Full-grown hornworm caterpillars can reach several inches in length and are nicely camouflaged with their green coloring. The two most common hornworms are the tomato hornworm which has white V-shaped markings on the sides and a blue or black horn and the tobacco hornworm which has diagonal white stripes and a red horn. Hornworms have five sets of prolegs that grip tightly to plant stems.
The adult moths can have a 4 to 5-inch wingspan and are grayish-brown. The adult tobacco hornworm is the Carolina sphinx moth. The adult tomato hornworm is the five-spotted hawk moth. The larvae causes the plant damage and indicate their presence with large dark green or black droppings on the leaves.
Tomato hornworms overwinter in the soil and emerge as adult moths in the spring to mate. Female moths lay eggs on plant leaves which hatch into caterpillars that will be full-grown within a few weeks. Mature caterpillars burrow into the soil to pupate and begin the second generation as moths emerge two weeks later. The second generation of caterpillars feed until late summer and then pupate until the spring moths emerge.
In addition to tomatoes, tomato hornworms have been known to cause damage to potato, eggplant and peppers as well as several varieties of weeds.
Monitor plants for tomato hornworms regularly to catch them before the damage is extensive. Keep the area around plants weed-free to remove host sites. The best control for hornworms is manual removal. Drop caterpillars into a bucket of soapy water or squish them to kill them. If you notice a tomato hornworm with white oval-shaped objects attached along its body, leave it in place. This caterpillar is being parasitized by a predatory wasp. The white objects are wasp cocoons. When the larvae hatch, they will feed on the inside of the hornworm, killing it, and then move to find more hornworms to eat.
Other control options include: Bt (Dipel, Thuricide), Spinosad (Conserve; Colorado Potato Beetle Beater Conc; Captain Jack’s Dead Bug Brew, Monterey Garden Insect Spray), cyfluthrin (BioAdvanced Vegetable & Garden Insect Spray) among other insecticides. As always when applying insecticides pay attention to the harvest interval between applying the treatment and harvest date. (Cynthia Domenghini)