Final Report for SW02-035

Project Type: Research and Education
Funds awarded in 2002: $31,280.00
Projected End Date: 12/31/2004
Region: Western
State: California
Principal Investigator:
Dr. Sean Swezey
Center for Agroecology and Sustainable Food System
Expand All

Project Information

Abstract:

A twice-weekly summer vacuuming treatment of alfalfa trap crops significantly lowered damage due to WTPB in associated organic strawberries in June 2003 and June and July 2004 compared with the organic strawberry grower’s standard whole-field vacuuming treatment. This vacuumed alfalfa trap crop treatment reduces grower’s vacuuming costs (tractor, tractor fuel, and driver time) by 78%. An economic analysis indicates that positive returns from the use of trap crops were observed in June 2003 and June and July 2004, and the overall positive return for the three months of trap cropping treatments in 2004 was calculated at +$734/acre.

Project Objectives:

Our specific objectives included:

1) Establish and evaluate the attractiveness to WTPB of on-farm trap crop plantings in organic strawberry fields by planting, irrigating, weeding, and maintenance of replicated culinary radish and alfalfa on beds directly adjacent to commercial strawberry beds.
2) Beginning in June of 2003, use tractor-mounted vacuum devices in a replicated experiment of alfalfa trap crops and organic strawberries including treatments that: a) vacuum only the trap crop vegetation, b) vacuum the whole strawberry field in the absence of the trap crop vegetation, c) leave trap crop vegetation and the field unvacuumed, and d) leave an untreated (unvacuumed) control without trap crop vegetation in 2003; or vacuum both the trap crop and the adjacent strawberries in 2004.
3) Monitor the weekly summer densities of WTPB and associated natural enemies in strawberries and trap crops as a function of distance from the trap crop.
4) Monitor seasonal damage caused to strawberry fruit as a function of treatment.
5) Calculate the relative economic costs and benefits of the treatments of interest to the grower-cooperator.
6) Partner with other experts in the field of on-farm habitat conservation and farmscaping to hold two extension/training workshops for extension of experimental results to coastal Central California agricultural professionals.

Introduction:

California organic strawberry growers declared a farm gate sales value of $28.4 million on 1382 acres in 2004, according to registration data from the California Organic Program in the California Department of Food and Agriculture. According to this data, over the past five years the value of California organic strawberries has more than tripled (from $8.7 million declared in 1999) and planted acreage has increased by over 70%. Many experienced conventional strawberry growers have recently converted some of their acreage to organic production in order to respond to increasing demand for organic strawberries. However, organic strawberry growers face significant key pest challenges (soil fungi, foliar and fruit pests) that pose varietal, rotational, and system redesign challenges for the success of organically compliant, non-chemical control. Organic strawberry growers cannot use most registered chemical pesticides nor do they routinely use any organically compliant insecticide products for key arthropod pest control due to expense and low efficacy of these products. Instead, California organic strawberry growers rely on rotation/field isolation from sources of key pests, adjustment of fresh-market planting and harvesting schedules, sanitation, and biological, physical and mechanical controls (Gliessman et al. 1996).

A key economic pest of strawberries in coastal Central California is the western tarnished plant bug (WTPB) or lygus bug, Lygus hesperus Knight (Hemiptera: Miridae)(Allen and Gaede 1963). This insect is a native species and it feeds on a broad range of winter broadleaved weeds in coastal central California, including wild radish, mustards, chickweed, shepard’s purse, common groundsel, lupine and other legumes, and knotweed (Strand, 1994; Barlow et al, 1999). More than 100 species in 24 plant families have been listed as WTPB hosts (Scott, 1977; Barlow et al. 1999). Leguminous cover crops (vetches, clover, and especially alfalfa, etc.) can also host WTPB in spring and summer. Alfalfa is consistently listed in these host inventories as a preferred host. Only the adult form of WTPB can fly from one plant to another, and they move from one host plant to another as each plant begins to flower. In coastal central California, WTPB overwinters as an adult on or near winter broadleaved hosts and reproduces in the spring on these hosts. In the late spring when rain ceases and these plant hosts dry out, the adults rapidly colonize flowering crops, including strawberries. Strawberries are not a preferred host of WTPB in California, but the absence of other more attractive host plants in early summer stimulates the colonization of strawberries. Based on heat unit accumulations typical of the central coast, two-three generations of WTPB develop between April and November in strawberries

WTPB adults are about 6 mm. long and variable in color. They are characterized by a conspicuous yellow or pale green ‘v’ on the scutellum. Females insert eggs into host plant tissues and often only the operculum or tip of the egg is visible externally and thus the eggs are not easily detectable. The first and second instars are tiny and pale green with a distinct red terminal antennal segment. The third through fifth instars are larger and have five black dots on the back (Sorensen, 1939).

Feeding by all five nymphal stages and by adult WTPB causes distortion of strawberry fruit, known as cat facing, rendering them unacceptable for fresh market sale. Distortion of the berries occurs when feeding by WTPB destroys developing embryos in achenes (seeds) during early fruit development, preventing growth of the fruit tissue beneath and surrounding the damaged achenes (Handley and Pollard, 1993).

WTPB damage is often called cat facing because it is characterized by depressed areas where all damaged achenes are contiguous and nearby undamaged (expanded) areas on the same fruit. Damage can also be centered on the apical area of the berry and for that reason is sometimes referred to as “apical seediness”. Allen and Gaede (1963) first showed that WTPB damage was not due to a toxic action but rather that feeding inside the achenes and the subsequent destruction of the enclosed embryo and endosperm was the reason for the deformed berries. Hormones responsible for receptacle enlargement are produced in the achenes and then translocated to the surrounding tissue. When WTPB feeds on the achenes, hormone synthesis and translocation is disrupted. Several studies indicate that WTPB feed on strawberry achenes from the time that the flower opens until the beginning of enlargement, when the achenes become too hard for their stylets (mouthparts) to penetrate. Handley and Pollard (1985) conducted electron microscope observations of WTPB feeding and damage and found that the holes in damaged achenes were consistent with the size of WTPB stylets. They also found that after selecting a feeding site, WTPB punctures the achene several times, causing considerable damage that results in a hollow achene. Damaged achenes range in size from completely undeveloped to fully developed. Allen and Gaede (1963) hypothesized that because WTPB bugs are able to feed on the achenes from 4 days before pollination to approximately 10 days after pollination that there should be more large hollow achenes than small ones on a damaged strawberry. Both WTPB adults and nymphs feed on developing achenes, but because nymphs appear to be quicker to select a site and are unable to fly, they tend to stay on a single seed/plant longer and thus cause more damage than adults.

