The crucifer flea beetle, Phyllotreta cruciferae, is a major pest of Brassica crops in the Pacific Northwest. Many growers rely on these crops as a major component of mixed-vegetable production, and flea beetle damage lowers marketable yields. In previous work we found that trap crops are effective in partially protecting broccoli from flea beetle damage. Companion plants intercropped with broccoli as an additional complementary flea beetle management option may provide an additional incentive for flea beetles to choose the trap crop rather than the broccoli, working synergistically to improve flea beetle control.
The crucifer flea beetle (CFB), Phyllotreta cruciferae (Coleoptera: Chrysomelidae), is an oligophagous pest of Brassica crops throughout North America (Palaniswamy and Lamb 1992). In the Pacific Northwest, many growers rely on Brassica crops as a major component of mixed-vegetable production; flea beetle damage lowers the marketable yields of these crops. In addition, Brassica crops can be planted and harvested season-long, providing a steady source of income throughout the year. Organic Brassica crops are valued at over $60 million annually (USDA NASS 2008) and include arugula, broccoli, cabbage, kale and mustard greens. Adult flea beetles scar foliage, resulting in produce that is unattractive to consumers and often kill seedlings and small transplants outright. Indeed, flea beetle damage sometimes leads to total crop loss (Newton 1928, Kinoshita et. al 1979, Turnock and Turnbull 1994), and for this reason many small-scale vegetable growers in the Pacific Northwest are unable to include Brassicas in their yearly rotations.
The crucifer flea beetle feeds only on Brassica species throughout North America (Palaniswamy and Lamb 1992). Adult flea beetles cause significant damage to crops in early spring when beetles emerge from overwintering sites to feed on newly planted Brassica plants, resulting in plant death, slowed growth and reduced marketable yields (Newton 1928, Kinoshita et. al 1979, Turnock and Turnbull 1994). During the fall the adult beetles move away from Brassica fields and overwinter in sites near field edges beneath hedgerows and trees (Burgess 1977, Wylie 1979, Lamb 1983). Because beetles are constantly moving into crops from these surrounding overwintering sites, it is extremely challenging to develop in-field management strategies. In addition, organic producers are very limited in their options for controlling flea beetles, these being limited to the use of floating row covers, which can be costly, and organic-approved insecticides, which must be applied frequently as flea beetles continuously move into the crop from surrounding vegetation. The limitations of these strategies have led the industry to look for alternatives.
In work we have already completed, we have found that trap crops, which are stands of plants highly-attractive to the pest and planted nearby, effectively draw flea beetles away from the broccoli plantings that we are trying to protect (Parker 2012). Trap crops manage pest populations by attracting insect pests, therefore reducing the likelihood that the pests will attack the target crop (Vandermeer 1989). Thus, trap cropping presents a new option for flea beetle control. However, trap cropping alone does not completely protect the broccoli from flea beetle damage, suggesting that additional controls are still needed. We investigated the use of companion plants and trap crops as a flea beetle management strategy.
Companion plants are interplantings of a second marketable crop within the protection target that can visually and/or chemically mask the ability of a pest to find its desired host plant (Cunningham 1998, Finch and Collier 2000). Trap cropping has received more attention than companion planting, and very little work has examined the combination of these two approaches for pest control. Companion plants have shown benefits in several cropping systems, by obstructing the pest’s ability to locate host plants. For example, fewer diamondback moths were found on Brussels sprouts when they were intercropped with malting barley, sage or thyme (Dove 1986). Additionally, fewer striped flea beetles were observed when Chinese cabbage (Brassica chinensis) was intercropped with green onions (Allium fistulosum) (Gao et. al 2004), while collards intercropped with spring onion (Allium cepa) (Brassica oleracea var. acephala) significantly reduced densities of the cabbage aphid (Brevicoryne brassicae) on the collards (Mutiga et. al 2010).
Flea beetle movement is also influenced by crop diversity. Flea beetles encounter non-host plants at a higher rate in mixed plantings, sometimes causing them to leave in search of a more suitable habitat (e.g., Elmstrom et. al 1998). This has positive implications for the combination of trap cropping with companion planting: a trap crop situated near a companion plant intercrop may visually confuse, repel, block or slow the movement of flea beetles into broccoli and consequently steer flea beetles to the trap crop. Therefore, trap crops and companion plants have the potential to work synergistically together, with their combined impacts greater than either single approach used alone.
