Using Nectar Cover Cropping in Vineyards for Sustainable Pest Management

Using Nectar Cover Cropping in Vineyards for Sustainable Pest Management

Summary

This progress report on work completed from one field season 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, that buckwheat is likely to be a better cover crop for use, cover crops may positively affect natural enemy and pest numbers, under some conditions cover crops may acquire grape pathogens but the possibility of transmission to vines needs closer study, and cover crops may have an impact on fruit yields and quality because of the additional irrigation needed to keep cover crops alive over summer.

Objectives/Performance Targets

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.

Accomplishments/Milestones

Accomplishments

Objective 1: Parasitoid survival and fecundity in the laboratory on nectar resources
In July 2007 we were scheduled to conduct laboratory trials to investigate if buckwheat flowers and cahaba vetch extrafloral nectaries increase longevity and fecundity of three natural enemies, G. ashmeadi, A. pseudococci and A. epos. We have completed these studies for A. pseudococci. Results showed that female A. pseudococci provided with vetch and buckwheat plants survived 4 and 5 days longer, respectively, compared with those females provided water only (Fig. 1). Similarly, the total number of offspring produced by female A. pseudococci was up to 4-fold higher when females were provided vetch and buckwheat compared with water only (Fig. 1). This suggests that vetch and buckwheat may be a suitable food source for A. pseudococci 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 through increased parasitism.
For G. ashmeadi, studies have been completed for the buckwheat and water treatments. The vetch study has not been completed. Since cahaba vetch is a winter cover crop, it has been very difficult to synchronize nectar production of greenhouse plants with the G. ashmeadi and glassy-winged sharpshooter (GWSS) colonies. These three factors were synchronized in October, but unfortunately experiments could not be conducted due to unforeseen circumstances (the project leader was forced to take maternity leave a month early). The vetch treatment will be completed in June 2009. Results to date showed that providing G. ashmeadi with buckwheat enhanced G. ashmeadi longevity by 9 days and increased offspring production more than 2-fold compared with water only (Fig. 2).
Finally, we were unable to establish an A. epos colony this season, therefore, trials with A. epos will be conducted in spring 2009.

Objective 2: Plant phenology
In June 2007 the phenology trial was set up at Ag. Ops., UCR. This trial involved sowing five buckwheat and vetch plots in the middle of each month for one year and measuring every six weeks plant height, the sowing to flowering times, and length of the flowering period. Preliminary results showed that mean six week height varied with sowing date for both plant species, with shorter plants occurring during the winter months. Such height information may be useful when selecting cover crops for crops that require an open canopy for prevention of moisture-loving diseases.
Fig 2a shows that the number of days required from sowing to flowering for buckwheat was shorter during the warmer summer months of July-August. 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. Vetch took between 0.6-28 days longer each month to start producing nectar compared with buckwheat. 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. This is particularly important in the summer months (June-August) where buckwheat produced nectar 17-28 days faster than vetch. Conversely, once extrafloral nectaries were present on vetch plants, they produced nectar for up to 146 days longer than buckwheat flowers throughout the year. 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. 6 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
A cover crop field study was set up at Bella Vista, Temecula where vetch and buckwheat were sown mid-February and early-May, respectively, in seven cover crop plots. Six plots maintained under current vineyard practices, which included cultivation between rows to remove unwanted weed vegetation, were used as controls. Monitoring of insects using weekly transparent sticky traps and bi-weekly 30 second funnel beat samples were conducted between May through until August. Leafhopper visual counts and 1 min sweep net samples were conducted every two weeks during June-August. Double-sided sticky tape was placed around the trunk and canes of two vines per plot and changed every two weeks during June-August to monitor mealybugs and spiders.
Sticky trap counts, funnel beat samples, sweep net samples and double-sided tape samples are currently being sorted and the number of pest and beneficial insects counted and recorded. Preliminary data for visual counts show that the number of pestiferous green and variegated leafhoppers counted per leaf was up to 42% higher in the cover crop plots when compared with controls (Fig. 7). We speculate that this may be due to the irrigation in the cover crop plots increasing vine vigor, which made these vines more attractive to leafhoppers. Data on weights of winter prunings will be conducted this winter to obtain a measure of vine vigor between treatment blocks. The number of predators counted per leaf was 150% higher on grapes in cover crop plots compared with controls, while there was no difference in the number of lacewing eggs between treatments (Fig. 7).

