraying buckwheat plants in a 30 m x 4.8 m plot with 5 liters of the triple marking solution via a 2-stroke 10 liter Stihl backpack sprayer (Andreas Stihl AG & Co., Virginia Beach VA). Four control plots were selected that were located in the same block as the buckwheat plots. Control plots were not treated with the triple mark to investigate the natural gradient of unmarked insects captured on sticky traps and to investigate the efficiency of buffer zones used to separate treatments by determining whether protein-marked insects are detected in the controls.

In additional to the sticky traps deployed as part of the insect monitoring study (see Objective 3; these traps were considered row 0 in this dispersal study), an additional four transparent sticky traps were placed on the 1st, 3rd, 6th and 10th row adjacent to the center of plot following the protocol previously outlined. Sticky traps were placed in each cardinal direction totally eight additional traps. Sticky traps were collected and replaced three and six days after marking to determine how long insects remained marked under prevailing field conditions. The number of pest and beneficial insects was counted on each trap in the same manner as was previously described for the sticky traps in Objective 3. Groups of beneficial insects that were counted included parasitic and predatory wasps, predatory thrips, pirate bugs, spiders and ‘other beneficials’ which included ladybugs, lacewings, big eyed bugs, predatory mites, carabid beetles, scarab beetles, anthicid beetles and staphylinid beetles. Groups of pest species that were counted on sticky traps were thrips, leafhoppers and ‘other pests’ which included miridae, psyllidae, sharpshooters, false chinch bugs, mites and aphids. Results for Leafhoppers, Other Pests and Thrips are not reported here because the main goal of this experiment was to determine the movement of beneficial insects from buckwheat plots.

To detect insects which were marked with yellow dye, traps were viewed under UV light for presence of yellow fluorescent dye. This was conducted by placing the parasitoid on a clean white 130 plastic vial lid under a dissecting microscope in a dark room. Two Croplands SARDI UV flashlights (SARDI, Urrbrae, SA, Australia) were held on either side of the dissecting microscope with the UV lights illuminating the parasitoids. Insects with yellow dye on their body were considered marked. Insects with yellow dye directly beside them on the sticky trap were also considered marked. Marked insects were circled with a yellow ultrafine-point Sharpie marker. Once the entire side of a trap was reviewed, marked beneficial insects were removed by peeling back a corner of the top acetate sheet and removing the insect with a toothpick (Greenbrier International Inc., Chesapeake VA). Toothpicks were broken in half and the tip containing the insect was inserted into a 200µL microcentrifuge tube (Eppindorf North America, Hauppauge, NY), sealed, and labeled. Microcentrifuge tubes were stored at -20oC until transport to James Hagler laboratory (USDA-ARS Phoenix Arizona) for ELISA testing to detect milk and egg proteins.

Three hundred and fourteen yellow dye marked leafhopper parasitoids (out of a total of 39,141 parasitic and predatory wasps) were removed from sticky traps across all treatments, replicates and sampling dates for ELISA analyses. All yellow dye marked and unmarked spiders (4 marked / total of 77), pirate bugs (3/70) and predatory thrips (5/343) were removed for ELISA testing. Additionally, a cohort of 8 leafhopper parasitoids per trap (totaling 2,349 parasitoids) that scored negative to yellow dye were removed for ELISA testing. All collected insects were packed and transported by car in a large cooler filled with dry ice from UCR to the USDA-ARS Arid-Land Agricultural Research Center in Maricopa, Arizona for detection of albumin and casein protein. Insects serving as negative controls were required as part of the ELISA methods where eight negative control insects are placed per plate of ELISA samples. Therefore, 272 parasitoids, 8 spiders, 8 pirate bugs and 24 predatory thrips were removed from sticky traps deployed on July 15, 2008 (before spraying of the triple mark) and sent to USDA-ARS to serve as negative controls. Insects scored positive for the presence of albumin or casein if the ELISA optical density reading exceeded the mean negative control reading by three standard deviations (Hagler 1997).

Statistical analyses

The effect of treatment (buckwheat or control), distance from middle of the plot (row 0, 1st row, 3rd row, 6th row and 10th row), row side (north or south side of row), presence of buckwheat (traps placed at row 0 on the side of buckwheat plots containing buckwheat were considered having buckwheat present while all other traps were considered having buckwheat absent), side of trap (open side versus foliage side) and treatment x distance interaction on the number of Total Pests, Total Beneficials, Parasitic and Predatory Wasps, Predatory Thrips, Pirate Bugs, Spiders and Other Beneficials counted on sticky traps was determined using Poisson Regression Model on log transformed data. Where a significant treatment x distance interaction existed, data was analyzed by treatment and distance. Pair-wise comparisons using a Bonferroni adjustment correlating to the number of comparisons were used to separate means for detecting significant differences between distances and treatments.

Logistic regression model was used to determine the effect of treatment, distance, row side, presence of buckwheat, side of the trap and treatment x distance interaction on the percentage of parasitoids marked with yellow dye, albumin, casein and a double mark. Non significant terms were removed and the model re-run. The odds ratio was used to determine the trend of significant row, buckwheat presence and trap side effects. The effect of distance from the middle of the plot on the percentage of parasitoids marked with yellow dye, albumin, casein and a double mark was determined for each treatment (buckwheat and control) using logistic regression. Distance was specified as a linear variable. The estimate was used to determine the trend of significant distance effect where a positive estimate equaled an increasing percentage of marked insects occurring with increasing distance and a negative estimate equaled a decreasing percentage of marked insects occurring with increasing distance. When data did not hold to linear fit assumptions, then data were specified as categorical data and Fishers Exact test was used to determine whether significant effect of distance existed.

The percentage of marked spiders, pirate bugs and predatory thrips was pooled together and called ‘other beneficials’. Logistic regression was used to determine the effect of treatment, distance, row side, presence of buckwheat, side of the trap, treatment x distance interaction and species of beneficial insect (spiders, pirate bug or predatory thrips) on the percentage of other beneficials marked with yellow dye, albumin, casein and a double mark. There was no significant effect of species of beneficial insects on the percentage of beneficial insects marked by yellow dye, albumin, casein and a double mark. Consequently, data are presented pooled over species. The effect of distance from the middle of the plot on the percentage of other beneficials marked with yellow dye, albumin, casein and a double mark was determined for each treatment using logistic regression in the same manor as described for parasitoids.