In their electron microscope observations, Handley and Pollard (1985) found that after achenes become too hard for WTPB to penetrate, they begin to feed on the receptacle near the achenes. Once feeding on the receptacle, WTPB targets the vessel conducting nutrients to the developing achene. Digestive enzymes from WTPB saliva can cause necrosis, deformation, and fruit abscission. Udayagiri and Welter (2000) observed that WTPB preferred to oviposit on the strawberry receptacle because of the complexity of its surface and the small distance between achenes on an enlarging berry. This reduces the accessibility of WTPB eggs to Anaphes iole, an egg parasitoid. WTPB oviposition into the receptacle alone can cause damage to the receptacle because of the wound created. WTPB adults that oviposit in strawberry may also feed at the same time, resulting in additional damage to the strawberry.

Insufficient pollination can also be responsible for the non-enlargement of some achenes, but these achenes are small with their embryo and endosperm intact. Riggs (1990) found that non-pollination alone did not have a significant effect on fruit deformity, but that the interaction between WTPB feeding and pollination was highly significant. He showed that WTPB feeding at the early stages of achene development had an effect on fruit deformity. After 16-17 days of development, WTPB feeding mostly affected only fruit weight. These sources suggest that the main criteria for distinguishing WTPB damage from other types of damage in strawberries are: (1) the emptiness of the damaged achenes, (2) the non- or partial enlargement of the surrounding receptacle, and (3) the color of the achenes: green (possibly damaged) or brown (damaged).

WTPB nymphal densities in strawberry fields can be consistently and most economically estimated by beating plants onto a clean enclosed surface, such as a white pan or white sheet of paper or cloth. Since adults are very mobile, their numbers are more accurately estimated by vacuuming plants using a leaf blower modified to act as a vacuum (Zalom et al. 1993). Economic damage is estimated to occur at densities of 1 or 2 WTPB per 20 strawberry plants sampled (Strand, 1994).

WTPB densities should be monitored throughout the season in organic strawberries. No organically compliant insecticides are very effective against the early instars; hence preventive control measures must be undertaken. These preventive measures must be timed especially to coincide with periods soon after egg hatch in each generation. Since WTPB adults migrate to strawberries from weeds in the spring, when strawberry flowering commences, plants need to be monitored in spring to determine the first appearance of adults. Egg hatch can be estimated using a degree-day (DDU) model (Pickel et al. 1990). Using a base temperature of 54 F, egg hatch is estimated to occur at 252 F DDUs, and on the central coast in California this translates to approximately 3 – 4 weeks after the first appearance of the adults in the trap crop, weeds or strawberries under cool spring temperatures. The second hatch of nymphs is predicted to occur 779°DDUs after the first nymphs are found in surrounding weedy host plants. The third hatch of nymphs is predicted to occur 799°DDUs after the first adults are found in the strawberries (Strand, 1994).

Naturally occurring native predators of WTPB eggs and nymphs include big-eyed bugs, Geocoris spp., minute pirate bugs, Orius spp., green and brown lacewings, Chrysoperla and Hemerobius spp., damsel bugs, Nabis spp., the convergent lady beetle, Hippodamia convergens Gurin-Mneville, and several species of spiders, which feed on aphids, white flies, and lepidopteran pests besides feeding on WTPB nymphs and eggs (Clancy and Pierce, 1966). Monitoring should take the presence of these beneficial insects into account. While these predators feed on WTPB eggs and nymphs, they often do not keep summer populations below the economic injury level. L. hesperus is also parasitized by a few native parasitoids including the egg parasitoid, Anaphes iole Girault (Hymenoptera: Mymaridae). A. iole appeared to have the greatest potential for suppressing WTPB populations in strawberries. Adult wasps are minute (0.6 mm) and black. The wasps were commercially available and have been released for WTPB suppression in strawberries and on small acreages in organic strawberries in California. In experimental plots in conventional strawberries augmentative releases of adult wasps @ 37,500/ha/wk provided a 43% WTPB reduction and a 22% reduction in berry damage (Norton and Welter 1996). Subsequent research has shown that A. iole releases can be as effective as insecticides for lowering WTPB densities (Udayagiri et al., 2000). However, efforts to modify release strategies and enhance performance of the parasitoid by integration of Anaphes releases into trap crops has not been recently feasible due to high release rates required, commercial unavailability, and high cost of repeated releases of parasitoids, which do not survive over multiple generations. Biological control of the tarnished plant bug in seed alfalfa on the east coast (Lygus lineolaris Palisot de Beauvois) has been achieved by releases of the single solitary endoparasitoid Peristenus digoneutis Loan, a parasitoid of the closely related European mirid Lygus rugulipennis Poppius (Day et al, 1990; Day, 1996). This braconid parasitoid is now widespread (Day et al. 2000) and populations of L. lineolaris are estimated to have been reduced by 75% in alfalfa. P. digoneutis has also been recently recovered from L. lineolaris in organic and conventional strawberries in western New York State, with up to 24% of nymphs parasitized in fields lacking insecticide applications. These parasitoids originated from releases into alfalfa and there was a positive relationship between rates of parasitism in strawberries and nearby alfalfa and old-field habitats. No nymphal parasitoids have been recorded for WTPB in coastal central California, and given results in New York, we have recently begun releases (in cooperation with the California Department of Food and Agriculture) of P. digoneutis into alfalfa in trap-cropped organic strawberries in coastal Central California.

