Combining trap cropping and natural-chemical lures to attract and kill crucifer flea beetles

Combining trap cropping and natural-chemical lures to attract and kill crucifer flea beetles

Summary

The crucifer flea beetle, Phyllotreta cruciferae Goeze (Coleoptera: Chrysomelidae: Alticinae), is an oligophagous pest of Brassica crops. In the Pacific Northwest, many growers rely on Brassica crops as a major component of mixed-vegetable production, and flea beetle damage lowers marketable yields of these crops. In plots both west (Mt. Vernon, WA) and east (Moscow, ID) of the Cascade Mountains, we have been evaluating different species-compositions of trap crop plantings to draw flea beetles out of broccoli (B. oleracae L. var. italica). In the 2010 field season we compared single- and mixed-species plantings of the three most attractive trap-crop species identified in a previous field trial (Brassica napus, Brassica juncea and Brassica campestris L. var. chinensis) for their abilities to attract flea beetles and protect adjacent broccoli. Flea beetle (P. cruciferae) populations in trap crops were tracked using D-vac suction-sampling, while visual observations were used to monitor flea populations and damage in broccoli. We found that the broccoli adjacent to diverse polycultures that included all three trap-crop species attained the greatest dry weight. Our results thus far suggest that multi-species trap crops are a particularly effective way to protect broccoli from flea beetle damage.

Objectives/Performance Targets

1. Investigate the use of simple and diverse trap crop plantings to protect broccoli from P. cruciferae.

2. Examine the use of a mustard trap crop monoculture to protect broccoli from P. cruciferae on paired farms versus without a trap crop.

3. Distribute information to growers about trap crops and how they can be used to manage P. cruciferae in mixed-vegetable farms.

Accomplishments/Milestones

Over the first two years of this project, we have made substantial progress on project objectives. In support of Objective 1 (investigating the use of simple and diverse trap crop plantings to protect broccoli from P. cruciferae), field experiments were conducted at two different locations in two different growing seasons (2009 and 2010). Given that P. cruciferae is a ubiquitous pest both east and west of the Cascade Mountains, in both years field sites were located at the University of Idaho’s Parker Plant Science Farm Moscow, ID (east) and at the Washington State University’s Mt. Vernon Research Center Mt. Vernon, WA (west). It is known that diversity improves ecosystem functioning in many ways, and Objective 1 focuses on how this “diversity benefit” can be exploited in trap-crop design and deployment. Results from our previous field season in 2009 showed that diverse trap crops were more effective at attracting flea beetles. We also found that the trap-crop species Brassica juncea (Pacific Gold Mustard), Brassica napus (Canola) and Brassica rapa subsp. pekinensis (Pac Choi) were most effective in attracting flea beetles. In 2010 we took these three most-attractive trap crop species and examined single- versus mixed-species plantings of these species. To examine the influence of plant species diversity in trap crops, we compared monocultures of these three species to two diverse polyculture treatments: medium diversity, consisting of each possible pair of two species; and high diversity, consisting of the most-diverse polyculture of all three species. Trap crop density was held constant across diversity treatments, such that any diversity effects were not confounded with changes in density (Fig. 1). Thus, in total there were eight different trap crop species compositions: three simple (each species in monoculture), three medium-diverse (all possible combinations of two species drawn from the pool of three species), one highly-diverse (all three species planted together) and one control where no trap crop was planted. Each treatment was replicated eight times in Mt. Vernon, WA and, due to space limitations at that site, just four times in Moscow, ID.

Trap crops were either direct-seeded or transplanted, depending on the variety. Seeds of B. juncea and B. napus were direct-seeded at a rate of 5g/foot in Moscow, ID on 10 May 2010 and in Mt. Vernon, WA on 12 April 2010. After the direct-seeded trap plants emerged, they were allowed to grow for two weeks before transplants were also added. Greenhouse-grown seedlings of B. rapa var. pekinensis were hand-transplanted in the field at recommended planting densities. This methodology assured that direct-seeded plants and transplanted plants were roughly the same size. Once trap plants were established, broccoli (Brassica oleracea var. italica) was transplanted. The experimental setup consisted of four rows of the target crop, broccoli, flanked on both sides by the different trap crop treatments. Each row was spaced 21’’ apart. Once planting was completed, trap plants were sampled several times throughout the season using a D-vac suction sampler, and P. cruciferae densities were recorded. Visual observations of P. cruciferae on broccoli were also made several times throughout the season. To track P. cruciferae movement, yellow sticky cards were attached to stakes in broccoli adjacent to each trap crop treatment. In late August, broccoli was harvested at both field sites. Broccoli plants were pulled directly from the soil, shaken to remove excess soil, labeled and placed into a paper sack. The broccoli was dried in an oven at 600C for seven days and dry weights were recorded for each sample. Another common Brassica pest, Plutella xylostella (diamond back moth), was also recorded when found in D-vac samples. All data were analyzed using SYSTAT 13: ANOVA was used to analyze broccoli whole-plant dry weight at both locations, and repeated-measures ANOVA was used to analyze the interaction between trap crop treatment and P. cruciferae density over the field season.

