Increasing biological control of brassica pests through overwintering

Increasing biological control of brassica pests through overwintering

Summary

Bryan O’Hara, a diversified organic farmer, has found that his pest problems in Brassicas have been much reduced since he began overwintering a substantial area of Brassicas under low tunnels and allowing them to flower in the spring. One possible explanation for this reduction in pest problems is that the overwintered Brassicas increase the numbers and diversity of parasitoids attacking his pests – either by providing habitat for the parasitoids to overwinter, by providing early season Brassica flowers as sources of nectar, or both. We compared Bryan’s farm to three other diversified organic farming areas: 1. with no overwintered Brassicas in the area, 2. with wild mustard weeds overwintering as rosettes, and 3. with a small area of overwintered winter kale. At each of these farms we took vacuum samples of any blooming Brassicas and other abundant flowering plants from early May to the end of June and again in September. We also collected weekly samples of caterpillars from Brassica crops (primarily collards and cabbage) and the wild mustard, and brought them back to the laboratory, reared them out to pupation, and recorded how many were parasitized.

Objectives/Performance Targets

These are the original performance targets from the proposal. They have been adapted as discussed below under “Accomplishments.”
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.

Accomplishments/Milestones

The parasitism tests with reared imported cabbageworm larvae did not work out. We had very high mortality of the imported cabbageworms, both in the field and after bringing them back to the laboratory for rearing. Thus, at the beginning of June we decided to change our approach and began collecting caterpillars in the field from Brassica crops at each location, and rearing those caterpillars to determine rates of parasitism. This was a much more successful way of determining parasitism. In addition, we collected much more information. We have information about which species of caterpillars were present on the Brassica crops on a weekly basis from early June to late September, and parasitism rates for the three major species (imported cabbageworm, diamondback moth, and cross-striped cabbageworm) over the season. However, it is unlikely that we will be able to get a clear answer to the original question about differences in parasitism with overwintered Brassicas, since so many factors (species of caterpillars, age and size of caterpillars, host plants, etc.) are varying over our sites and over the season.

In addition to the caterpillar samples, we also collected vaccum samples of flowering plants on a weekly basis through all of May and June, a single mid-summer sample in July, and then weekly samples again in September. These were collected on a broad range of flowering plants at the four sites. Our plan to evaluate these samples is to first identify the parasites reared from caterpillars so that we have a good reference collection of the parasite complex on Brassica caterpillars, and then use that reference collection to get beyond general taxonomic categories (families and subfamilies) of parasitoids on flowering plants and see if we can link specific parasitoids of Brassica caterpillars to flowers on which they feed as adults.

Impacts and Contributions/Outcomes

In the coming year, we will be making presentations at the CT NOFA “Cultivating an Organic Connecticut Conference” in March and at the regional NOFA summer conference in August, and writing up information about overwintering Brassicas and about the Brassica pest complex for publication. This should result in impacts on the wider community of farmers.

Collaborators: