Combining trap cropping and natural-chemical lures to attract and kill crucifer flea beetles
The crucifer flea beetle is a damaging pest of Brassica crops. On farms both west (Mt. Vernon, WA) and east (Moscow, ID) of the Cascade Mountains, we have been evaluating different species/varieties of trap crops and different bio-diverse mixtures of trap crops to draw flea beetles out of broccoli. In year one we examined five possible trap crops based on literature and grower recommendations (Fig. 1). Trap crops were planted either as single species or in mixes of four different species. We found that trap crop species differed in attractiveness, and that diverse mixes were more attractive than single species.
1. Determine the varieties of mustard (B. juncea) that produce the highest levels of AITC, the plant chemical that crucifer flea beetles (CFB) use to locate their host plants and the size and shape of trap crop plantings most efficient for attracting CFB.
2. Determine whether flaying the trap crop to damage plants and cause release of AITC or adding mustard seed meals that produce abundant AITC enhance attraction of CFB.
3. Determine whether releasing CFB aggregation pheromone within the trap crop enhances attraction of CFB to the trap crop.
4. Compare applications of the insect pathogen B. bassiana, flaming, and tilling under the crop to kill CFB once they are drawn into the trap crop.
5. Use participant interviews to determine economic costs and benefits of traditional versus these new control options. Detailed budgets will be available on-line in a spreadsheet form that will allow growers to compare among new versus traditional controls.
6. Transmit what we learn to growers through an extension program including farm visits that use hands-on learning as the key training tool; also, transmit information to growers through a wide variety of more traditional outreach strategies to maximize coverage.
Our key achievement was attracting Joyce Parker as the first PhD student supported by this project. Joyce comes to us with an MS in Entomology from New Mexico State University. Joyce has directed the research and extension efforts in this first field season of our project. Joyce is a PhD student in Entomology at Washington State University, with PI Snyder as her major advisor.
This year we made progress on several of the research and extension objectives. In support of Objective 1, which investigates the use of simple and diverse trap crop plantings to protect broccoli from crucifer flea beetles, we conducted a field experiment at two locations. Because crucifer flea beetle is a pest both east and west of the Cascade Mountains, one set of plots was established at the University of Idaho’s Parker Plant Science Farm in Moscow, ID (east), while a second set of plots was established at Washington State University’s Mount Vernon Research Center in Mt. Vernon, WA (west). It now is well-known that greater biodiversity (for example, increasing the number of species present) often improves ecosystem functioning, and inspired by this work, we examined whether diverse trap-crop plantings that include several species are more effective at attracting flea beetles than any single trap crop species. To do this we compared plantings of single trap crop species (SIMPLE) to diverse plantings that included four species (DIVERSE). In our experiment both plot size and total trap-crop density did not differ between SIMPLE and DIVERSE plots. The SIMPLE treatments consisted of separate monocultures of each of five putative trap crop species: yellow rocket (Barbarea vulgaris), mustard (Brassica juncea), canola (Brassica napus), collards (Brassica oleracea var. acephala) and pac choi (Brassica rapa subsp. pekinensis) (Fig. 1). The DIVERSE treatment included separate plantings of each of the five unique possible combinations of four trap species drawn from this pool of five species (Fig. 1). Each treatment was replicated four times at the Mount Vernon, WA, site and two times at the Moscow, ID, site.
Trap crop plantings were both direct-seeded and transplanted, depending on the requirements of each species. Seeds of mustard and canola were direct-seeded at a rate of 5g/foot at the Moscow site on 5 May 2009 and at the Mt. Vernon site on 4 April 2009. Two weeks after emergence of the direct-planted species, we transplanted greenhouse-grown seedlings of yellow rocket, collards and pac choi to fill out the remaining SIMPLE and DIVERSE treatments. This way, direct seeded plants and transplants were roughly the same size. In all cases, rows were spaced 18’’ apart.
