Combining trap cropping and natural-chemical lures to attract and kill crucifer flea beetles
The crucifer flea beetle (CFB), 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 previous work we found that a diverse trap crop consisting of Brassica napus, Brassica juncea and Brassica rapa subsp. pekinensis, were particularly effective at protecting broccoli (Brassica oleracea L. var. italica) from CFB damage. In plots both west (Mt. Vernon, WA) and east (Moscow, ID) of the Cascade Mountains, we have been evaluating optimal trap crop distances that will effectively draw flea beetles out of broccoli, yet prevent over-spilling from occurring. Flea beetle (P. cruciferae) populations in trap-crop species were tracked using D-vac suction sampler, while visual observations were used to monitor flea beetle populations and damage in broccoli. Our results thus far suggest that multi-species trap crops protected broccoli planted at varying distance by inducing subtle changes in CFB behavior. Therefore, trap crops drew away CFB feeding.
1. Investigate trap crop proximity: does trap cropping alter CFB distribution in broccoli?
2. Examine CFB removal: does killing CFB in the trap crop improve control?
3. Distribute information to growers about trap crops and how they can be employed to manage CFB in mixed vegetable farms.
We have made progress on several of the objectives to date. In support of Objective 1 (investigating trap crop proximity) and Objective 2 (CFB removal), large field experiments were conducted at two different sites across the state. Since CFB is a ubiquitous pest both east and west of the Cascade Mountains, field sites were located at the University of Idaho-Parker Plant Science Farm Moscow, ID (east) and at the Washington State University – Mt. Vernon Research Center Mt. Vernon, WA (west) in summer 2011. We discovered, from previous studies, that a diverse trap crop protected broccoli from flea beetle damage; however, we were still finding CFB on broccoli. Therefore, we decided to take a closer look at CFB distribution in broccoli plantings.
We simultaneously assessed the effects of trap crop proximity and CFB removal in its ability to protect adjacent broccoli. The experimental design consisted of five rows of trap crops adjacent to two rows of three broccoli plots. The trap crop mixture included Brassica juncea (Pacific Gold Mustard), Brassica napus (Rape) and Brassica rapa subsp. pekinensis (Pac Choi). Trap crop plantings were both direct seeded and 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 June 1, 2011 and in Mt. Vernon, WA on May 25, 2011. After the direct seeded trap plants emerged, they were grown for two weeks. Greenhouse grown seedlings of B. rapa var. pekinensis were then hand transplanted into the rape and mustard at the recommended planting densities. Therefore, both direct seeded and transplanted trap plants were roughly the same size. Once trap plants were established, broccoli (Brassica oleracea var. italica) were transplanted. Broccoli plots were planted at three different distances away from the trap crops: 59 cm, 3.5 m and 7 m. With this design, we were able to observe if over spilling (CFB over spilling back into the broccoli) was occurring. Broccoli plants were spaced 18’’ apart within rows, and both broccoli and trap crop rows were spaced 23’’ apart between rows. Randomly selected trap crop plots were treated with an insecticide (Mustang Max 174) to kill CFB populations in the trap crop. Control plots consisted of no trap crop (only broccoli). All plots were spaced 3.5 m apart and separated by bare ground.
Once planting was completed, trap plants were sampled several times throughout the season using a D-vac suction sampler, and CFB densities were recorded. Visual observations of CFB on broccoli were also made several times throughout the season. To track CFB movement, yellow sticky cards were attached to stakes in broccoli planted at each distance. All plots were maintained weed-free by hand weeding/hoeing. In mid-September, 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. All data were analyzed using JMP 9 SAS, an MANOVA was used to analyze broccoli (distance nested in treatment) whole plant dry weight at both locations. A repeated measures MANOVA was used to analyze CFB densities in trap crops and CFB densities in broccoli (distance nested in treatment), as well as the interaction between trap crop treatment (sprayed, not sprayed, control) and CFB density over the field season.
Results from CFB in broccoli showed that we collected significantly fewer flea beetles out of our control plots where there was no trap crop. We also collected significantly fewer CFB in our treated trap crop plots (sprayed) (F = 112.35, P = < 0.001, Fig. 1). Therefore, in the control plots we had very few CFB. When a trap crop was present, we saw significantly more CFB, and when we killed CFB in the trap crop we found significantly fewer. Results from broccoli whole plant dry weight showed that broccoli planted in the presence of a trap crop attained the greatest dry weight (F = 16.47, P = < 0.001, Fig. 2). We also found no effect of trap crop distance on broccoli yield. Results of CFB on broccoli were more complex. Since broccoli yields were greater adjacent to trap crops, we would expect to find an increase in CFB on broccoli in our control plots and a decrease in CFB on broccoli in the trap crop plots; however, this was not the case.
Despite causing strong and clear differences in yield, trap cropping had surprisingly little effect on where CFB were found in the broccoli crop (treatment: F = 4.106, P = 0.0020; distance: F = 0.312, P = 0.0204, Fig. 3). In the low density west site, CFB were found at the edge of the broccoli crop regardless of whether a trap crop was present or sprayed. Patterns were intriguingly different at the high beetle density eastern site. Here, CFB were evenly distributed across treatments and locations in the crop in all situations but one: when the trap crop was sprayed, CFB densities fell in adjacent broccoli. Our data suggests three points about CFB behavior. First, when densities are low, CFB concentrate at the crop edge, but move further inside the crop as densities increase. Second, CFB are not necessarily heavily feeding on the plants where they are found; broccoli was protected by trap crops without changing pest densities in adjacent broccoli, regardless of whether flea beetles were at high or low densities. Finally, at least at in the high-density site, killing CFB in the trap crop reduced CFB densities in the adjacent broccoli crop (although, again this change in beetle density did not impact broccoli yields). Altogether, these results suggest that CFB are regularly moving between the trap crop and broccoli, but when given a choice will do most of their feeding on the trap crop. Apparently, because of this movement, the trap crop protected broccoli without changing apparent densities of the pests in the crop.
In support of Objective 3, we held a natural pest management field day at Greentree Naturals Certified Organic Farm in Sandpoint, ID on July 17, 2011. We utilized the evaluations from our previous field day in 2009 and 2010 and expanded our presentations to cover different aspects of organic pest management, including introducing growers to the benefits of species diversity and species evenness, entomopathogenic nematodes, beneficial arthropod identification and trap cropping. Field day participants were able to observe the trap crop farm trial, d-vac collection technique and were treated to a lunch made with local produce provided by Greentree Naturals Certified Organic Farm.
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.
King County Extension
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Sustainable Systems Analyst
Washington State University
Center for Sustaining Agriculture and Natural Res.
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University of Idaho
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Small Farms Extension Educator
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