We examined the use of companion plantings to control the crucifer flea beetle in combination with trap crops. Companion plantings may interrupt host finding by flea beetles and other broccoli pests and enhance densities of beneficial predators and parasitoids.
Our objectives were to:
1)Determine the most effective companion crop species to discourage flea beetle colonization of broccoli and improve marketable yield.
2)Examine how the companion plants will affect colonization by other broccoli pests and beneficial insects (and spiders).
3)Examine the combination of companion planting and trap cropping for flea beetle control
4)Distribute what we learn to growers through an extension program including a substantial extension presence, along with field days and other outreach strategies that maximize coverage.
Because the crucifer flea beetle is a ubiquitous pest both east and west of the Cascade Mountains, this project included a research study at the Washington State University (WSU) Mt. Vernon Research and Extension Center in Mt. Vernon, WA (west) and at the WSU Tukey Horticultural Orchard in Pullman, WA (east). In addition, an educational program at Greentree Naturals Certified Organic Farm in Sandpoint, ID was conducted to introduce growers and interested community members to the benefits of trap crops and companion plants as alternative pest management strategies.
In support of the first three objectives, we examined companion plants that may repel flea beetles, and thus protect broccoli plants, and how these companion plants interact with a mustard trap crop at the WSU Research and Extension Center in Mt. Vernon, WA. Companion plant species were chosen based on their aromatic qualities, size, shape, published literature and grower recommendation. Therefore, we chose companion plants based on what organic vegetable farmers would likely incorporate on their farms in the Pacific Northwest. Companion plants included spring onion Allium fistulosum x cepa, Dill cv. Superdukat Anethum graveolens, Yukon Gold potatoes Solanum tuberosum, and marigold cv. Golden Guardian Tagetes patula. The experimental design consisted of three rows of the target crop, broccoli (Brassica oleracea cv. Italica), intercropped with four rows of the different companion plant treatments. These plots were flanked on both sides by a monoculture trap crop of Pacific Gold mustard (Brassica juncea) (Figure 1). Potatoes and dill were directed seeded. Spring onion, marigold and broccoli were grown in the greenhouse and transplanted in the field. In addition, the trap crop was direct seeded two weeks prior to transplanting the companion plants and broccoli. The trap crop was planted earlier in order for it to be well established and attractive to flea beetles. There were a total of five treatments: each of the four companion plant species listed above, along with the target crop (broccoli) intercropped with broccoli as a control.
Using results from our first experiment, we conducted a second experiment to examine ratios of protection target (broccoli) to companion plant. This was conducted at the WSU Mt. Vernon Research and Extension Center in Mt. Vernon, WA and at the WSU Tukey Horticultural Orchard in Pullman, WA. Our goal was to find a ratio of companion plants that could enhance the trap crop’s effectiveness. For this experiment, we selected two companion plants that collected significantly fewer flea beetles in adjacent trap crops at one sampling period of the first field experiment: spring onion (Allium fistulosum x cepa) and Yukon gold potato (Solanum tuberosum). Here we compared companion:broccoli ratios of 0 – 100% companion-onions in increments of 10% at both sites and a range of 10, 20, 40, 60 and 80% companion-potatoes at the Mt. Vernon, WA site only (Figure 2). Potatoes were not included at the Pullman, WA location due to space limitations and inclement weather.
All plots were kept weed-free by regular hoeing and hand-weeding. For both experiments, samples within the trap crop were taken with a D-vac suction sampler (Rincon Vitova, Ventura, California) at two week intervals. In addition, Flea beetles were counted visually on the broccoli by carefully examining all broccoli leaves and florets. At the end of the season, approximately 73 days after planting, subsets of mature broccoli in each treatment were harvested to determine final whole plant biomass.
All data were analyzed using JMP, version 9 (SAS Institute Inc., Cary, North Carolina). For our first field experiment repeated measures ANOVA were used to analyze the interactions between flea beetle numbers in trap crops and broccoli and broccoli yield for companion plant treatments. For our second field experiment regressions were used to analyze the relationship between companion plant ratio and flea beetle density in broccoli and trap crops as well as broccoli dry weights. In both experiments flea beetle counts were log10 transformed while broccoli whole plant dry weights were log transformed to homogenize variances in experiment II.
