By Nancy Hinkle
University of Georgia

Volume XXXIII Number 1 Page 22

Anyone who has grown tomatoes has probably seen a bright green caterpillar with a red horn on its posterior. This could be either a tomato hornworm, which has eight white lines on its side, or a tobacco hornworm, which has only seven lines. The species are very similar in appearance. Both feed on tomato plants.

Tobacco hornworms are one of the most common tomato pests. Because they are so large, a few tobacco hornworms can rapidly destroy a tomato plant.

Goodbye, hornworms

The most effective control strategy is to physically remove hornworms and crush them. Despite their large size, it is often difficult to see these cryptic caterpillars. Get your kids involved and use their young eyes.

If there are too many plants to efficiently examine and manually protect, spray Bacillus thuringiensis, commonly known as Bt, to control tobacco hornworms. Bt should be used while the larvae are small, because larger larvae need to consume a lot of Bt – and thus a lot of tomato plant – to pick up a lethal dose.

Often a hornworm will be seen with dozens of little white cocoons stuck to its back. These cocoons contain parasitic wasps that are very effective in killing hornworms. These larvae should be removed from the plant, but not destroyed. A parasitized hornworm will die in a couple of days.

Back again

The adult hornworm is a large hawk moth. These moths are fast flyers. They are capable of hovering as they feed from flowers and are occasionally mistaken for hummingbirds.

The female moth deposits eggs on the tomato plant. These eggs hatch within a week, and the tiny caterpillars begin feeding. Initially they are so small that their feeding damage is not noticeable. However, they grow rapidly, and the full-grown larva can be as large as your little finger. Their color and markings camouflage them so that they look like tomato stems and foliage.

The horn is harmless and too flexible to stick in skin. Its actual purpose is unknown.

Once it has reached its largest size, the larva descends from the plant and burrows into the soil to pupate.

Because the mature larva does not migrate far from the tomato patch when it gets ready to pupate, a gardener will often turn up a dirt-encrusted pupa in the soil during spring garden preparation. A large loop, which contains its developing mouthparts, extends from the front of its head and identifies this as the pupa of a hawk moth.

Left undisturbed, next spring the adult moth will emerge from its underground cell and crawl back to the surface. The adults will fly around, feed, mate and start the next season’s hornworm population.