Objective 4. Grape yield and quality
On September 18th, 2008, the number of grape clusters present within a 3 m section of vine in the center of each plot was counted, and 10 clusters were harvested from each section and transported to the laboratory for grape yield and quality measurements. The mean weight per cluster was 32% higher for those harvested from cover crop plots compared with controls, while there was no difference in number of clusters or number of berries per cluster between treatments (Fig. 7). Mean Brix content was 3 degrees higher in control plots compared with the cover crop treatment (Fig 7). This was probably attributable to the extra irrigation the cover crop plots received which may have diluted sugars in the berries or caused excess vine vigor, thereby decreasing the amount of sunlight reaching the berries.
Berries from each of the 10 harvested clusters were removed and the number of ‘shriveled berries’ and berries damaged by feeding insects were counted. Additionally, 25 berries were randomly selected from each cluster and berry diameter was measured using calipers. The percentage of berries scarred through insect feeding damage and those stained with leafhopper excreta was calculated. Results showed that the number of berries that were shriveled due to dehydration was 452% higher in control plots compared with the cover crop plots (Fig 8). This likely illustrates the effect of extra irrigation the cover crop plots received at berry maturity. The number of berries with broken skin from insect damage was 1033% higher in cover crop plots compared with controls (Fig. 8) suggesting that the nectar provided by the cover crop 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 cover crop plots during harvest. Berry size was equivalent for both treatments (Fig. 8). The percentage of scarred berries was 31% higher in cover crop plots compared with controls (Fig. 8). 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. Sticky trap data, sweep net sample data and funnel beat sample data will be used to determine whether densities of thrips were higher in cover crop plots compared with controls. Thrips samples will be slide mounted and species identifications will be made where possible.
Sprinkler irrigation was installed on the existing grower’s grape irrigation in the cover crop plots. This watering was supplemened via water sprayer and 4WD motorbike as the sprinkler irrigation was insufficient to germinate seeds and maintain the growth of young plants. The amount of water applied to cover crop plots was recorded during the field trial and the grower’s water bills are currently being used to calculate the cost of irrigating the cover crop. This entire cover crop field trial will be repeated in 2009 and cover crop and control plots will be reallocated randomly.

Objective 5. Vine vigor
The influence of cover crops on vine vigor will be investigated this winter by measuring the weight of winter prunings from two vines in the center of each cover crop and control plot.

Objective 6. Grape pathogens, pathogen vectors and grape pests
Approximately 25 buckwheat and vetch plants were needle inoculated 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. 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 1). This demonstrates that X. fastidiosa can successfully infect and replicate in buckwheat. Results from the vetch needle inoculations showed that 8% of plants became infected with X. fastidiosa as detected by ELISA tests (Table 1). None of the vetch plants tested positive for X. fastidiosa using the culture technique (Table 1). However, due to premature death of many of the culture test plants in the greenhouse the number of replicates was only two, therefore further testing is necessary to confirm vetch as a host for X. fastidiosa.
Since buckwheat tested positive to X. fastidiosa, further testing was conducted to determine whether GWSS could acquire X. fastidiosa from buckwheat and successfully transmit the pathogen to grape vines. If GWSS can transfer the pathogen from buckwheat to grapes then cover crop plants may act as a potential reservoir of X. fastidiosa and be detrimental to 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 5 GWSS were placed into a sleeve cage on each of 5 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 grape to cover crop were tested. Preliminary results show that GWSS can not transmit X. fastidiosa from buckwheat plants to buckwheat plants. Further greenhouse studies will be conducted using vetch if this plant also tests positive to needle inoculation. Results for the other studies are pending.
Finally, 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 3 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.
125 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 alive 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 3 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 6 buckwheat plants were tested with ELISA and culture. The vetch plants were sampled at 4-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. Preliminary results show that transmission from grapevine to buckwheat was successful in the field (Table 3). We are still waiting on results from the other transmission studies.