Research results and discussion:

Objective 1: Parasitoid survival and fecundity in the laboratory on nectar resources

Results showed that female A. pseudococci provided with vetch plants survived four days longer compared with those females provided water only (Fig. 1). Buckwheat enhanced survival of female A. pseudococci three days compared with water, although this result was not statistically significant. The total number of offspring produced by female A. pseudococci increased by up to 152% when females were provided vetch and buckwheat compared with water only (Fig. 1). Similarly, providing G. ashmeadi with buckwheat and vetch plants enhanced G. ashmeadi longevity by nine and six days, respectively, compared with water only (Fig. 2). Offspring production was increased by up to 142% when female G. ashmeadi were provided buckwheat and vetch plants compared with water only (Fig. 2). This suggests that vetch and buckwheat may be a suitable food source for A. pseudococci and G. ashmeadi for enhancing longevity and fecundity in the field when sown as a cover crop. Increased fitness because of access to floral resources could, in turn, enhance biological control of mealybugs and GWSS through increased parasitism.

Objective 2: Cover crop phenology

There was a highly significant effect of sowing date (F = 93.92, df = 12, p < 0.0001), plant species (F = 2089.87, df = 1, p < 0.0001) and sowing date* plant species interaction (F = 16.29, df = 12, p < 0.0001) on mean six week height. Sowing date had a significant effect on six week height of buckwheat (F = 83.29, df = 12, p < 0.0001) and vetch (F = 32.67, df = 12, p < 0.0001) with shorter plants occurring during the winter months (Table 1). For each sowing date, vetch was significantly shorter than buckwheat (p < 0.001) (Tables 1 & 2). Such height information may be useful when selecting cover crops for crops that require an open canopy for prevention of moisture-loving diseases.

There was a highly significant effect of sowing date (F = 33.73, df = 12, p < 0.0001), plant species (F = 77.15, df = 1, p < 0.0001) and sowing date* plant species interaction (F = 7.17, df = 12, p < 0.0001) on the number of days from sowing to flowering. Sowing date had a significant effect on days until flowering of buckwheat (F = 22.31, df = 12, p < 0.0001) and vetch (F = 18.65, df = 12, p < 0.0001). Table 1 demonstrates that the number of days required from sowing to flowering for buckwheat was significantly shorter during the warmer summer months of July through to August compared with the winter months of November through to January. From April through to September it took buckwheat under 30 days from sowing to produce nectar-producing flowers. This information is important for growers intending to synchronize buckwheat nectar production to the phenology of natural enemies of key pests. Table 2 demonstrates that the number of days required from sowing to nectar production for vetch was significantly shorter during the spring months of March through May compared with the winter months of November through January. In August 2007, November 2007, June 2008, July 2008 and August 2008 vetch took significantly longer (14-32 days longer) to start producing nectar compared with buckwheat (Tables 1 & 2). This indicates that buckwheat may be a better cover crop for growers that require a quick growing plant that provides nutrition for natural enemies that are likely to contribute to the suppression of an identifiable pest problem.

There was a highly significant effect of sowing date (F = 7.16, df = 12, p < 0.0001), plant species (F = 98.86, df = 1, p < 0.0001) and sowing date* plant species interaction (F = 12.39, df = 12, p < 0.0001) on the length of nectar production. Sowing date had no significant effect on the length of nectar production of buckwheat flowers (F = 1.72, df = 12, p = 0.07) and a highly significant effect on length of nectar production of vetch extrafloral nectaries (F = 20.67 df = 12, p < 0.0001). Length of nectar production in vetch was significantly longer (up to 206 days longer) when seeds were sown in July and August compared with the rest of the year (Table 2). In contrast to results for number of days until flowering, once extrafloral nectaries were present on vetch plants, vetch plants produced nectar for up to 196 days longer than buckwheat plants in August 2007, September 2007, October 2007, November 2007, July 2008 and August 2008 (Tables 1 & 2). There is a trade off here, speed to floral production vs. longevity of floral production. This result suggests that mixed species sowings may be useful to simultaneously take advantage of quick flowering species and those that have long flowering periods.

Consequently, information on days to nectar production and the length of nectar producing period can be used to construct guides to assist growers with cover crop sowing decisions. For example, Fig. 3 portrays a guide growers can use for strategizing buckwheat plantings. Growers could select a month along the x-axis where they require increased biological control for a particular pest problem, then the duration of flowering information could be used to determine which month the grower would need to sow buckwheat to maximize nectar production for natural enemies when the pest occurs.

Objective 3: Natural enemy enhancement and pest population suppression

Results from sticky trap data

There was no significant (p > 0.05) effect of treatment on total number of pests, pestiferous leafhoppers, thrips and other pests captured on sticky traps deployed at the beginning of the trial (between June 10, 2008 and August 5, 2008). For the last time period (August 12, 2008 – August 19, 2008), treatment had significant effect on total pest counts (F = 5.70, p < 0.01), leafhoppers (F = 5.70, p < 0.01), other pests (F = 4.17, p < 0.01). Total pest counts per trap side was significantly higher (81% higher) in irrigated plots compared to controls (non-irrigated plots) (Fig. 4). We speculate that these results may be due to the irrigation increasing vine vigor, which made these vines more attractive to leafhoppers. Mean cane weight was 222% and 170% for vines in the buckwheat and irrigated treatments compared with controls (see Objective 5 below; Fig. 16). When buckwheat was present in the plot, but not the trap row, total pest counts were 77% higher than in control plots, however, this difference was not significant (Fig. 4). The number of total pests was equivalent between controls and plots with buckwheat present in the same row as the trap (Fig. 4). Leafhopper and other pests counts were significantly higher (80% and 127% higher, respectively) in irrigated plots compared with controls (non-irrigated plots). No further significant differences occurred between treatments (Fig. 4). Treatment had no significant effect on numbers of thrips captured on sticky traps in the last time period (F = 3.18, p = 0.06) (Fig. 4).

There was no significant effect of treatment on total number of beneficial insects captured on sticky traps deployed at the first three time periods (between June 10, 2008 and July 15, 2008). For the last two time periods, treatment had significant effect on total number of beneficials (time period 4: F = 5.34, p < 0.01; time period 5: F = 11.29, p < 0.001). For both of these time periods, total numbers of beneficials was significantly higher (127%-167% higher) in irrigated plots compared to controls (Figs. 5 & 6). For the last time period (August 12, 2008 – August 19, 2008), the number of beneficial insects were significantly higher (105% higher) in plots containing buckwheat near the trap (but not in the trap row), compared with control plots (Fig. 6). Additionally, the number of beneficial insects was equivalent between controls and plots with buckwheat present in the same row as the trap (Fig. 6). There were no other significant differences between treatments for both dates (Figs. 5 & 6).