Results for broccoli whole-plant dry weights showed that broccoli adjacent to the diverse polyculture containing all three trap crop species attained the greatest dry weight, and that this occurred at both locations (F= 5.295, P=0.006, Fig. 2). We found no significant differences in the number of P. cruciferae in our different trap crop treatments across the two locations (F= 1.400, P=0.240, Fig. 3). This outcome is not completely unexpected since we chose the top three most attractive trap crop species from the season before, such that trap crop mixes and monocultures would all be expected to be highly-attractive to flea beetles. However, it was surprising that broccoli yields increased even where we did not see differences in flea beetle densities in the adjacent trap crops; this suggests that flea beetle behavior was somehow by altered by diverse trap crop polycultures.

Results from P. cruciferae densities in broccoli adjacent to the different trap crop treatments were even more complicated. As diversity increased so too did the density of P. cruciferae on broccoli, such that we found significantly more P. cruciferae on broccoli adjacent to the diverse three trap crops (F= 114.489, P=<0.000, Fig. 3). Thus, flea beetle densities were highest on broccoli plants adjacent to diverse trap crops, even though these same broccoli plants were largest at harvest. This again suggests that flea beetle behavior changed when near high-diversity (three species) trap crop plantings, such that broccoli was protected even when flea beetle densities were high. To examine this possibility, we took a closer look at where flea beetles were congregating in broccoli plantings. In our experiment there was a total of four rows of broccoli, with two of those located closer to the trap crop (outer rows) and two further away from the trap crop (inner rows). So, we next examined whether flea beetles were more likely to be found in inner versus outer rows across the different trap crop treatments. Indeed, when near diverse trap crop polycultures, P. cruciferae tended to congregate in broccoli closer to the trap crops (outer rows), compared to the other trap crop diversity treatments (F= 27.607, P<0.000, Fig. 4). This suggests that flea beetles near diverse trap crops may be spending most of their time in the trap crop, with the higher beetle densities in broccoli simply reflecting “spillover” into the adjacent broccoli. Results from the yellow sticky cards (which have not yet been counted) may reveal whether flea beetles do indeed move more when near more diverse trap crop plantings.

To examine the effectiveness of a mustard trap crop monoculture under real farm conditions, we collaborated with several organic farms located east and west of the Cascade Mountains (Objective 2). East collaborators included Greentree Naturals Certified Organic Farm (Sandpoint, ID), GT’s Farm Foods (Moscow, ID), University of Idaho Soil Stewards (Moscow, ID), Washington State University Organic Farm (Pullman, WA) and Washington State University Tukey Orchard (Pullman, WA). West collaborators included Zestful Gardens (Tacoma, WA) and Terry’s Berries (Tacoma, WA). The distance at which this trap crop attracts P. cruciferae is not known. Therefore, we used paired farms located near each other, one with a trap crop planting and the other without, to examine trap-crop effectiveness. Paired farms were randomly assigned either a mustard trap crop treatment adjacent to a broccoli monoculture (trap crop treatment) or served as a no-trap-crop control where broccoli was not paired with a trap crop (control). We compared and recorded P. cruciferae densities in broccoli adjacent to the trap crop treatment and broccoli without the trap crop, using the same procedure for sampling and documentation as utilized in the larger field trials described above. Results of these experiments showed no significant differences in P. cruciferae density recorded in broccoli adjacent to the trap crop treatment versus broccoli without a trap crop (F=0.334; P=0.570). Natural farm-to-farm variability was considerable due to different farming practices, location, plot layout, etc., which likely would make it difficult to detect any treatment effects that might occur. This is always a challenge with on-farm work.

In support of Objective 3, we held a “natural pest management” field day at Greentree Naturals Certified Organic Farm in Sandpoint, ID on 18 July 2010. We utilized the evaluations from our previous field day in 2009 to expand our coverage into new areas of interest to growers, in addition to trap cropping. These new topics included: the benefits of flowering plants as resources for beneficial insects, the use of entomopathogenic nematodes to control soil pests and the identification of beneficial arthropods. In addition to the oral presentations, field day participants observed the trap crop farm trial, practiced using the D-vac suction sampler to collect insects and enjoyed a “farm walk” led by the sponsoring grower. Additionally, attendees enjoyed a lunch made with local produce provided by Greentree Naturals Certified Organic Farm. We had 30 participants in the field day. We intend to expand our outreach into western Washington in 2011.

Our results this past season will help guide us in future directions addressing P. cruciferae intervention, which involves exploring different killing methods once P. cruciferae is in the trap crop and the use of repellent or deterrent plants to drive P. cruciferae into trap crops and out of broccoli.

Impacts and Contributions/Outcomes

Our project has established new control tactics for crucifer flea beetle, where trap crops can (at least partially) replace insecticide applications. We have conducted extension field days and on-farm trials working with a half-dozen local growers.

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