Our results indicated that none of the companion plants (dill, marigold, onion and potato) improved broccoli dry biomass (F4,34 = 0.228, P = 0.921) (Figure 3). In addition, the companion plant species never influenced flea beetle densities on broccoli (F4,35 = 0.382). However, whenever we saw higher numbers of flea beetles in trap crops, we did see significantly lower numbers of CFB in broccoli (Figure 4); again, this did not influence broccoli dry weights (F1,78 = 4.536, P = 0.036). When looking at flea beetle numbers in the mustard trap crop we collected significantly fewer flea beetles out of trap crops adjacent to potato and onion companion plants in late June (F4, 35 = 475.128, P = < 0.0001). However, this only occurred once during the sampling period throughout the field season. Due to these results we decided to expand our study to include manipulating companion:broccoli ratios. Similarly, results from our second experiment demonstrated that manipulating the amount of companion plant ratio failed to increase the number of flea beetles collected in the trap crop or improve broccoli dry weight (Figures 5 and 6; Table 1).
Educational & Outreach Activities
Our outreach program focused on a field day where we concentrated on natural pest management strategies that emphasized the use of trap crops and companion plants, supporting Objective 4. Evaluations from a previous field day revealed that basic entomology knowledge, such as beneficial insect and pest identification, is lacking and highly desired by attendees. We incorporated this information, and also discussed the use of insectary plants, during our Natural Pest Management Field Day. Our field day started with three teaching components: 1) common pest and beneficial insect identification; 2) attraction of beneficial insects with insectary plants; and 3) implementation of trap crops and companion plants as alternative pest control strategies. A visual display was set up which showed pictures of the trap crop and companion plant experiment throughout the field season. This allowed the attendees to observe flea beetle management through time. In addition, the field day attendees participated in a “farm walk,” during which the host grower provided a guided tour of that year’s experimental trap crop plots, discussing what worked well, what didn’t and what they have learned so far (Figure 7). We demonstrated the D-vac suction sampler, which is used to collect insects, and assisted attendees in identifying collected insects, which included common beneficials and pests found on the farm.
When the “work” portion of the field day concluded, all of the participants gathered for a meal featuring local food prepared by a caterer. This approach of including locally-produced foods in the communal meal has been successfully used by our participating grower (Greentree Naturals Certified Organic Farm) at a series of field days at her farm and by one previous field day we have collaborated on. We had over 30 growers and interested community members attend.
Currently, we are preparing a manuscript for submission to the Journal of Sustainable Agriculture that will report the results of our companion plant experiments.
Understanding interactions between trap crops, companion plants, farm biodiversity and pest management allows growers to utilize more sustainable management options to control flea beetle and other pests on small-acreage farms. We expanded on our earlier work by characterizing and investigating interactions between trap crops and companion plants and searching for possible synergies in cultural control techniques. By utilizing companion plants and/or plants that farmers are already growing, farmers can benefit from “ecological engineering” on their farms that suppresses pests. Although the companion plants in our study did not enhance the trap crop’s effectiveness, they contained important market value and our experiments will add to the growing literature of companion plant research.
Implementing trap crops and companion plants have the potential to save growers money relative to the high costs of pesticides. In addition, companion plants may provide an additional opportunity for growers as they can be sold at market for profit. Furthermore, because companion plants are generally not related to the target crop or trap crop, they are left undamaged by the targeted pest.
The adoption of our technique could allow for the replacement of chemically-intensive flea beetle control – giving new options to organic, no-spray and other low-input growers. Growers that attended our field day learned about trap crops and companion plants and how to incorporate these techniques into their current pest management plan. In addition, our Natural Pest Management Field Day and trap crop field trial showed hands-on the trap crop and companion planting design.
Areas needing additional study
Currently few studies, if any, have used companion plants with trap crops to enhance pest control. There is potential for combining cultural techniques like companion plants and trap crops. However, as our results show, more empirical data needs to be collected before specific recommendations about companion plants can be made as an alternative pest control strategy.