Vegetable Crops News

Controlling Caterpillars on Cabbage and Broccoli

Caterpillars chewing on cabbagge or broccoli are one of the most common insect problems in home gardens. In small gardens with few plants, floating row covers can prevent adults from laying eggs on the plants. Timely releases of Trichogramma wasps may prevent caterpillar populations from reaching damaging levels. Inspecting each plant every few days and destroying any eggs or caterpillars observed is also effective. Small caterpillars, especially the imported cabbageworm and diamondback moth, blend in with cabbage leaves very well and are often difficult to find. Be sure to check the heart leaves for damage, as small larvae are easily overlooked there. Check plants carefully even if no feeding damage is apparent to look for eggs that will hatch into small caterpillars several days later. Also check for signs of natual enemies. Natural populations of parasites, predators, or disease may help reduce caterpillar populations to tolerable levels.

If physical removal is not practical, regular Bt sprays will control caterpillars without affecting natural enemies that may be providing some control of the caterpillars as well as of other pests, such as aphids. Good coverage of the lower leaf surfaces, where many caterpillars feed, is important. Plants can tolerate varying amounts of damage depending on the maturity of the plant. Seedlings cannot tolerate much defoliation; heading or mature plants are also very vulnerable, when caterpillars may feed on the developing head or flowers. Apply Bt when 10% of the plants are infested at these times. However, plants can tolerate moderate feeding after the seedling stage until cupping or first flower. During this time apply Bt when 30% of cabbage plants or 50% of broccoli are infested. Just before heading, reduce the threshold for caterpillars on cabbage to 20% infestation since damage at this stage affects the quality of the future heads.

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