Increasing biological control of brassica pests through overwintering

Project Overview

Project Type: Partnership
Funds awarded in 2006: $9,903.00
Projected End Date: 12/31/2008
Region: Northeast
State: Connecticut
Project Leader:
Kimberly Stoner
Connecticut Agricultural Experiment Station

Annual Reports


  • Vegetables: broccoli, greens (leafy)


  • Pest Management: biological control, row covers (for pests)

    Proposal abstract:

    Crops in the cabbage family (Brassicaceae, which I will refer to throughout this proposal as brassicas) are very important economically to diversified organic vegetable growers, but also require more attention to pest management than most other vegetable crops. This project will look at a method of growing these crops, particularly brassica salad greens, in a way that may reduce the need for pest management inputs. The most difficult pest for organic growers of brassicas to manage is the crucifer flea beetle (Phyllotreta cruciferae). Flea beetles have become more important pests in recent years as growers have become more interested in growing brassicas for mixed salad greens because: 1) the Asian greens and arugula used in salad mixes are highly attractive to the beetles, 2) the beetles cause direct cosmetic damage (small “shot-holes) to the leaves used in the mix, and 3) growers often have multiple plantings of salad greens to be able to supply their markets through as much of the year as possible, allowing the beetle population to build up through multiple generations. None of the approved organic pesticides registered for use against flea beetles is very effective in controlling them. Caterpillars (including imported cabbageworm, Pieris rapae, diamondback moth, Plutella xylostella, cross-striped cabbageworm, Evergestis rimosalis, cabbage loooper, Trichoplusia ni, and occasionally other species) are ubiquitous pests wherever brassicas are grown, but are also more easily controlled with organic pesticides. Materials based on the active ingredients Bacillus thuringiensis and spinosad work well against caterpillars. But they also require close monitoring and planning to be used effectively. Bacillus thuringiensis is most effective when used on early instars of caterpillars, and is a very short-lived material, so monitoring and timing are important. Spinosad (Entrust is the only organically approved brand) can be used a maximum of three times per season, due to concerns about development of insecticide resistance, and is also expensive at present. There are a variety of natural enemies, both specialist parasitoids and generalist predators, attacking these pests, but their impact, even on organic farms, is minimal. Biological control experts generally attribute the low effectiveness of insect natural enemies in annual crops to the delay in re-establishment each year. Specifically, studies of parasitism of caterpillars on brassicas in the Northeast have found that levels of parasitism are very low until late in the fall, confirming this general pattern (e.g. Godin & Boivin 1998). A normal cropping cycle for annual crops calls for complete destruction of the crop at the end of the growing season, with tillage either in fall or spring destroying crop residues, overwintering sites, and, in the case of brassicas, which are mostly winter annuals, the spring flowering stage of the plant. Each year, when the crop is planted again, the new crop is recolonized by insects – first the pests, and then the natural enemies. What generally happens is that the pests arrive first and thrive for a long time before the natural enemies arrive. The question addressed by this project is: By carrying brassicas over the winter and through their flowering stage in the spring, can we increase the effectiveness of natural enemies of brassica pests, particularly early in the season, and thus reduce the need for pest management inputs on brassica crops? The idea for our proposed solution came from the experiments and observations of an organic farmer, Bryan O’Hara. Crucifer flea beetles were a key pest in his system, because his farm relies substantially on salad green production and marketing for its income. Bryan experimented for several years with the recommendations for flea beetle management that came from reports of organic farming practices and research around the region – covering his spring crops with row cover, avoiding planting any new brassica crops outside the row cover until the end of June, separating his plantings to avoid movement between crops, etc. He does not use any pesticides, not even organically approved insecticides, so he occasionally also had problems with caterpillar pests, such as the imported cabbage worm and the cross-striped cabbageworm, that other growers would control with organic materials. A few years ago, he began carrying his brassica crops through the winter outdoors, under two layers of row cover, one of clear plastic and the other of fiber. He was interested in producing his own seed, and he reasoned that brassicas are winter annuals, so they might grow better and have fewer pests when grown in their natural cycle. He seeds brassica salad greens in the summer (July to early August) and harvests these crops through the fall and early winter. The crops destined for overwintering are seeded in late September and October. These crops are harvested and sold in late winter to early spring, and allowed to go to flower in April and May, and then set seed in June. He does not plant any brassicas, not even long-season cole crops like Brussels sprouts, until late June. Bryan’s observation after carrying this cycle through for a few years is that the pests that plagued him in the past have disappeared. He still grows cole crops in the traditional cycle, planting late spring to early summer and harvesting in the fall, but he no longer has the flea beetle or caterpillar pests he had in the past. The overwintered salad greens are not damaged by flea beetles. The flowering brassicas attract a wide range of beneficial insects. Our hypothesis, which we will test through this project, is that the overwintered brassicas provide a habitat for insect natural enemies that reduce the pest pressure on all brassicas on Bryan’s farm. They may do this partly by providing overwintering habitat for the natural enemies of pests, and partly by attracting the natural enemies to feed early in the spring, when few pollen and nectar sources are available, on the brassica flowers. There is evidence to suggest that overwintering habitat may be important for key parasitoids of brassica pests. The Cotesia spp. parasitoids, which attack imported cabbageworms and cross-striped cabbageworms, overwinter as cocoons on host plants (either singly or in groups, depending on the species), so overwintering the plants intact would be expected to increase the survival of these wasps considerably. Overwintering the plants under row covers might also increase winter survival of the wasps by reducing their exposure to harsh physical conditions or predation. In the case of Microctonus vittatae, which parasitizes crucifer flea beetle, the parasitoid overwinters inside the adult flea beetle host. The parasitized flea beetles emerge considerably earlier than normal, unparasitized hosts, (Wylie 1982), and continue feed while the parasite develops within. Having suitable host material for the adult beetles to feed on so early in the season could be important, so that the wasps develop to adulthood. Availability of nectar and pollen resources, particularly early in the spring when these are not yet abundant, is another known factor in the effectiveness of parasitoids. Oddly, the suitability of brassica flowers for parasitoids of brassica pests, has not been adequately studied.

