Increasing biological control of brassica pests through overwintering

• 2006 annual report
• 2007 annual report

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.