Efforts to suppress WTPB populations using tractor-mounted vacuum devices, including the “bug vac” technology originating on the California central coast, have been successful in lowering adult and nymphal densities and reducing damage (Pickel et al. 1994, Vincent and LaChance, 1993, Bancourt et al., 2003). Depending on suction force generated and operational method, vacuum machines are an important and common tool for suppression of WTPB in organic strawberries in coastal central California. Since most vacuum machines remove larger instars and adults in the upper canopy, but may have limited impact on the early instars or insects located in the lower canopy (Vincent and Chagon, 2000), weekly use of the vacuum is necessary for effective control, and repeat vacuuming during each week may be necessary. WTPB adults are mobile and can rapidly move back into strawberries after passage of the vacuum. Growers should routinely check that under the airflow and speeds of operation of their weekly vacuuming, that the use of the vacuum is effective in reducing WTPB numbers and damage, and make adjustments in their vacuuming program according to field sampling information. Vacuuming should begin as soon fields are dry in the spring, and can be timed to coincide with intervals in which nymphs and adults are abundant, according to degree-day unit calculations mentioned above.

Trap cropping as a management strategy for key agricultural pests has been reviewed by Hokkanen (1991), and although documented for soybeans, cotton, and several other cropping systems, successful key pest trap cropping programs do not appear numerous in the literature. However, as more recent attention has been focused on this idea, effective trap cropping of green vegetable bug (Rea et al., 2002) and pepper maggot (Boucher et al. 2003) has been documented. A WTPB bug trap cropping strategy that has been historically documented in cotton in the San Joaquin Valley of California is the establishment of alfalfa trap crops adjacent to cotton. (Stern et al. 1964, 1969; Sevacherian and Stern 1974; Godfrey and Leigh 1994.) Since alfalfa is a preferred host for WTPB, cotton is protected by close proximity to alfalfa when properly managed. Blackmer et al. (2004) have recently shown that alfalfa with other WTPB adults present, damaged by previous WTPB feeding, and flowering alfalfa are highly attractive to WTPB adults in an olfactometer bioassay due to a number of plant odors that were identified.

We hypothesized that this relationship could also be true for an alfalfa/strawberry association for organic strawberry production, given the necessity to employ non-chemical cultural methods for control of WTPB. An alfalfa trap crop could be planted on beds at the field border and within the field to attract WTPB. Once the insects are concentrated in the trap crop, we hypothesized that they can be controlled by increased numbers of beneficial insects, and/or they could be vacuumed with a tractor-mounted vacuum machine that generates a suction force sufficient to remove the insects from the trap crop, killing them in the fan housing. This strategy would also seek to avoid vacuuming the in-field adjacent strawberry rows as much as possible (grower’s current practice), since the vacuum machines are non-selective in their effect and also remove beneficial insects from the strawberry crop (Vincent and LaChance, 1993). Repeated in-field vacuuming may also increase problems with mold and mildew by spreading spores of these diseases (Strand, 1994). Our hypothesis included the theory that it should be possible to reduce WTPB numbers and damage in unvacuumed strawberry rows associated with alfalfa trap crops. Any control measures (vacuuming, release of beneficial insects, etc.) would be concentrated in the trap crop vegetation. Previously, Easterbrook and Tooly (1999), in England, reported that trap crops of alfalfa and plants in the family Asteraceae did not consistently reduce Lygus rugulipennis in strawberries. However, they did not incorporate vacuum treatments of the trap crop into their experiment. We report here our final evaluation of our management hypotheses concerning suppression of WTPB in alfalfa trap crops with tractor-mounted vacuums in replicated studies in a certified organic strawberry field in Salinas in the 2003 and 2004 production seasons.

Research

Materials and methods:

Materials and Methods

In October, prior to each of the two study years (2003, 2004), the principal investigator and two field assistants assigned to the project met with the president, pest control advisor, and farm manager at Pacific Gold Farms, Watsonville, CA, to plan the experimental design and management responsibilities. The research team consisted of the principal investigator, Sean L. Swezey; field assistants, Janet Bryer and Diego Nieto; Pacific Gold Farms president, Larry Eddings; Pacific Gold Farms manager, Ramon Serrano; and Pacific Gold Farms pest control advisor, Joe Valdez. All experiments were carried out at Eagle Tree Farm on a 35-acre certified organic strawberry farm located several miles north of Salinas, CA.

The experimental design was set up each year as completely randomized plots consisting of four treatments replicated four times: 1) unvacuumed, non-trap-cropped strawberry rows (untreated control) in 2003, and in 2004 non-trap-cropped whole field vacuumed strawberry rows (grower’s practice control) with a strawberry row (row 0) in the place of the alfalfa trap crop, 2) unvacuumed alfalfa trap crop and unvacuumed adjacent strawberry rows, 3) vacuumed trap crop with unvacuumed adjacent strawberry rows, 4) vacuumed trap crop and vacuumed adjacent strawberry rows (in 2004). Each treatment plot consisted of 16 rows of strawberries (numbered 1 through 16 based on proximity to the trap crop vegetation; row 1 directly adjacent), with each row measuring approximately 150 ft. in length. Strawberry beds were planted on 48-in. centers, and were 18 in. in width and 12 in. in height. Strawberry varieties included in experimental plots across the farm were a mixture of day-neutral Seascape, Aromas, and Diamante varieties in each year. In 2003, four replicates were lost in July and August due to the damaging effects of root rot pathogens to the strawberries in those plots. We treated these data as “missing” in our statistical analyses of data for those months. Plots were distributed over an area of approximately 15 acres. Plots were separated by at least 25 feet in all cases. In the trap-cropped treatments of 2003, two edge beds were left unplanted to strawberries, and, instead of strawberry crowns, were planted to seeds of a planted trap crop consisting of culinary radish (outside trap crop bed) and alfalfa (inside trap crop bed). In 2004, only alfalfa was planted as a trap crop in all treatments. All strawberry and trap crop beds of the experiment were planted by the end of October in each year, and germination and development with fall rains was excellent. In 2004, culinary radish was eliminated from the experimental design due to relatively poor attractiveness for WTPB and horticultural difficulties (size of plants, lack of consistent flowering, weediness). All beds were drip-irrigated and fertilized with a single sub-surface tape, as needed.

From April-September 2003 and June-September 2004, we monitored the trap crop plants in each replicate weekly with a hand-held vacuum suction device (modified reversed Stihl BG75 leaf blower) (Zalom et al., 1993) with a 5-inch insect-netted intake orifice secured by rubber bands. Each week, a whole sample consisting of 50 one-second suction points was taken from a continuous line walked along the trap crop vegetation. We also monitored strawberry row 1 in the same fashion.

In June of each sample year, the collaborating grower began treating the replicated plots in the described fashion, with a 65 hp tractor, mounted with three rectangular vacuum collector inlets (6X24 in). The inlets were mounted on 48-in. centers in order to match up with strawberry bed dimensions. Each inlet had an associated fan that generated an average negative pressure at each inlet surface of approximately 28 mi/hour (40 km/hr) based on the average of several measurements taken with a portable wind speed indicator. The tractor was driven at a speed on approximately 1.5 mi/hour when vacuuming the rows. Where required by treatment, one inlet was passed over the alfalfa trap crop row two times (30 minutes between passes) at approximated 2/3 canopy height, two days a week between 10 a.m. and noon each week until September. On all strawberry rows that required treatment, the tractor-mounted vacuum was passed over the plants at canopy height once a week. In June of each sample year, we began monitoring insects in rows 1, 2, 4, 8, and 16 with the hand-held vacuum as previously described. On May 28, 2003, the radish trap crop plants were completely removed and the drip irrigation to that row tied off, due to senescence and termination of flowering. In the first week of June in both sample years, we established three randomly selected clusters of four strawberry plants as permanent “pick plots” in rows 1, 2, 4, 8, and 16. Each week, developing berries that showed signs of distinct WTPB damage were counted and removed. New berries with no evidence of damage were tagged with a colored twist-tie to avoid double counting, and then counted as undamaged upon maturity. Percent damaged berries were calculated as a cumulative percentage by month of harvest during the WTPB “damage season” (June= early, July=mid, August=late).

All statistical analyses and comparisons of means were performed with the ANOVA (repeated measures where relevant) program of STATISTICA software, by Stat Soft Inc., Tulsa, OK at an alpha level = 0.05. A least significant difference test was performed to separate means except where indicated. All insect counts were transformed by the square root of n + 1 transformation and percent damage estimates by the arcsine transformation before statistical analysis.

Research results and discussion:

Figure 1 shows average total (adult and nymphs) weekly accumulation of WTPB in the trap crops from April-May, 2003. A highly significant difference was found when the radish, alfalfa, and strawberry rows were compared. For seven weeks in April and May, alfalfa attracted or retained over 7 times more WTPB than radish. Radish was also significantly more attractive than the adjacent strawberry row. Alfalfa was a significantly better trap crop than radish in terms of number of total WTPB attracted. This is a fundamental result, because tractor-mounted vacuum management of a trap crop can begin in mid May, when all winter drainage ditches are closed and the threat of heavy spring rain and muddy row conditions has diminished.

Figure 2 illustrates mean total of WTPB in the unvacuumed alfalfa/strawberry treatments by date throughout the main harvest season (June-August) in 2003. WTPB densities in alfalfa were substantially higher than in adjacent strawberry rows and peaked in the trap crop on 27 July. At this time, WTPB densities begin to decline, and it is possible that alfalfa quality or some other feature of the attractiveness of the trap crop is diminished. Similar results were recorded in 2004 (data not shown), and these data validate the relatively high attractiveness and effectiveness of alfalfa plantings as a WTPB trap crop in organic strawberry systems until mid-late August.

Figure 3 shows mean total accumulated WTPB in the trap crops in vacuumed and unvacuumed alfalfa trap treatments. In 2003 and 2004, the use of a tractor-mounted vacuum device reduced WTPB populations in the vacuumed trap crops by approximately 68% and 88%, respectively, from June-September each year. This is a significant result indicating that the tractor-mounted vacuum can effectively remove large percentages of WTPB from an alfalfa trap crop.

Figure 4 illustrates total (adults and nymphs) accumulated WTPB by treatment and row means in June 2003. Weekly tractor-mounted vacuuming of the alfalfa reduced total WTPB in the alfalfa trap crop row by 70% in June. Except in row 4, the vacuumed trap crop treatment also had the same accumulated WTPB as either the whole-field vacuuming or the untreated control.
However, why total WTPB in the untreated control was consistently low is not clear, unless general whole-field vacuuming in the commercial strawberry fields surrounding this experiment lowered the general level of WTPB pressure in these small plots. “Sinking” of WTPB to nearby trap crop treatments could also explain this effect. It is possible that the experimental plots were not located far enough away from each other to diminish this effect. It is clear from these data that an un-vacuumed trap crop/strawberries treatment in June will result in significantly increased presence of WTPB in the strawberry rows 1-8. It is also clear that vacuuming an alfalfa trap crop (but not vacuuming the associated strawberries) did not result in increased in-field WTPB counts, when compared with the grower’s whole-field vacuuming program. No effect due to treatment was seen at row 16, indicating a maximum distance of any trap crop effect on WTPB counts could not be detected past row 8 to row 16 in June 2003.

Figure 5 shows overall mean strawberry damage by WTPB due to treatment in June 2003. The vacuumed trap crop treatment (11.1% damage) had significantly lower percent damaged strawberries than either the whole-field vacuuming (41% reduction) or the untreated control (48% reduction). It is interesting that the unvacuumed trap crop treatment has not yet shown a significant difference in damage in this early sample, even though it accumulated significantly more WTPB in rows 1-8. The fact that the highest damage was seen in the untreated control, which consistently accumulated fewer WTPB than the unvacuumed trap crop, is also interesting. This could indicate that WTPB in June associated with or feeding on nearby trap crops do less per capita damage to strawberries, i.e. there may be a WTPB physiological habituation effect after feeding in alfalfa to explain why abundance alone doesn’t correspond well with damage.
Figure 6 shows accumulated WTPB by treatment and row means in July 2003. Vacuuming of the alfalfa reduced total WTPB in the alfalfa trap crop row by 79% in July. Except in row 4, the vacuumed trap crop treatment also had the same accumulated number WTPB as either the whole-field vacuuming or the untreated control. Again, the reason why the total WTPB in the untreated control plots was consistently low is not clear, unless general whole-field vacuuming and “sinking” to nearby trap crop treatments explains this effect. It is clear that an unvacuumed trap crop treatment in July will result in significantly increased presence of WTPB in the strawberry rows 2-16, when compared with a grower’s whole-field vacuuming program. This is also a significant result in that it shows that an alfalfa trap crop can affect WTPB activity at least as far as 16 strawberry rows away. It is also clear that vacuuming a trap crop, but not the associated strawberries, does not result in increased WTPB counts in July, when compared with the grower’s whole-field vacuuming program.

Figure 7 shows overall mean strawberry damage by WTPB due to treatment in July 2003. In the mid-season, vacuuming of the trap crop reduces total damage in associated strawberries by 49% when compared with the un-vacuumed trap crop. However, no significant differences in damage could be detected between the vacuumed trap crop and either whole-field vacuuming or untreated control treatments. It is clear that in July, an unvacuumed trap crop will accumulate significantly higher damage than other treatments, indicating that the trap crop must be managed with the vacuums to diminish this effect before it occurs. The next highest damage due to treatment is seen in the untreated control, but the magnitude of this treatment damage is not significant (as in June).
Figure 8 shows mean accumulated WTPB by treatment and row in August 2003. Differences between vacuumed and unvacuumed alfalfa trap crops were not statistically different. Treatment differences were recorded through row 16. From late July through the remainder of our sampling season, our grower cooperator experienced several operational problems on the farm that prevented him from timely and consistent vacuuming each week, and we documented three weekly trap crop vacuumings that were not performed during late July-August.
This time of operational problems also corresponded to a production emphasis on picking processing (as opposed to fresh market) strawberries on the farm. Processing strawberries do not have the high cosmetic standards for low WTPB damage required of fresh market strawberries. This lack of consistent weekly trap crop vacuuming may have resulted in the significantly higher WTPB densities associated with trap crops when compared to whole field vacuuming witnessed in rows 2-16 in August. Total WTPB in the untreated control plots were similar to trap-cropped treatments in all strawberry rows, indicating that even sporadic whole-field vacuum passes in August, 2003 significantly lowered WTPB populations in the strawberries.

Figure 10 illustrates overall mean strawberry damage by WTPB due to treatment in August 2003. Damage percentages greatly increased in August, possibly due to the lack of consistent vacuuming of the trap crops. Whole field vacuuming had the lowest damage rate (15.9%). The vacuumed trap crop treatment and the untreated control had equivalent damage rates, while unvacuumed trap crops had the highest rates of damage. Again, there is a strong possibility that the grower-cooperator’s inability to fully comply with the vacuuming program contributed, in part, to the increased damage in the strawberries adjacent to the vacuumed trap crop. In general, the elevated damage rates associated with unvacuumed trap crops indicate that if alfalfa is not aggressively managed to remove or reduce WTPB populations, it can actually increase WTPB-induced damage in associated strawberries.
Figure 11 illustrates total (adults and nymphs) accumulated WTPB by treatment and row means in June 2004. Weekly tractor-mounted vacuuming of the alfalfa reduced total WTPB in the alfalfa trap crop row by approximately 83% in June 2004. The only recorded difference in WTPB density was in row 1, where strawberries adjacent to unvacuumed trap crops possessed significantly higher accumulated WTPB than either vacuumed trap crop or whole-field vacuuming treatments. Overall abundance levels of WTPB were very low compared with 2003 and below economic thresholds of approximately 3-5 WTPB/50 plant sample).
Figure 12 shows overall mean strawberry damage by WTPB due to treatment in June 2004. The vacuumed trap crop treatment had significantly lower percent damaged strawberries when compared with either the whole-field vacuuming (36% reduction) or the unvacuumed trap crop (46% reduction).

Figure 13 shows total accumulated WTPB by treatment and row means in July 2004. Exclusive weekly tractor-mounted vacuuming of the alfalfa reduced total WTPB in the alfalfa trap crop row by 87%. As in June, the only recorded difference in WTPB density was in row 1, where strawberries adjacent to unvacuumed trap crops had higher accumulated WTPB than either vacuumed trap crop or whole-field vacuuming treatments. In July and August 2004, a new fourth treatment (vacuuming the trap crop and strawberries) was included in the experiment, instead of the unvaccumed non-trap-cropped control strawberry treatment included in 2003. This treatment reflects the 2004 trap-cropping and vacuuming program being employed by the grower cooperator on the entire farm, and the unwillingness of the grower cooperator to allow untreated control damage to replicate plots on the farm. In this treatment, both the strawberries and alfalfa were treated by a single vacuuming pass per week.

Figure 14 shows overall mean strawberry damage by WTPB due to treatment in July 2004. During this mid-season sample, vacuuming only the trap crop significantly reduced total damage when compared with all other treatments. Damage reduction ranged from 25-43% between treatments. Again, while overall mean abundances of WTPB in strawberries were statistically similar during this time period, damage associated with the vacuumed trap crop was statistically lowest, possibly indicating additional natural enemy effects or WTPB-alfalfa feeding habituation.
Figure 15 shows mean total accumulated WTPB by treatment and row means in August 2004. Exclusive weekly tractor-mounted vacuuming of the alfalfa trap crop reduced total WTPB in the alfalfa trap crop row by approximately 86%. Treatment by strawberry row differences was recorded in rows 1 and 2, where strawberries adjacent to unvacuumed trap crops had the significantly highest amounts of accumulated WTPB. Relative to other August samples in previous years, overall WTPB was abundance very low and remained below economic thresholds.
Figure 16 illustrates overall strawberry damage by WTPB due to treatment in August 2004. During this late season, damage associated with unvacuumed trap crops was significantly greater than all other treatments. The entire 2004 damage data set indicates that the grower-cooperator expended extra time and effort to consistently apply the weekly vacuuming treatments, and that vacuuming the strawberry rows together with the trap crop did not result in reduced damage compared with exclusive trap crop vacuuming.