The numbers of parasitic and predatory wasps captured on sticky traps showed similar trends to total numbers of beneficial insects. There was a significant effect of treatment on numbers of parasitic and predatory wasps for the last two time periods (time period 4: F = 5.33, p < 0.05; time period 5: F = 11.26, p < 0.001). For time period 4, numbers of parasitic and predatory wasps were 127% higher in the irrigated treatment compared with controls (Fig. 5). No other significant differences existed between treatments for this time period. For time period 5, numbers of parasitic and predatory wasps were 168% higher in the irrigated treatment compared with controls, and 106% higher in plots containing buckwheat near the trap (but not in the trap row), compared with controls (Fig. 6). The number of parasitic and predatory wasps was statistical equivalent between controls and plots with buckwheat present in the same row as the trap (Fig. 6).

There was a significant effect of treatment on the number of predatory thrips captured on traps between July 8th, 2008 and July 15th, 2008 (time period 3) (F = 3.94, p < 0.01). Predatory thrip counts were significantly higher (up to 126% higher) in plots containing buckwheat near the trap (but not in the trap row), compared with controls and the irrigated treatment (Fig. 7). There were no further significant differences between treatments for this time period and all remaining time periods (p > 0.05) (Figs. 5-7). Additionally, treatment had no significant effect on numbers of other beneficial insects captured on sticky traps deployed on all dates (p > 0.05) (Figs. 5-7).

In summary, results from the sticky trap data indicated that irrigated plots were attractive to leafhoppers and other pests and resulted in higher pest numbers compared to controls due to increased vine vigor. However, if irrigation was coupled with presence of a buckwheat cover crop, pest numbers were similar to controls. The combined effect of buckwheat and irrigation may have been partially due to increased levels of predatory thrips in the middle of the trial (time period 3); however, buckwheat failed to increase populations of parasitic and predatory wasps and other beneficial insects since numbers were equivalent between irrigated plots and plots containing irrigated buckwheat. The combined effect of buckwheat and irrigation, which canceled the negative effects of the irrigation on pest populations, may have been attributable to buckwheat plants supplying pollen and nectar to beneficial insects and subsequently increasing longevity and fecundity, leading to higher parasitism, predation and pest control. Our laboratory studies (see Objective 1 above) demonstrated the positive effect of buckwheat on longevity of beneficial insects and parasitism of grape pests. Alternatively, only four replicated buckwheat plots could be established for this study, which may have increased difficulty of detecting significant differences between pest numbers in buckwheat plots and controls.

The placement of sticky traps on the north or south side of the vine row had no significant effect on total pest counts, leafhopper counts, numbers of thrips, other pests, total beneficial insects, parasitic and predatory wasps, predatory thrips and other beneficial insects counted on sticky traps from all deployment dates. The side of the trap had no significant effect on total pest counts, leafhopper counts, other pests, total beneficial insects, parasitic and predatory wasps and predatory thrips counted on sticky traps from all deployment dates. In contrast, numbers of thrips counted on traps deployed during the last four time periods (June 6, 2008 – August 19, 2008) was significantly higher (37% – 65% higher) on the ‘open’ side of the trap, which was positioned furthest from the vine foliage compared with the ‘foliage’ side of the trap, which was opposite the grape canopy (time period 2: F = 15.72, p < 0.001; time period 3: F = 16.47, p < 0.001; time period 4: F = 31.50, p < 0.001; time period 5: F = 50.88, p < 0.001). This indicates that populations of thrips were predominately immigrating into the grape canopy. There was no significant effect of trap side on number of thrips captured on sticky traps deployed during the first time period (June 10, 2008 – June 17, 2008). Additionally, the numbers of other beneficial insects counted on traps deployed during time periods 2 (F = 17.26, p < 0.001) and 3 (F = 6.99, p < 0.01) were significantly higher (60-67% higher) on the ‘open’ side of the trap compared with the ‘foliage’ side. There was no significant effect of trap side on number of other beneficial insects captured on sticky traps deployed during the remaining time periods. These results indicate that populations of other beneficial insects and thrips were predominately immigrating into the grape canopy.

Results from funnel beat data

Date had a significant effect on numbers of total pests (F = 18.74, p < 0.001), leafhoppers (F = 19.09, p < 0.001), other pests (F = 6.69, p < 0.01) and parasitic and predatory wasps (F = 48.73, p < 0.001) captured in funnel trap samples conducted between June 17, 2008 and July 9, 2008. There was no date effect for the remaining insect groups (p > 0.05). Total number of pests, leafhoppers, other pests and parasitic and predatory wasps significantly increased (by 382% – 8,977%) from June 17, 2008 until July 29, 2008 (Fig. 8). Treatment and the interaction between date and treatment had no significant (p > 0.05) effect on any of the eight insect groups.

Results from visual counts data

Table 3 shows the effect of month, treatment, month x treatment and row on numbers of leafhoppers, predators and lacewing eggs counted during visual leaf inspections. Month had a significant effect on numbers of leafhopper and lacewing eggs, with maximum numbers of leafhoppers occurring in August, 2008 and maximum lacewing eggs occurring during July – August 2008 (Fig. 9). Predator counts did not significantly differ between months (Fig. 9; Table 3). There was a significant row effect only for lacewing egg counts, where the number of lacewing eggs was 63% higher on the south side (mean = 0.39 ± 0.04) of the row compared to the north side (mean = 0.24 ± 0.02) (Table 3). South sides of vines generally have higher sunshine hours and less protection from prevailing south westerly winds. Treatment and month x treatment had a significant effect for leafhopper and predator counts, but not for numbers of lacewing eggs (Table 3). Re-analyzing data by month and treatment demonstrated that treatment had a significant effect on leafhopper and predatory counts in August (leafhopper: F = 5.23, p < 0.05; predator: F = 12.44, p < 0.001), but not for June and July. For August, the number of leafhoppers counted on grape leaves was significantly higher (129-240% higher) in plots where buckwheat was present in the same row as the trap and in irrigated plots compared with controls (Fig. 10). Leafhopper counts were 145% higher in plots containing buckwheat near the trap (but not in the same row as the trap) compared with controls, although this result was not significant, probably due to low replication making it difficult to detect significant differences between treatments. Plots containing buckwheat near the trap contained significantly higher numbers of predators (up to 1150% higher) compared with the three remaining treatments (Fig. 10). There were no other significant differences in predator counts between treatments (Fig. 10).