    Project objectives from proposal:

    We will carry out sampling on four farms, two that overwinter brassicas and one that does not, and one with two fields, almost a mile apart, where brassicas will be overwintered in one field, but not in the other. All four will be farms that are or could be certified organic. (Bryan O’Hara has been certified organic in the past, but chooses not to be at present.) All will be in eastern Connecticut, within 40 miles of each other.
    On all farms, the samples from “other plants flowering at the same time” will be from overwintered or transplanted parsnips or parsley, and ubiquitous weeds flowering at the same time, such as dandelions and chickweed.

    Samples to be taken:
    Spring vacuum samples: Vacuum samples will be taken weekly during the period the overwintered brassicas are flowering. Using a modified leaf blower, 10 randomly chosen plants per overwintered brassica plot will be vacuumed for 10 seconds each. Similar samples will be taken for other plants flowering at the same time. For farms without overwintering brassicas, just the second set of samples, from other flowering plants on the farm, will be taken.

    Spring pitfall samples: Pitfall traps (cups buried in the ground with the lip level with the soil surface) will be set out over a 48-hour period once each week during the period the overwintered brassicas are flowering. Insects caught in the pitfall traps will be preserved in alcohol for later sorting and analysis. Two pitfall traps will be used for each overwintered brassica plot, and two will be put in a nearby area of other plants flowering at the same time. On farms without overwintered brassicas, just the second set of samples, from the other flowering plants, will be taken.

    Spring parasitism test: Once per week, 40 imported cabbageworm larvae (2nd-3rd instar) on 10 potted collard plants will be set out for 48 hours once each week during the flowering period on each farm. The potted plants will be near, but not in direct contact with, overwintered brassicas or other overwintered vegetation. At the end of 48 hours, the remaining caterpillars will be counted and brought back to the laboratory for rearing to determine what percentage of them have been parasitized.
    Late Summer – Fall parasitism test: The above experiment will be repeated 3 times during a period when all farms have brassica crops present in the field.

    Analysis of the above samples: Vacuum samples and pitfall samples will be sorted with a focus on identifying and counting the parasitoids known to attack specific brassica pests (Microctonus vittatae attacking flea beetles, Cotesia glomerata and closely related species Cotesia rubecula attacking imported cabbageworm, and Cotesia orbenae attacking cross-striped cabbageworm), and a few groups of generalist predators (including generalist lady beetles such as Coleomegilla maculata and Harmonia axyridis, and carabid beetles in predominantly predacious genera such as Pterostichus).

    Numbers of target natural enemies will be compared across farms using analysis of variance. Percentage parasitism of caterpillars will also be compared using analysis of variance after appropriate transformation.

    Project timetable:

    Summer 2006: Establish a colony of imported cabbageworms at the Connecticut Agricultural Experiment Station, and test the above protocol for parasitism tests to determine if this number of larvae and length of exposure will be sufficient to measure parasitism levels on organic farms.

    Fall 2006: Plan with the farmers the location of the overwintered brassicas and other flowering plants (such as parsley or parsnips) to be sampled the following spring. Collect information from farmers about when they plant and harvest their brassica crops, the methods used in overwintering brassicas, and the cost of materials for overwintering brassicas.

    Winter 2006-2007: Gear up imported cabbageworm colony so that it can produce 200 larvae per week in the spring of 2007.

    April – June 2007: Collect spring vacuum samples and conduct spring parasitism tests as described above. The spring parasitism tests require rearing out 1000 imported cabbageworm larvae (assuming the tests are conducted for 5 weeks) in order to determine rates of parasitism.

    August – September 2007: Conduct late summer – fall parasitism tests.

    June 2007 – December 2007: Sort vacuum samples, organize and enter data from the vacuum samples and parasitism tests.

    December 2007 – February 2008: Analyze data from all experiments, write up results for publication and presentation.

    March 2008: Outreach through grower meetings (conference of the Connecticut chapter of the Northeast Organic Farming Association ) and publication in Gleanings (CT NOFA newsletter) and on the CT Agricultural Experiment Station website.

    August 2008: Additional outreach through a workshop at the regional NOFA Summer conference.

    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.