The weather has been kind to tomato plants this year. The rain and mild temperatures this summer helped my tomato plants survive and continue producing through July and even into August. I am enjoying the abundant tomato harvest, and so are my coworkers, neighbors, friends, and even random people on the street.
Last week, a lot of leaves were stripped off my tomato plants, and I knew the culprit before I spotted it. The tobacco hornworm is a three to four-inch caterpillar in the moth family Sphingidae. It is bright green with diagonal white stripes and a red horn on the end. The tomato hornworm looks very similar but has a black horn. Both can be found on vegetable plants in our area, and they feed on blossoms, leaves, and fruit on the plants.
Despite the damage they do to tomatoes, eggplant, peppers, and potatoes, the hornworm is one of my favorite caterpillars. They are so fat and look menacing with a horn, but they are harmless to humans. Hornworms overwinter as pupae in the soil, and the adult moths emerge in the summer. The moth from a hornworm is a large sphinx moth that is grayish brown. It has a wingspan of four to five inches and feeds on nectar of flowers.
Female moths lay eggs on the underside of host plant leaves, and the caterpillars can emerge in about seven days. The caterpillars go through five stages in three or four weeks, and then they pupate in the soil. An adult moth can emerge in about three weeks unless it overwinters in the soil. We can see one or two generations per year.
If you find hornworms in your vegetable garden, the easiest method of control is to pick them off and drop them in a bucket of soapy water. Bacillus thuringiensis (Bt) is an effective spray for young caterpillars if you have an infestation.
Hornworms have a very interesting natural predator. The Cotesia congregates is a tiny wasp that lays eggs just beneath the skin of the hornworm. As the eggs hatch, the wasp larvae feed on the internal organs of the hornworm and eventually chew their way out of their host. Once they get outside, the wasp larvae spin tiny white oval-shaped cocoons along the back and sides of the hornworm. The hornworm dies shortly after that, preventing it from eating anymore tomato plants.
For more information about lawn and garden topics, contact Kate Whitney, Williamson County Horticulture Extension Agent, at 512-943-3300.