Results from sweep net sample data

Results from the X. fastidiosa transmission studies demonstrated that buckwheat is a host of X. fastidiosa and that GWSS can successfully transmit X. fastidiosa from buckwheat to grapevines (see Objective 6 below). Therefore, data from sweep net samples conducted on flowering buckwheat plots are particularly important for determining the possible risk buckwheat might pose to being a source of X. fastidiosa in the vineyard. Sharpshooter counts were 3536% higher in grape foliage compared with flowering buckwheat plants (?2 = 2.98, df = 1, p < 0.05) (Fig. 11A). That is, only one GWSS was captured during sweep netting of buckwheat flowers across all dates and replicates (16 plots), whereas, sweep netting grape foliage resulted in 50 GWSS (22 plots). These results indicate that while it is possible for GWSS to feed off buckwheat cover crops and transmit X. fasidiosa from infected buckwheat plants to grapevines, it may be unlikely that a buckwheat cover crop would act as a significant reservoir of X. fastidiosa in the vineyard since GWSS prefer feeding on grape foliage and high populations would not be found in buckwheat cover crops. Choice and no choice trials investigating GWSS feeding preferences between grape and buckwheat are required to aid further speculation. Numbers of sharpshooters captured in sweep net samples was not effected by time (?2 = 1.04, df = 1, p = 031).

The interaction between time and treatment had a significant effect on numbers of total pests (Table 4). When analyzed by time and treatment, treatment had a significant effect on total pest counts at both sampling times (<9am: F = 59.38, df = 1, 7, p < 0.001; > 9am: F = 6.12, df = 1, 7, p < 0.05). For both sampling times, total pest counts were significantly higher (305-505% higher) from grape foliage in controls plots compared with buckwheat foliage (Fig. 12). Time had a significant effect on total pest counts when sampled from buckwheat plots (F = 18.93, df = 1, 7, p < 0.01), but not from control plots (F = 0.48, df = 1, 7, p = 0.51). In buckwheat plots, a significantly higher (57% higher) number of total pests were captured after 9am compared with before 9am (Fig. 12).

The interaction between date and treatment had a significant effect on numbers of leafhoppers (Table 4). When analyzed by date and treatment, treatment had a significant effect on leafhopper counts for all four sampling dates (June 19, 2008: F = 45.12, df = 1, 7, p < 0.001; July 10, 2008: F = 19.15, df = 1, 5, p < 0.01; July 30, 2008: F = 19.70, df = 1, 7, p < 0.01; August 14, 2008: F = 7.81, df = 1, 7, p < 0.05). For all sampling dates, counts of leafhoppers from grape foliage was significantly higher (up to 1760% higher) compared with buckwheat foliage (Fig. 13). Time had no significant effect on leafhopper counts for any sampling date (p > 0.05).

The interaction between date and treatment had a significant effect on numbers of ants (Table 4). When analyzed by date and treatment, treatment had a significant effect on ant counts for samples collected on July 10, 2008 (F = 48.01 df = 1, 4, p < 0.01) and August 14, 2008 (F = 6.56, df = 1, 7, p < 0.05). There was no treatment effect on ant counts for samples collected June 19, 2008 (F = 1.34, df = 1, 6, p = 0.29) or July 30, 2008 (F = 1.34, df = 1, 6, p = 0.29). For July 10 and August 14, counts of ants from grape foliage was 4748% and 4536% higher, respectively, in buckwheat foliage compared with grape foliage (Figs. 13B & D). Time had a significant effect on numbers of ants captured in sweep net samples collected on July 10, 2008 (F = 9.13, df = 1, 4, p < 0.05), but not on remaining sampling dates (p > 0.05). On July 10, 2008, the number of ants captured in samples conducted before 9am (mean = 10.8 ± 5.6) was 4232% higher compared with samples conducted after 9am (mean = 0.3 ± 0.3). Ants are known to feed on nectar of flowering plants. The presence of ants in the vineyard may disrupt biological control of scales, mealybugs and aphids.

Treatment (?2 = 0.94, df = 1 p = 0.33) and time (?2 = 0.08, df = 1, p = 0.78) had no significant effect on the number of thrips captured in sweep net samples (Fig. 11). The interaction between time and treatment had a significant effect on numbers of other pests (Table 4). When analyzed by time and treatment, treatment had a significant effect on other pest counts when sampled after 9am (F = 11.45, df = 1, 7, p < 0.01), but not when sampled before 9am (F = 0.24, df = 1, 7, p = 0.64). When sampled after 9am, number of other pests counted in buckwheat foliage was significantly higher (281% higher) compared with grape foliage in control plots (Fig. 12). Time had a significant effect on counts of other pest when sampled from buckwheat plots (F = 8.85, df = 1, 7, p < 0.05), but not from control plots (F = 0.31, df = 1, 7, p = 0.59). In buckwheat plots, a significantly higher (57% higher) number of other pests were captured after 9am compared with before 9am (Fig. 12).

The interaction between date and treatment had a significant effect on numbers of total beneficials (Table 4). When analyzed by date and treatment, treatment had a significant effect on total beneficials for the June 19, 2008 sampling date (F = 13.43, df = 1, 7, p < 0.01). There was no treatment effect for the later sampling dates (p > 0.05). On June 19, 2008, numbers of total beneficials was 2667% higher in flowering buckwheat foliage compared with grape foliage (Fig. 13). Time had no significant effect on counts of total beneficials for any sampling date (p > 0.05).

Numbers of parasitic and predatory wasps captured in sweep net samples was significantly effected by treatment (?2 = 8.01, df = 1, p < 0.01), but not time (?2 = 0.58, df = 1, p = 0.45) (Fig. 11). The number of parasitic and predatory wasps was significantly higher in grape foliage compared with flowering buckwheat foliage (Fig. 11). Treatment (?2 = 0.45, df = 1, p = 0.50) and time (?2 = 1.88, df = 1, p = 0.17) had no significant effect on the number of predatory thrips captured in sweep net samples (Fig. 11).

The interaction between date and treatment had a significant effect on numbers of other beneficials (Table 4). When analyzed by date and treatment, treatment had a significant effect on other beneficials for the June 19, 2008 sampling date (F = 23.6, df = 1, 7, p < 0.01). There was no treatment effect for the later sampling dates (p > 0.05). On June 19, 2008, numbers of other beneficials was 2400% higher on buckwheat foliage compared with grape foliage (Fig. 13A). Time had no significant effect on counts of other beneficials for any sampling date (p > 0.05).

