The Best Way to Kill a Hornworm Pest on a Tomato

The tobacco hornworm has diagonal lines on its sides and often feeds on tomato plants.

Tomato hornworms are the larvae of the Five-Spotted Hawkmoth, or Manduca quinquemaculata. They are related to the tobacco hornworm, or Manduca sexta, and the two species often eat the same host plants. Hornworms are bright green with white diagonal or V-shaped markings along their sides, and a red or black horn-like projection on the last segment. Like all caterpillars, they grow through several stages, called instars, increasing in size until they reach 3 to 4 inches long and more than one-half inch in diameter. These large caterpillars can devastate a garden, as they are ravenous feeders. With careful preparation and observation, you can catch and destroy these pests before they decimate your tomato plants.

Things You'll Need

• Rototiller
• Bacillus thuringiensis spray
• Bucket of soapy water or garden shears
• Braconid wasps

Instructions

1. Till the garden soil at least 6 inches deep before planting. Tomato hornworm larvae burrow into the ground to pupate and can overwinter in your garden's soil. To prevent adult moths from emerging and laying eggs on your plants, till the soil after harvest or in early spring.

2. Look for pupae when planting. Hornworm pupae are large, brown, elongated and have a thin "handle" that looks like the handle of a pitcher on one side. If you find these in the planting soil, remove and destroy them by squashing them or cutting them in half with garden shears.

3. Spray your crop with Bacillus thuringiensis, or Bt, as a preventive measure, if you wish. Bt is a natural insecticide used by organic farmers to deter crop-eating insects. Judy Sedbrook, Master Gardener for Colorado State University's Cooperative Extension, recommends precautionary spraying with Bt.

4. Observe plants for caterpillar activity. Hornworms are most active in the early morning and late evening, when there is less sunlight. They can be difficult to see, as their protective coloring helps them blend with their host plant. Look for green or black frass -- caterpillar droppings -- or signs of defoliation, such as partially eaten leaves and stem ends.

5. Remove and kill any hornworms that you find. Because of their large size, hornworms are easy to remove. They do not bite, and the horn is not dangerous. Remove them from the tomato vine and kill them by squashing them, dropping them into a bucket of soapy water, or cutting them in half with garden shears.

6. Release braconid wasps in your garden to parasitize the caterpillars. These wasps lay their eggs on the caterpillars, and the wasp larvae consume and destroy them. If you find a hornworm that has white oval projections on its skin, do not remove or kill them. The caterpillar will stop eating and die while the wasps mature, naturally repopulating your garden with a beneficial insect.

Tips & Warnings

• If you plant a variety of vegetables in your garden, be aware that hornworms can infest any of the plants in the genus Solanaceae, or Nightshades, including potatoes, eggplant, tomato, tomatillo, and peppers. They can also infest tobacco crops and weeds like nightshade and jimson weed.

References

• University of Minnesota VegEdge, "Tomato Hornworm"; S.J. Wold-Burkness and W.D. Hutchison.
• University of Maryland Extension: Tomato Hornworm
• Ohio State University Extension, Garden News and Notes: Check Plants for Tomato Hornworm
• Colorado State University Cooperative Extension; "The Tomato Hornworms"; Judy Sedbrook.
• University of California San Deigo; Bacillus Thuringiensis.
• Green Life: Organic Tomato Gardening Tips.

Resources

• University of Nebraska Dept. of Entomology: Tobacco Hornworm