Our plot layout consisted of four rows of the target crop, broccoli (Brassica oleracea var. italica), flanked on both sides by the different trap crop treatments. The broccoli that served as our protection target was transplanted after the trap-crops had established. Once broccoli planting was completed, the trap-crop plots were sampled for insects several times throughout the season, using a D-vac suction sampler. At each sample date, flea beetle densities on broccoli were recorded using visual observations. Another common Brassica pest, the diamondback moth (Plutella xylostella), was also collected in the D-vac samples and numbers of this pest will also be tabulated. Finally, to track flea beetle movement, yellow sticky cards were attached to stakes in each trap crop treatment. In late August, broccoli was harvested at both field sites. Harvested broccoli plants were pulled directly from the soil, shaken to remove excess soil, labeled and placed into paper sacks. The broccoli was then dried in an oven at 600C for seven days, and dry weights were recorded for each sample. Repeated measures MANOVA was used to analyze trap-crop diversity and location main and interactive effects on flea beetle densities and how these effects changed through time.
Increasing diversity significantly improved the ability of trap crops to attract flea beetles and to protect broccoli crops at both sites (Fig. 2). However, diversity improved beetle attraction to trap crops most strongly early in the season at the Moscow (east) site but at mid-season at the Mount Vernon (west site), leading to a significant diversity x site x time interaction (F= 2.996, P=0.032; Fig. 2). It is important to note that small broccoli transplants are most susceptible to flea beetle damage, and so early-season protection of the broccoli plants is most important. Thus, diversity likely delivered the greatest benefit for broccoli plants at the east site. Consistent with beetle densities in the trap crops, when more beetles were collected in diverse trap crops, there were fewer flea beetles in the adjacent broccoli (trap crop diversity main effect, F= 1.549, P=0.020; Fig. 2). Figure 3 takes a closer look at flea beetle densities in monocultures of each trap crop species compared with flea beetle densities in the average of the DIVERSE treatment. Three monocultures were significantly less effective trap crops than the DIVERSE average: collards, canola and yellow rocket (Fig. 3). Across all aspects of the study, we have found that trap crop species, the diversity of trap crop plantings, site and time of year all influence how effective a trap crop will be at protecting broccoli plants.
In support of Project Objective 2, we examined the effectiveness of amending trap crops with mustard meal. This work was conducted in plots at the farms of four cooperating growers: Greentree Naturals Certified Organic Farm (Sandpoint, ID), GT’s Farm (Moscow, ID), Garden Treasures Nursery and Organic Farm (Arlington, WA) and Rents Due Ranch (Stanwood, WA). At each site we planted a trap crop plot of the species mustard, and then further amended one-half of each plot with mustard seed meal (the meal releases a chemical attractive to crucifer flea beetles). At each farm, broccoli was planted adjacent to the trap crop. Insect densities and plant damage were assessed as described above. Results of this experiment are currently being analyzed.
Finally, in support of Objective 6 we held a “Natural Pest Management” field day at Greentree Naturals Certified Organic Farm in Sandpoint, ID on 12 July 2009. Our instruction exercises focused on introducing growers to the use of entomopathogenic nematodes and trap cropping for pest control and basic ID of beneficial arthropods. Field day participants also observed the trap crop farm trial ongoing at that site and tried their hand at using the D-vac suction sampler to collect pest and beneficial insects. Finally, all participants were treated to a lunch featuring produce grown on the sponsoring farm. We attracted ca. 30 registered participants, which we considered to be “sold out” (based on how many attendees the farm could host at once); additional interested attendees had to be turned away.
Work Left to do:
A second Ph.D. student, Summer Lindzey, was recruited into the project in August 2009. She will be supervised by co-PI Eigenbrode, in the Plant, Soil and Entomological Sciences Department at the University of Idaho. Since joining the project, Summer has been reviewing the literature on “push-pull’ systems. The “push-pull” method combines an attractive trap crop that draws in the pest with a second repellent/deterrent that drives the pest out of the protection target (e.g. Cook et al. 2006). The findings from this review are guiding design of field and laboratory studies to be conducted in spring and summer of 2010. These studies will focus on two goals:
1. Identifying specific push and pull semiochemicals to augment botanical pushes and pulls (in support of Objectives 2 and 3).
2. Determining the best spatial arrangement of trap and target crop to manipulate crucifer flea beetle behavior and distribution in the field (in support of Objective 1).