Objective 4. Grape yield and quality

There was no significant difference in number of clusters between treatment plots (F = 0.63, df = 2, 10, p = 0.55). The mean weight per cluster was up to 39% higher for those harvested from buckwheat plots and the irrigated treatment compared with controls, and this result was marginally significant (F = 1.80, df = 3, 105, p = 0.07) (Fig. 14). Treatment (F = 1.41, df = 3, 105, p = 0.24) had no significant effect on mean number of berries per cluster (Fig. 14). Mean Brix content was up to 3.2 higher in control plots compared with the buckwheat plots and the irrigated treatment (F = 3.91, df = 3, 105, p < 0.01) (Fig 14). Berries harvested from the side of the row containing buckwheat plants were 0.67 mm larger compared with berries harvested from control plots (F = 2.97, df = 3, 3096, p < 0.05) (Fig. 15). Increased berry size and Brix content was probably attributable to the extra irrigation the buckwheat and irrigated treatments received, which may have increased berry size and diluted sugars in the berries, or caused excess vine vigor, thereby decreasing the amount of sunlight reaching the berries.

The percentage of berries that were shriveled due to dehydration was up to 11% higher in control plots compared with the buckwheat plots and the irrigated treatment (?2 = 288.35, df = 3, p < 0.0001) (Fig 15). This illustrates the effect of extra irrigation grapevines received at berry maturity in the buckwheat and irrigated plots. The percentage of berries with broken skin from insect damage was up to 2% higher in buckwheat plots compared with controls and the irrigated treatment (?2 = 153.14, df = 3, p < 0.0001) (Fig. 15). This result suggests that the nectar provided by the buckwheat plots may have attracted insects, such as bees and yellow jackets, which then fed on ripened berries. Bees and yellow jackets were observed feeding from berries in buckwheat plots during harvest. The percentage of scarred berries was 10% higher in plots containing buckwheat, but not in the same row of the grapes, compared with controls (F = 2.71, df = 3, 105, p < 0.05) (Fig. 15). Feeding by thrips adults and larvae can scar immature berries and scar damage becomes noticeable as berries mature. Aesthetic damage resulting from thrips feeding may not be important for wine grapes when compared with table grapes. The side of the row that the grapes were harvested from had no significant effect on any grape yield and quality variable (p > 0.05). Similarly, there was no significant treatment x row side interaction effect for any grape yield and quality variable (p > 0.05).

Objective 5. Vine vigor

Treatment had significant effect on mean cane weight (F = 12.85, df = 2, 35, p < 0.0001) (Fig. 16). Mean cane weight was 222% and 170% higher in the irrigated and buckwheat treatments, respectively, compared with the control plots. This difference can be attributed to the extra irrigation the vines received in the buckwheat and irrigated treatments.

Soil analyses

There was no significant effect of treatment on the percentage of sand (F = 1.22, df = 2, 13, p = 0.32), silt (F = 0.96, df = 2, 13, p = 0.41) and clay (F = 1.23, df = 2, 13, p = 0.32) present in the soil of each plot. Treatment had a significant effect on soil moisture content at 0.3 ATM (F = 3.83, df = 2, 13, p < 0.05), but not at 15 ATM (F = 1.55, df = 2, 13, p = 0.25). When saturated soil was allowed to equilibrate under 0.3 ATM, moisture content of soil in buckwheat plots was 3% higher compared with controls plots (Fig. 17). Buckwheat plots had the highest moisture content which may illustrate why buckwheat established in four of the seven plots originally allocated ‘cover crop plots’ for this trial. Furthermore, inconsistencies in moisture retention between plots demonstrated why we randomly re-allocated buckwheat and control plots for the second year of the cover crop field study. There was no significant effect of treatment on the percentage of organic matter present in soil from each plot (F = 0.50, df = 2, 13, p = 0.62).

Objective 6. Grape pathogens, pathogen vectors and grape pests

Results from the buckwheat needle inoculations showed that 63% and 53% of plants became infected with X. fastidiosa as detected by ELISA and culture tests, respectively (Table 7). This demonstrates that X. fastidiosa can successfully infect and replicate in buckwheat. Results from the vetch needle inoculations showed that 45% and 15% of plants became infected with X. fastidiosa as detected by ELISA and culture tests, respectively (Table 7). This demonstrates that X. fastidiosa can also successfully infect and replicate in vetch.

The greenhouse transmission results showed that GWSS can transmit X. fastidiosa from buckwheat plants to buckwheat plants, and from buckwheat plants to grapevines (Table 8). The grapevine to grapevine controls were also positive. Greenhouse transmission results for vetch were inconclusive. Vetch plants grown in the greenhouse were susceptible to pest problems and were extremely difficult to keep alive long enough to allow adequate testing. Four cohorts of vetch plants were set up during two years of testing, and only plants from the last cohort survived long enough for testing. Results from the ELISA testing showed that 40-50% of the vetch to vetch and vetch to grape tests were positive for X. fasidiosa transmission (Table 8). However, the culture technique resulted in 0% transmission from vetch to vetch and vetch to grape (Table 8), and this test is more reliable since it detects alive X. fasidiosa cells rather than ELISA testing which detects dead X. fasidiosa cells, sometimes resulting in false positives. Additionally, the grape to grape controls resulted in no transmission (Table 8), which may indicate there was a problem with the GWSS used for this cohort. Given these issues with greenhouse testing, field transmission tests were conducted as previously mentioned.

Results from the field transmission studies showed that transmission from grapevine to buckwheat and grapevine to vetch were successful in the field (Table 9). It is interesting to note that 3/10 field grown vetch acquired X. fasidiosa by GWSS, but only 3/24 greenhouse grown vetch acquired X. fasidiosa by needle inoculation. This may indicate there is an unknown element of the insect-pathogen-plant interface that is required for successful transmission between GWSS and vetch, or that vetch is a poor host for X. fasidiosa.

Objective 8: Dispersal of natural enemies

Results for counts of insects on sticky traps

Side of the row had a significant effect on the number of total pests, total beneficials, parasitic and predatory wasps, pirate bugs and predatory thrips, but no significant effect on the abundance of spiders and other beneficials (Table 5). The number of total pests, total beneficials, parasitic and predatory wasps and pirate bugs was up to 34% higher on the south side of the row compared with the north side (Fig. 18). Conversely, the number of predatory thrips was 44% higher on the north side of the row (Fig. 18). Presence of buckwheat had a significant effect on total pests, total beneficials, parasitic and predatory wasps, thrips and spiders, but no significant effect on the abundance of pirate bugs and other beneficials (Table 5). The number of total pests, total beneficials, parasitic and predatory wasps, thrips and spiders was up to 252% higher when buckwheat was absent compared with sticky traps placed directly by buckwheat plants (Fig. 19). The side of the trap had a significant effect on the number of total pests, total beneficials, parasitic and predatory wasps, predatory thrips, pirate bugs and other beneficials, but no significant effect on the abundance of spiders (Table 5). The side of the trap facing away from the grape foliage (open side) contained up to 55% more total pests, total beneficials, parasitic and predatory wasps, predatory thrips, pirate bugs and other beneficials compared with the side of the trap facing the grape foliage (Fig. 20). This indicates that these insect groups were predominately immigrating into the grape canopy.

The treatment x distance interaction had a significant effect on numbers of total pests, total beneficials, parasitic and predatory wasps, pirate bugs and other beneficials (Table 5). When analyzed by treatment and distance, results showed there was a significant effect of distance from the middle of the plot on numbers of total pests in buckwheat plots, where up to 76% more pests were captured 0, 1 and 10 rows from the middle of buckwheat plots compared with rows 3 and 6 (Fig. 21). Total pest counts did not significantly differ between distances in control plots (Fig. 21). At rows 0, 1 and 10, 72% more total pests were captured in buckwheat plots compared to controls (Fig. 21). Similar to total pest counts, the abundance of total beneficials and parasitic and predatory wasps were up to 126% higher 0 and 1 row from the middle of buckwheat plots compared with rows 3 and 6 (Fig. 21). This indicates that beneficial insects disperse from buckwheat refuges into the adjacent row, or that beneficial insects were highly attracted to flowering buckwheat or the irrigation in these plots. However, for control plots, highest numbers of total beneficials and parasitic and predatory wasps were also captured on the 1st row compared with remaining rows. Consequently, results comparing percentage of marked beneficial insects between distances in buckwheat plots may be more reliable for speculating about dispersal of beneficial insects compared to relying on insect counts. Interestingly, the abundance of total beneficials and parasitic and predatory wasps was significantly higher 10 rows from the buckwheat plot compared with rows 3 and 6, and this difference was not demonstrated in control plots. It is unknown why more beneficial insects were captured so far from buckwheat plots, but this may have been attributable to an ‘edge effect’ where often higher numbers of insect occur on the edges and boundaries of crops. Due to the small size of the vineyard used in this study, the 10th row from the middle of buckwheat plots was often close to the edge of a block of grapes and was therefore adjacent to a road, citrus grove or a riparian habitat that ran through the vineyard. Similar to total pest counts, rows 0, 1 and 10 resulted in up to 51% more total beneficials and parasitic and predatory wasps in buckwheat plots compared to controls (Fig. 21). This supports sticky trap and visual counts data which suggested that the flowering buckwheat, higher pest numbers and the extra irrigation in buckwheat plots enhanced numbers of these beneficial insects compared with controls.

Although the treatment x distance interaction was significant for counts of pirate bugs and other beneficials, pair-wise comparisons using Bonferroni adjustments showed there were no significant differences between distances or treatments for these insect groups. There was no significant treatment x distance interaction, treatment or distance effects on the number of spiders (Table 5). The number of predatory thrips captured in buckwheat and controls plots was not significantly effected by treatment x distance interaction (Table 5). Distance had a significant effect on predatory thrip counts (Table 5) with 59-65% less predatory thrips occurring in the middle of buckwheat and control plots compared with 1, 3, 6 and 10 rows from the middle of plots (Fig. 21). Treatment had no significant effect on predatory thrip counts (Table 5).

Results for marked insects on sticky traps

Side of the row had a significant effect on the percentage of parasitoids marked with albumin, casein and a double-mark (Table 6). The odds ratio (< 1) showed that the north side of the row contained a significantly higher number of parasitoids marked with albumin, casein and a double-mark compared with the south side of the row. The presence of buckwheat had a significant effect on percentage of parasitoids marked with yellow dye, albumin, casein and a double-mark, and on the percentage of other beneficials marked with yellow dye, albumin and a double-mark (Table 6). The odds ratio (> 1) showed that a significantly higher percentage of these marked insects were captured near buckwheat plants compared with sticky traps deployed in the absence of buckwheat plants (Table 6). The side of the trap had a significant effect on the percentage of parasitoids marked with albumin and a double-mark (Table 6). The odds ratio (< 1) showed that a significantly higher percentage of parasitoids marked with albumin and a double-mark were captured on the foliage side of the trap, indicating that marked insects were predominately immigrating from the grape canopy into the row (Table 6).

The percentage of parasitoids, spiders, predatory thrips and pirate bugs marked by yellow dye, albumin, casein or a double mark generally decreased from the middle of buckwheat plots sprayed with yellow dye, albumin and casein solution (Figs. 22 & 23). For all marks, the highest percentage of marked parasitoids, spiders, predatory thrips and pirate bugs were captured at row 0 in buckwheat plots. Results comparing the percentage of parasitoids marked by yellow dye between distances in buckwheat plots indicated that parasitoids may disperse at least 10 rows adjacent to buckwheat refuges (Fig. 22A). Results comparing the percentage of other beneficials marked by yellow dye, albumin or a double mark between distances in buckwheat plots indicated that marked spiders, predatory thrips and pirate bugs generally dispersed into the 3rd row adjacent to buckwheat refuges (Fig. 23). Furthermore, up to 17% of parasitoids and other beneficials marked with yellow dye, albumin or casein were captured in control plots (Figs. 22 & 23). This may indicate that either (1) parasitoids, spiders, pirate bugs and predatory thrips were able to disperse the 36 m between buffer zones used to separate buckwheat and control plots, and/or (2) that wind carried dried flakes of the marking spray containing yellow dye, albumin and casein into control plots marking parasitoids on sticky traps and resulting in false positives. Laboratory studies investigating the use of the triple mark containing yellow dye, albumin and casein for marking parasitoids demonstrated that relying on a double mark where any two of the three marks test positive may be more reliable and result in a lower percentage of false positives compared with relying on any one mark. Results comparing the percentage of parasitoids marked with a double mark between distances in buckwheat plots indicate that parasitoids disperse up to 10 rows from buckwheat refuges, while no double marked parasitoids were captured in controls (Fig. 22). Results comparing the percentage of other beneficials marked with a double mark between distances in buckwheat plots indicate that parasitoids disperse at least 3 rows from buckwheat refuges (Fig. 23). These results may suggest that grape growers could sow buckwheat refuges every 6th or 10th row to reduce costs of irrigating and maintaining a cover crop, while still gaining the potential benefits associated with providing beneficial insects with a flowering buckwheat refuges, such as enhancing longevity and fecundity of beneficial insects and increasing biological control of grape pests.