Objective 7: Weed competitiveness
The ability of buckwheat and vetch to outcompete weeds was not investigated this year 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. Next year vetch will be sown earlier (December 2008 instead of mid-February) to ensure sufficient establishment before the high spring and summer temperatures occur in southern California.

Objective 8: Dispersal of natural enemies
In July 2008, insects were triple marked with yellow fluorescent SARDI pigment and a 80% milk: 20% egg white mix by spraying plants with this mixture via a 2-stroke backpack sprayer. Marking insects in this manner by spraying the plants they are visiting and living on was intended to investigate the dispersal of natural enemies from cover crop plots into the vineyard. 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. Control plots were untreated 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. An additional 6 transparent sticky traps were placed on the 1st, 3rd, 6th and 10th row adjacent to the center of 4 replicates of each treatment, Cards were placed in each cardinal direction. Sticky traps were collected and replaced three and six days after marking to determine how long insects remained marked under prevailing field conditions. Sticky traps have been placed in the freezer waiting processing. In the laboratory, sticky traps will be scanned with a UV light to determine the presence of the yellow fluorescent pigment on beneficial insects and approximately 1,000 beneficial insects (marked with the fluorescent pigment and unmarked) will be removed from sticky traps across all treatments and replicates for each sampling date (2,000 insects in total). Insects will be removed using a toothpick, placed into individual 1.5 ml microcentrifuge tubes, labeled and frozen. Samples will be sent to James Hagler (USDA-ARS Phoenix Arizona) for ELISA testing to detect milk and egg proteins.
The number of marked and unmarked beneficial insects will be compared between treatments, distances and dates using three-way ANOVA. Data from this experiment will be used to determine: (1) how far marked beneficial insects disperse from cover crop refuges, (2) how many cover crop rows are required to assist natural enemy dispersal in vineyards, (3) whether control plots contained marked insects from neighboring cover crop treatments (i.e., how efficient the 36 m buffer zones were).

Objective 9: Outreach and education
This project includes a comprehensive outreach plan to extend the results of this research to growers in Temecula, Lodi and Coachella Valley. Funding was awarded from Hansen Trust to extend outreach efforts to Ventura grape growers. As part of these outreach efforts, a survey (reviewed and approved by Western SARE in an earlier progress report) was mailed out in June 2007 and this will be repeated in June 2010 after this work and associated outreach are 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 (http://www.biocontrol.ucr.edu/irvin/Research/WSARE.html) and growers could download the survey and return it via email. We had 225 replies from growers which is a 27.4% response rate. The surveys are currently being collated and information transferred into an Excel spreadsheet. The field work funded by WSARE will be conducted over the next 2 years and results will be extended to growers in Ventura, Temecula, Lodi and Coachella Valley following the comprehensive outreach plan detailed in the grant. In June 2010, the survey will be mailed again to determine rate of adoption and the percentage reduction in pesticide use.
Table 4 shows the schedule of outreach originally proposed. However, since the project leader went on maternity leave in October 2008, the scheduled outreach for October 2008, November 2008 and February 2009 will be delayed until the following year. This will not effect the project’s end date.

Impacts and Contributions/Outcomes

Impacts and contributions:

Work completed and reported here is the first significant set of studies that have investigated the strengths and limitations of cover cropping under the unique growing conditions representative of grape producing areas of southern California. Consequently, this project is making 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 contributions this work had delivered here in this progress report are:
1) Identification of cover crop species that can be used in southern California
a. Growth time required to flowering and duration of flowering and how this host plant phenology can be manipulated and used by growers during different times of the year
b. Improved understanding of how cover crops influence pest and natural enemy abundances
c. Identification of the risk cover crops pose in relation to harboring PD and insects that vector this pathogen
d. Quantification of the effects cover crops have on grape yield, quality, and quantity
2) The economic and practical feasibility of cover crops for pest control in vineyards in southern California will be quantified. This is particularly important with respect to water usage.
3) The vineyard-wide effect cover crops have on natural enemy activity and pest suppression as spillover occurs from resource rich areas with cover crops into resource poor areas devoid of cover crops occurs.
4) Quantification of the competiveness of cover crops over undesirable weed species will provide additional information of the potential benefits of cover cropping in addition to natural enemy conservation and enhancement.

Collaborators: