The Best Way to Kill Caterpillars on Tomatoes

Trap Crops

• Trap cropping employs sacrificial plants to lure unwanted pests away from ornamental plants and food crops. Tomato hornworms feed on solanaceous plants, including tomatoes, peppers and eggplants. Many hornworms, however, prefer dill over tomato plants. Planted close to tomatoes, dill acts as a trap crop to entice caterpillars. Gardeners simply hand-pick worms off dill plants and toss to open areas for nearby wildlife to eat. Frogs, toads, snakes and birds eat hornworms. Cardinals, bluebirds, sparrows and woodpeckers enjoy this delicacy.

Bt and Pyrethrum

• Bacillus thuringiensis, Bt for short, is a spore-producing bacterium with insecticidal properties. Different strains of Bt control different pests. Bt kurstaki is a caterpillar killer with effectiveness against hornworms in their early stages of instar development. It contains an endotoxin that kills caterpillars after ingestion. Pyrethrum is a "knock-down" insecticide, working on contact with pests to paralyze their nervous systems. Adherence to label directions ensures that the insecticide does not threaten honeybees, which are susceptible to Bt's toxicity.

Spinosad and Neem

• Dr. Michael Merchant, Texas Cooperative Extension entomologist, says that spinosad is a safe organic insecticide that is especially effective against caterpillars. Its residual effect lasts up to four weeks, compared to Bt's residual effect of one to two days. Spinosad is a soil-derived bacterium that delivers toxic action after ingestion. Neem oil, from leaf and nut extracts of the Indian neem tree, delivers suffocation action after contact application to caterpillars. Merchant calls spinosad "an insecticide to make organic gardeners smile."

Site Cultivation

• Hornworms are caterpillar larvae of large moths. Female moths lay eggs on leaves of host plants, including tomatoes. After hatching, first-generation caterpillars progress through five instar larval stages before pupating in soil. Moths emerge from soil and lay second-generation eggs in the same season, to repeat the metamorphic cycle with one difference. Second-generation larvae pupate in soil and emerge the following spring. In autumn, after tomato harvest, rototilling, discing or cultivating soil with a long-handled hoe is an effective control measure. Dan Pavuk, University of Michigan Extension educator, says post-harvest tillage destroys up to 90 percent of pupating caterpillars.