Figures and Tables – SW07-022

Research conclusions:

This project is the first significant set of studies that have investigated the strengths and limitations of summer cover cropping under the unique growing conditions representative of grape producing areas of southern California. Consequently, this project has made significant contributions to our understanding of the potential that cover crops offer when used as a pest management tool in vineyards in southern California. The main contributions from this work are:

  1. Demonstrating the phenology of cover crop plants in southern California. Growth time required to flowering and duration of nectar production and how this plant phenology can be manipulated and used by growers during different times of the year.
  2. Illustrating the difficulty of establishing cover crops in southern California. The phenology study contributed valuable information on the susceptibility of vetch to mites, aphids, thrips, the three-cornered alfalfa hopper (Spissistilus festinus [Say]) and the false chinch bug. This information was the first indication that vetch may not be a suitable cover crop for growing in southern California. The 2008 field trial at Bella Vista vineyard where we were unable to successfully establish vetch plots confirmed that vetch was unsuitable as a summer cover crop in southern California. The 2008 and 2009 trial also demonstrated the difficulty of establishing a buckwheat cover crop due to issues with irrigation, birds, rabbits, poor quality seed, extreme summer temperatures and severe damage to cover crop plants from tractor and vineyard workers during routine vineyard maintenance.
  3. Identification of the risk buckwheat and vetch cover crops pose in relation to harboring Xylella and insects that vector this pathogen. Results showed that X. fastidiosa can successfully infect and replicate in buckwheat and vetch ,and that transmission of X. fastidiosa from grapevine to buckwheat and grapevine to vetch by GWSS is possible in the field. However, GWSS counts were 3536% higher in grape foliage compared with flowering buckwheat plants, indicating that while it is possible for GWSS to feed off buckwheat cover crops and transmit X. fasidiosa from infected buckwheat plants to grapevines, it may be unlikely that a buckwheat cover crop would act as a significant reservoir of X. fastidiosa in the vineyard since GWSS prefer feeding on grape foliage and high populations would not be found in buckwheat cover crops.
  4. Improved understanding of how cover crops influence pest and natural enemy abundances. Laboratory studies demonstrated that vetch and buckwheat may be a suitable food source for key natural enemies of grape pests for enhancing longevity and fecundity in the field when sown as a cover crop. We speculated that increased fitness because of access to floral resources could in turn, enhance biological control of mealybugs, GWSS and other grape pests through increased parasitism. Sweep net sampling from the 2008 field trial showed that flowering buckwheat was extremely attractive to natural enemies, resulting in 2667% higher abundance of beneficial insects compared with grape foliage. Results from sticky trap and visual counts data showed that a buckwheat cover crop may show potential for enhancing abundance of predatory thrips and predators in southern California vineyards. However, results also showed that the additional irrigation required to keep the cover crop alive during the summer may also lead to increased populations of pestiferous leafhoppers and other pests. Additionally, sweep net sampling showed that flowering buckwheat plants may harbor ants which feed on nectar of flowering plants. The presence of ants in the vineyard may disrupt biological control of scales, mealybugs and aphids.
  5. Quantification of the effects cover crops have on grape yield, fruit quality and vine vigor. A buckwheat cover crop may increase berry size and weight, reduce sugar content of berries and delay berry maturity. Increased berry size and Brix content was probably attributable to the extra irrigation the buckwheat and irrigated treatments received. Extra irrigation may have directly increased berry size and diluted sugars in the berries, or indirectly affected these parameters by increasing vine vigor, thereby decreasing the amount of sunlight reaching the berries. Increased vine vigor and reduced sugar content of grapes may not be desirable for wine and table grape growers. Additionally, a buckwheat cover crop may lead to reductions in berry aesthetic quality, such as increasing the percentage of berries with broken skin caused by bees and yellow jackets and the percentage of scared berries caused by thrips. Aesthetic damage may not be important for wine grapes when compared with table grapes.
  6. The economic and practical feasibility of irrigated cover crops for pest control in vineyards in southern California. This is particularly important with respect to water usage. The amount of water applied to cover crop plots was recorded during each year’s field trial and the grower’s water bills were used to calculate the cost of irrigating the cover crop (see Economic Analysis below).
  7. Improved understanding of the dispersal of natural enemies from cover crop refuges. Results suggested that grape growers could sow buckwheat refuges every 6th or 10th row to reduce costs of sowing, irrigating and maintaining a cover crop, while still gaining the potential benefits associated with providing natural enemies with floral resources, such as enhancing longevity and fecundity of natural enemies and increasing biological control of grape pests.

Results from this four-year study suggest that nectar cover cropping during the summer months may not prove to be a viable option for grape growers in southern California due to the difficulty of establishing cover crops in southern California climate, cost of irrigation water, and both cover crops testing positive for harboring Xylella. Even with costly supplemental irrigation, it is not guaranteed that a buckwheat cover crop will consistently establish and flower in the southern California climate. Results from sticky trap and visual counts data showed that additional irrigation required by the cover crop may lead to increased populations of pestiferous leafhoppers and other pests. Additionally, an irrigated buckwheat cover crop may also result in reductions in berry quality and increased vine vigor.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Results from this four-year study suggest that nectar cover cropping during the summer months may not prove to be a viable option for grape growers in southern California. Results from our study did not justify production of a chapter for Code of Sustainable Winegrowing Workbook, however, this project made significant contributions to our understanding of the potential that cover crops offer when used as a pest management tool in vineyards in southern California by investigating the strengths and limitations of cover cropping under the unique growing conditions representative of grape producing areas of southern California. It is important to extend the results of our project to grape growers throughout California, especially since results demonstrate that cover crops may act as reservoirs of Xylella which can be transmitted to grapevines by GWSS. Results from the previous progress report were included in the January 2011 newsletter issued to Coachella Valley Grape Growers via Carmen Gisbert [UC Cooperative Extension Specialist, Coachella Valley]. In March 2011, we developed a website on cover crops for Southern California growers outlining the results of our research and listing the pros and cons of cover cropping in arid Southern California. This website can be accessed at the following address:

http://biocontrol.ucr.edu/irvin/research/wsare.html

We are currently updating this website with final results and conclusions presented in this final report. This final report is also being summarized into an informative summary report outlining the results of this research for distribution to leading grower advisers and UC extension specialists who supported this project (Carmen Gisbert [UC Cooperative Extension Specialist, Coachella Valley], Ben Drake [PCA, Temecula Valley], Nick Toscano [Extension Specialist & Area-Wide GWSS Management Team, Temecula Valley], Cliff Ohmart [IPM Director, Lodi Woodbridge Winegrape Commission; now works at SureHarvest], Peggy Evans [Executive Director, Temecula Winegrowers Association] and Phil Phillips [UC Cooperative Extension Specialist, Ventura County]. Additionally, a copy of the summary report and website address will be emailed to interested Viticulture Farm Advisors and UC Cooperative Extensions specialists around California (Napa County – Monica Cooper; Kern County – Jennifer Hashim-Buckey; Mendocino County – Glenn McGourty ; Sonoma County – Rhonda Smith; San Luis Obispo County – Mark Battany). We are also currently compiling results from this project into two manuscripts for submission to leading journals. One manuscript will focus on the potential of vetch and buckwheat as cover crop options in southern California vineyards and includes results from laboratory trials investigating effects of buckwheat and vetch on survival and fecundity of key natural enemies of grape pests (Objective 1), cover crop phenology (Objective 2) and the potential of buckwheat and vetch as hosts for X. fastidiosa (Objective 6). The second manuscript will focus on the results from the field trials at Bella Vista vineyard, including investigating the effect of buckwheat cover cropping on the abundance of pests and natural enemies (Objective 3), grape yield and quality (Objective 4) and vine vigor (Objective 5), plus the dispersal study (Objective 8), soil analyses and water usage statistics.

Project Outcomes

Project outcomes:

During the 2008 study, the seven designated cover crop plots were irrigated via sprinkler irrigation installed on existing grape irrigation, plus supplemental watering up to three times a week using a 16 gal water sprayer. Sprinklers were rated at 12 gals per hour and five sprinklers were installed each side of the 60 m2 plot (which encompassed two rows). On grape irrigation days, sprinklers irrigated for six hours emitting 360 gals per side of the plot. Sprinkler and supplemental watering days were recorded and the number of gallons of water each plot received was calculated per month. One 30 m2 side of each plot received 72-803 gals of water each month (Table 10), and by May 2008 both sides of the 60 m2 plot were watered. The estimated total number of gallons the seven irrigated plots used during the trial was 76,984 gals (Table 10). The monthly cost of the water used by our trial was calculated using Bella Vista vineyard water bills obtained from Rancho California Water Board (Table 10). Cost of Bella Vista water was approximately 0.0014 cents per gal, amounting to $110.42 cents for the entire trial.

Additionally, the Rancho California Water Board allocates monthly water restrictions and charges considerable penalties for exceeding these water allocations. During June-September 2008, Bella Vista winery was charged a total of $10,019 for exceeding monthly water allocations (Table 10). In 2007 and 2009, there were no penalties charged during these months. It may be unlikely that the water requirements of our trial caused the 2008 penalties, since water usage by our trial only amounted to 0.7% of Bella Vista’s water consumption during March – August. However, our trial consisted of small 60 m2 plots, whereas, vineyard growers would likely sow cover crops down the entire row, spacing cover crop refuges every 6-10 rows. This would considerably increase water usage and cost of maintaining the cover crop. To illustrate this, water usage and cost of water required to maintain cover cropping throughout Bella Vista vineyard (40 acres of grapes approximately 176 rows x 485.2 m long), using only sprinkler irrigation installed on existing vineyard irrigation (no supplemental watering). This was calculated based on two strategies devised from results pertaining to objective 8 (which demonstrated that parasitoids and other beneficial insects may disperse up to 10 rows from cover crop refuges):

  1. Sowing one cover crop row in every seven (i.e., having 6 rows between refuges);
  2. Sowing one cover crop row in every 11 (i.e., having 10 rows between refuges).

Results showed that for strategy (1), water usage was 2,086,156 gallons, costing $2,959 (Table 11). This water usage would increase Bella Vista water consumption during March-August by 18%, which may result in further water allocation penalties. For strategy (2), water usage was 1,327,705 gallons, costing $1,884 (Table 11). This water usage would increase Bella Vista water consumption during March-August by 11%. Increasing vineyard water consumption by 11-18% may lead to further costly penalties for exceeding monthly water allocations. Therefore, nectar cover cropping during the summer months may not prove to be a viable option for grape growers in southern California. These estimated costs are for water only and do not include cost of seed, labor for cultivating soil and drilling seed, irrigation sprinklers and labor to install and maintain irrigation.

Tables 10 & 11 – Economic Analysis – SW07-022

Farmer Adoption

A survey (reviewed and approved by Western SARE in an earlier progress report) was mailed out in June 2007, and it was intended that this survey would be repeated in June 2010 after this work and associated outreach were completed to measure the rate of adoption and percentage reduction of pesticide use resulting from utilization of our study cover crop plants. In June 2007, this survey was mailed to 100% of growers located in Ventura (5 growers), Lodi (740 growers), Coachella Valley (30 growers) and Temecula (45 growers) with help of cooperative extension specialists Phil Phillips and Carmen Gispert, and Cliff Ohmart (Lodi Woodbridge Winegrape Commission) and Linda Kissam (Temecula Winegrowers Association). Information about the cover crop project was posted online in June 2007, and growers could download the survey and return it via email. We had 225 replies from growers which is a 27.4% response rate. Preliminary results from the 2008 field trial suggested that cover cropping using buckwheat and vetch in southern California may not prove to be a viable option for grape growers in southern California. Therefore, we could not promote this technology to grape growers in southern California or Lodi where additional irrigation is required to sustain cover crops throughout the summer. Additionally, our results may have limited applicability to growers in northern California and other western states where cover crops do not require irrigation because results from our field trial were heavily dictated by adverse effects of supplemental irrigation. Consequently, the second survey was not mailed to grape growers in Ventura, Lodi, Coachella Valley and Temecula in order to measure grower adoption.

Recommendations:

Areas needing additional study

Our study investigated the use of buckwheat and vetch as cover crop options in southern California where high temperatures and zero rainfall during the summer month mean these plant species require supplemental irrigation for successful establishment. Even with costly supplemental irrigation, vetch did not establish in our field trials, and we had considerable difficulty establishing a buckwheat cover crop in both years. Investigating the use of drought tolerant plants which do not require additional irrigation for establishment may be a more successful option for supplying natural enemies floral resource for enhance pest control in southern California vineyards. One such plant may be the California poppy. Further investigation into potential drought tolerant and Californian native plants bearing nectar producing flowers or extrafloral nectars is required. Such research should investigate whether key natural enemies of grape pests can successfully access flowers and utilized nectar for increased survival and fecundity, and how these plants can be practically incorporated into current vineyard maintenance practices.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.