To address goal 1, target plants (broccoli) will be grown in the greenhouse at the H. C. Manis laboratory at the University of Idaho, providing plant tissue for screening materials repellent to field-collected flea beetles. We will collaborate with a Nez Perce County educator, Lydia Clayton, who is located in Lewiston, ID, to identify possible sources of flea beetles at the low elevation farms that are part of her responsibility. This will allow biossays to begin before the beetles are active in eastern and western Washington and northern Idaho.
Materials to be tested to contribute to ‘pushing’ flea beetles away from the broccoli that we are trying to protect will be drawn from those previously shown to have potential repellent properties against crucifer flea beetle (e.g. Gruber et al. 2009) or those with possible deterrent properties. These will include crude extracts of Crambe abyssinica (e.g. Henderson et al. 2004). Deterrents will be sprayed onto plants, while repellents will be presented in slow-release vials. Any effects observed in the pre-season greenhouse trial will be replicated in a field setting. Broccoli monoculture strips will be planted at the Plant Science Farm in Moscow ID for these experiments. Materials to be tested for the “pull” will include a synthetic flea beetle pheromone, as described in Project Objective 3. Collaborators at the Max Planck Institute for Chemical Ecology are preparing this pheromone for their own work and will make some available to us.
In both greenhouse and field studies, whole-plant beetle counts on controls and treated plants will be compared. In addition, focal observations of beetles will be performed to quantify effects of the materials on beetle behavior. These data will be collected using a computer program (Noldus Observer). The resulting knowledge of mechanisms will help us understand the likely best strategies for spatial deployment of deterrents, repellents and attractants under Project Objectives 1-3.
To further address Project Objective 1, the most effective trap crop identified in the 2009 field trial will be tested for its capacity to accumulate flea beetles under different relative spatial arrangements of the trap and target crops. We will examine, separately, two aspects of trap crop design: size and shape. In both sub-experiments our trap crop test plots will be placed next to a standardized Brassica planting consisting of three 12-m long rows of newly transplanted broccoli. The broccoli will serve as our protection target. The trap crop will be planted along the long, 12-m edge of the broccoli, on the side towards the outer edge of the farm so that the trap crop is in place to intercept flea beetles as they move towards the protection target. To study the effect of trap-crop shape, we will compare two shapes for planting the trap crop; a 3 m x 3 m square planting including four evenly spaced rows (“Square”) placed along the center of the broccoli rows versus a single, 12-m long trap-crop row. To study the effect of size, we will compare smaller rectangle plantings (1 m x 1.5 m, three rows) versus similarly shaped plantings three times as long and wide (3 m x 13.5 m, nine rows).
Cook SM, Khan ZR, Pickett JA. 2007. The use of push-pull strategies in integrated pest management. Annual Review of Entomology 52:375-400.
Gruber MY, Xu N, Grenkow L, Li X, Onyilagha J, Soroka JJ, Westcott ND, Hegedus DD. 2009 Responses of the crucifer flea beetle to Brassica volatiles in an olfactometer. Environmental Entomology 38(5):1467-1479.
Henderson AE, Hallett RH, Soroka JJ. 2004. Prefeeding behavior of the crucifer flea beetle, Phyllotreta cruciferae, on host and nonhost crucifers. Journal of Insect Behavior 17(1):17-39.
Impacts and Contributions/Outcomes
Short-term we will generate awareness of trap cropping and chemical attractants for CFB control among cooperators and their neighbors. Additionally, we will challenge growers to use these newly-acquired tools to develop a better understanding of their specific pest problems. We also will provide the skills needed to use mustard seed meals or other natural attractants to draw beetles into trap-crop strips.
Medium-term impacts will be the adoption of our techniques regionally, replacing chemically-intensive crucifer flea beetle control and giving new options to organic, no-spray and other low-input growers. Our economics worksheet will allow growers to calculate whether the new controls make financial sense, so that growers can make an educated decision on whether to switch from synthetic chemicals or row covers to our new flea beetle control options. Our evaluation plan will be used to chart progress towards these outcome goals.
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