The big-eyed bug (Geocoris spp.) (BEB) is an important predator of WTPB eggs and nymphs. Figures 17 and 18 show plots of the decline of big-eyed bug abundance during each harvest season in the vacuumed trap crop treatments. In each year, this decline coincided with increasing populations of WTPB nymphs. This trend, which occurs in all treatments (data not shown), may explain why WTPB damage generally increases throughout the organic strawberry growing season. In the early season, when big-eyed bugs are more numerous than are WTPB nymphs, WTPB damage is the lowest. In August, however, when that trend is reversed, damage levels increase.

Figures 19 and 20 show accumulated mean BEB abundance by row and treatment for 2003 and 2004, respectively. In 2003, significantly greater BEB abundance is associated with vacuumed trap cropping up to 16 rows away from a trap crop. In 2004, statistical differences were only recorded in row 1, where treatments associated with trap cropping had higher BEB abundance than the whole-field vacuuming program. In 2003 and 2004, overall BEB densities were significantly higher in strawberry rows (all distances pooled) adjacent to vacuumed trap crops (3.21±0.34; 1.65±0.24) when compared with strawberry rows in a whole-field vacuuming program (1.57±0.17; 1.16±0.11), (Students T-test; p<0.05) respectively. These differences in BEB densities between treatments may explain, through increased potential predation on WTPB, some of the lower damage rates associated with vacuumed alfalfa trap crops.

Figures 21 and 22 illustrate the effects of vacuuming trap crops and strawberry rows on BEB and other WTPB predators. Mean abundance of both BEB and the minute pirate bug Orius spp. (MPB) were not significantly diminished by the application of a tractor-mounted vacuum over an alfalfa trap crop in either year. Only the damsel bug Nabis sp.(DB) was diminished by this type of disturbance.
Applying a vacuum over strawberry rows decreases mean accumulated BEB populations in 2003, while this treatment did not impact MPB or DB abundance in 2003. This result for BEB is significant because it documents the negative effect of whole-field vacuuming on BEB populations in organic strawberries.

Discussion/Milestones

A number of field-based research milestones for the effective application of alfalfa trap cropping and tractor mounted-vacuums for control of WTPB in organic strawberries were accomplished with the completion of this project. Alfalfa is an attractive trap crop for WTPB, and accumulated high numbers of WTPB throughout the harvest season compared with adjacent strawberry plants. There were significantly more WTPB attracted to alfalfa than to either culinary radish or strawberry plantings in a comparative test performed in April-May 2003. Weekly vacuuming of alfalfa can remove up to 88% of total WTPB. In five of six harvest months (June-August each year) observed, vacuuming of an alfalfa trap crop (but not vacuuming the associated rows of strawberries) resulted in reduced or equivalent WTPB damage to associated organic strawberries when compared with the grower’s non trap-cropped whole-field vacuuming program. This is a key result, because a program of exclusive vacuuming of the trap crop constitutes a 78% reduction in the time and cost of vacuum treatments (on a per acre basis) compared with a whole-field vacuuming program common in organic strawberry production in coastal central California. The comparative WTPB damage results of August 2003 in which trap-crop vacuuming showed increased WTPB damage may be attributed to the inability of the grower-cooperator at Eagle Tree Farm to consistently implement the trap crop vacuuming treatment. This was due to several operational challenges experienced from late July 2003 through the end of that sampling season.
The higher damage rates in August might also be generally attributable to diminished BEB (big-eyed bug) populations, an important predator of WTPB. These populations declined in both years in August in all treatments without recovery during the remainder of the season. This dynamic might have affected late-season between-treatment damage rates due to absence of predation on WTPB.

Trap cropping with alfalfa can influence WTPB and increase natural enemy populations as far as the 16th strawberry row outward from the trap crop, and possibly at farther distances. Vacuuming has variable effects on natural enemies, reducing damsel bug populations in the alfalfa trap crop (but not big-eyed bugs or minute pirate bugs), and reducing big-eyed bugs in strawberry rows. This documented reduction of big-eyed bugs in vacuumed strawberry rows is an important stimulus for our commitment to limiting application of tractor-mounted vacuums to trap crop vegetation only.

Consistently higher WTPB damage of strawberries adjacent to unvacuumed trap crop treatments in both years indicates the need to use the tractor-mounted vacuums consistently on a fixed schedule in the trap crop each week during the harvest months.

Preliminary between-treatment economic analyses (trap crop vacuuming vs. whole field vacuuming with no trap crops) were conducted for both study years (see below). Trap crop vacuuming constitutes a 78% reduction in machine energy/effort usually expended by organic strawberry growers in whole-field vacuuming programs, and positive $/acre savings were recorded for trap crop vacuuming in June 2003 and June and July 2004. Relative to a whole-field vacuuming program, vacuumed alfalfa trap crops must reduce damage rates by a marginal 1.2% or more to obtain a comparative positive return per acre. Vacuumed trap crops in August did not produce relatively lower damage rates or higher net economic returns in either study year. Alternately, as alfalfa ages, it could become decreasingly attractive to WTPB, thereby creating a source, rather than a sink for WTPB in August. Some of this August WTPB damage is negated, however, by the organic processing fruit contracts that are often entered into by August, as buyers of processing fruit are not concerned with the cosmetic appearance of the strawberries.

Research conclusions:

On April 9, 2004, the principal investigator presented project results at an extension meeting of 30 farmers hosted by the Community Alliance with Family Farmers Foundation (CAFF) in Watsonville, CA. The CAFF sub-contractor surveyed the signed-in attendees prior to the presentation concerning their knowledge and opinions concerning trap crops. A subsequent extension meeting with approximately 35 farmers in attendance was held on November 12 to re-assess client knowledge and opinions. The survey data were then reported to the principal investigator concerning the change in knowledge reflected by the attendees after the presentations. The following report by the sub-contractor details these activities in order to document an increased level of awareness and adoption of trap cropping in organic strawberry crops in coastal central California.

Project Report: Control of western tarnished plant bug (WTPB) Lygus hesperus Knight in organic strawberry production systems with trap crops and tractor-mounted vacuums

Results of two surveys conducted by Community Alliance with Family Farmers (CAFF) of farmers and agricultural professionals concerning trap crops and farmscaping practices

As part of the Western SARE project entitled: “Control of Western Tarnished Plant Bug, Lygus hesperus Knight, in Organic Strawberry Production Systems with Trap Crops and Tractor-mounted Vacuums,” CAFF conducted surveys at two workshops, held in Watsonville, California on April 9, 2004, and on November 12, 2004. The surveys were designed to determine the knowledge and opinions of farmers and agricultural resource professionals concerning the use of trap crops in organic strawberry systems and of farmscaping practices. Initially, the surveys were intended to provide an initial baseline gathering of information, with a follow-up to assess changes in knowledge and opinions over time. However, since there was virtually no overlap in workshop attendees from the April workshop to the November workshop, the survey instead served to assess the state of knowledge concerning these topics, as well as to solicit useful comments and suggestions for further research and attention. Additionally, with the widespread media coverage of both workshops and the increased activity among farmers concerning non-crop vegetation management practices, such as trap crops and grassed waterways, the surveys provided a sample look at the change in attitudes that farmers and agricultural professionals are undergoing as a result of field trials, workshops, and farm tours.

Survey Questions

The following page is a copy of the questions contained in the survey. This survey was conducted at the meetings in April and November.

Survey Results

Survey results will be discussed under three topics: 1) current and projected use of trap crops and farmscaping practices; 2) Sources of information about trap crops and farmscaping practices; and 3) Information needed to help make a decision to plant a trap crop or farmscape practice.

1) Current and projected use of trap crops and farmscaping practices

Thirty-five surveys were collected, 19 in April and 16 in November. Of the survey respondents, 16 (84%) in the April survey and 14 (88%) in November said that they had planted a trap crop, filter strip, hedgerow, grassed waterway, or other non-crop vegetative practice.
Twelve (63%) survey respondents at the April workshop and 7 (44%) in November said they would consider planting trap crops or other farmscape practices.

Of the respondents, 14 (74%) in April and 14 (88%) in November had heard of trap cropping, and 4 people in each group (29%) had actually planted a trap crop. In response to the query whether planting a trap crop would be considered, 11 (79%) in April and 7 (50%) in November responded positively. Additionally, one individual in April who had not heard of trap cropping said he would consider planting a trap crop.

All respondents who had planted trap crops noted benefits from the plantings in the areas of increased beneficial insects and pest management, soil erosion control, weed management, improved water infiltration and runoff control, wind protection, and increased biodiversity.

2) Sources of information about trap crops and farmscaping practices

Survey respondents noted that they obtained information about trap crops from various sources including: Sean Swezey (3); UC SAREP; NRCS; ATTRA; Santa Cruz RCD (2); CAFF

SURVEY OF TRAP CROP/ FARMSCAPE CULTURAL PRACTICES

Have you ever planted a:
trap crop
hedgerow
filter strip
grassed road/waterway
other non-crop vegetation

If you checked any of the above:

– What is the purpose of your planting?

– What specific benefits have you noticed?

– Have you noticed any benefits such as increased beneficial insects, weed control, or erosion control?

– Have you heard that farmscaping can help with erosion control, attract beneficial insects, serve as a windbreak, and help create on-farm biodiversity?

– Do you think there is any benefit to planting non-crop vegetation on the farm?

– What kind of information would you need to make a decision to plant a trap crop, hedgerow, grassed waterway, filter strip, or other non-crop vegetation?

– Do you think it is important to create on-farm biodiversity?

– Have you ever heard of planting trap crops to help control Lygus in strawberries?

– Where have you heard about trap crops?

– Would you consider planting any of the following: trap crop, hedgerow, filter strip,grassed waterway, other non-crop vegetation

– If you checked any of the above, why?

– If you checked none of the above, why?

(2); Farm newspapers and magazines (3); IPM classes (2); UCCE (2); EcoFarm Conference (3); Winegrower Association; UCSC (2); Rich Merrill, Cabrillo College; UC Davis; and peers.

3) Information needed to help make a decision to plant a trap crop or farmscape practice

For the question, “What kind of information would you need to make a decision to plant a trap crop the responses fell into five categories:

Category Information needed
1. Trap Crop 1. How efficient are trap crops?
2. Do trap crops increase pests attracted?
3. More study on matching trap crops to specific pests
4. Long-term effectiveness

2. Plant 1. What plant species attract what insects?
2. What combination of plants work best?
3. How effective are plants at attracting insects?
4. Site specific information for which plants to use

3. Insect 1. How far do insects travel into the crops from the trap crops and from the insectary plantings?
2. Do trap crops and habitat plantings provide over-wintering sites for pests?

4. Cost 1. How do costs play out in trap crops over the long term?
2. How much does it cost to install and maintain a trap crop?
3. Cost/benefit analysis
4. More evaluation of costs

5. General 1. Do trap crops lead to an increase in weeds in crops?
2. Do they increase pests in crops?
3. How effective are these plantings?
4. More scientific study

Discussion

These surveys were useful for gathering information from farmers and agricultural resource professionals. In addition, the process of completing the survey encouraged participants to think about these issues and gave them the opportunity to express their knowledge and opinions. There was a high and increasing level of knowledge about trap cropping between the meetings and many of the respondents referred to the importance of vegetation plantings in contributing to the management of natural resources and pest control on farms. Twenty-nine percent of attendees who had heard of trap crops had actually planted one. On average, over half of attendees who had heard of trap crops would consider planting one. Both of the workshops and tours were reported in local and state media, and these data show that farmers are becoming more aware of the importance of non-crop vegetation for pest management. The information on trap cropping was new to some of the strawberry growers attending, and they expressed their interest in learning more and experimenting with plantings on their farms. The data indicating that a total of 19 farmers (54%) would consider planting trap crop suggest that the presentations were effective in delivering information on trap crops that would influence growers’ practices.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

A scientific article entitled: “Control of western tarnished plant bug (WTPB) Lygus hesperus Knight (Hemiptera: Miridae) in organic strawberries with alfalfa trap crops and tractor-mounted vacuums” is in preparation for immediate submission to Environmental Entomology. Copies of the following publications resulting from or related to this project are appended.

Carol, B. 2003 Mining organic gold. California Farmer 286(10)10-11

Swezey, S.L. 2004. Trap cropping the western tarnished plant bug, Lygus hesperus Knight, in California organic strawberries. Proceedings California Conference on Biological Control: California Organic Production and Farming in the New Millennium. Berkeley, pp. 58-66

Brown, M. 2004 Trap crops show potential to reduce pest damage, save time and energy in organic strawberry production. The Cultivar (Spring/Summer): 1-3;12,17-18).

On April 9, 2004, the principal investigator presented project results at an extension meeting of 30 farmers hosted by the Community Alliance with Family Farmers Foundation (CAFF) in Watsonville, CA. The CAFF sub-contractor surveyed the signed-in attendees prior to the presentation concerning their knowledge and opinions concerning trap crops. A subsequent extension meeting with approximately 35 farmers in attendance was held on November 12 to re-assess client knowledge and opinions. The survey data were then reported to the principal investigator concerning the change in knowledge reflected by the attendees after the presentations. The following report by the sub-contractor details these activities in order to document an increased level of awareness and adoption of trap cropping in organic strawberry crops in coastal central California.

Project Outcomes

Project outcomes:

Table 1 shows sample economic costs and returns associated with the vacuumed trap crop treatment and the non-trapped, whole field vacuuming treatment in June 2003. This comparison was the most important to the grower collaborator in order to evaluate modification of his whole-field vacuuming WTPB control practices. All costs and production values were estimates per acre provided by the grower cooperator and WTPB damage impact was based on the statistically significant treatment differences in damage due to WTPB observed in experimental plots. We assumed all other overall expected yields and costs were the same, and that differences in the treatments were due only to additional costs or savings due to the trap crop occupying 2.5% of the acreage (i.e. $300 of strawberry production is lost; but $131 of additional monthly production costs for those berries accrue to the non-trapped whole-field vacuuming treatment). Under these assumptions, it was calculated that even with production losses associated with the area devoted to the vacuumed trap crop, the lowered damage losses and lowered costs of vacuuming only a trap crop associated with the strawberries made the vacuumed trap crop treatment economically advantageous in June 2003.

Table 2 illustrates the difference in returns, based on the previous assumptions, for vacuumed trap crop and whole-field vacuuming treatments in all production months studied. Positive returns for trap cropping, in which the vacuumed trap crop treatment was calculated to be more profitable than whole-field vacuuming, occurred in June of both study years and July of 2004. In 2003, due to late-season difficulties with trap crop vacuuming, a negative total balance of -$659 for trap cropping was observed, whereas in 2004, a positive total balance of +$734 was calculated. We believe that, among other factors, the grower-cooperator greatly improved the application of the vacuuming treatments in 2004.
These data indicate that a trap-cropping program can economically decrease WTPB-induced strawberry damage in June and July. During these periods, when damage rates were more than 1.2% lower in vacuumed trap crop treatments compared with whole-field vacuuming, positive dollar differentials in favor of trap cropping were achieved. Current costs of trap cropping, including reduction in area planted to alfalfa trap crops and number of vacuuming passes per week, are being analyzed for optimal profitability.

In August 2003, when the grower-cooperator was unable to reliably follow a trap crop vacuuming schedule, damage rates and costs associated with trap crops rose dramatically. It is essential to consistently and proactively remove WTPB from the trap crops to reduce damage in adjacent strawberry rows. When performed consistently, vacuuming alfalfa can effectively and profitably reduce WTPB populations in associated, unvacuumed strawberry rows.

Farmer Adoption

The farmer-collaborator expanded his trap-cropping program to over 75 acres of organic and 50 acres of conventional strawberry production in three farms in Monterey County. In June 2005, he implemented a “trap-crop only” vacuuming program on this acreage with excellent economic results. He has repeatedly expressed great satisfaction with the project (see attached article: “Mining Organic Gold”). All members of the research team and cooperators have indicated their willingness to continue research activities associated with trap cropping in organic strawberries. We expect to assist a new organic strawberry grower, Dan Schmida, owner/ operator of Sandpiper Farms, with alfalfa trap crops on his farm in 2005-2006. Due to the high mobility of WTPB adults, we recommend initial trap cropping of at least 10 acres of organic strawberries with the alfalfa trap crop rows planted every 45-50 rows, for rows of 150 ft or greater in length. This results in recommended average per acre trap crop coverage of approximately 2.5% of the total planted area.

Recommendations:

Areas needing additional study

Based on the farmer survey response information from the extension meetings and our own observations, the most important areas needing more study are methods of lowering the economic costs associated with trap crops, making their operation more efficient, and enhancement of any effect on beneficial insects that could contribute to pest control. For example, we do not know the ideal density of trap crops on an organic strawberry farm that results in optimal WTPB suppression. How far do WTPB move between trap crops? Do trap crops habituate WTPB to alfalfa, causing them to do less damage to strawberries? Introduction and over-wintering of specific WTPB natural enemies (parasitoids) in the trap crops should be tested. New varieties of alfalfa with a more dwarfing and earlier flowering habits should be evaluated. Management-based refinements to the vacuuming system, such as increasing air flow of the vacuum and reducing the number of vacuuming passes over the trap crop each week, could still control WTPB and increase natural enemy abundance, while reducing costs, making the per acre income differential of trap cropping more competitive. Finally, is there a “landscape” level effect of large-scale alfalfa trap cropping in organic strawberries? Are larger areas or continuous rows planted to trap crops better than smaller dispersed patches?

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.