Tomato Hornworm

About a week ago I noticed it: a tiny nubbin of a tomato. My first. I was so happy, as most of my gardening friends as well as my parents were already harvesting their tomato crop. Each day as I've picked lettuce or snow peas for dinner, I've checked in on my nubbin ... and one day there were two! Out of the corner of my consciousness I thought I saw something. I thought a branch of the tomato plant was missing. Naw!

But then, sure enough, on the next day, about two-feet-worth of leaves were gone. Bare green branches were all that remained. And there he was, eating as fast as he could! The dreaded Tomato Hornworm, devouring my plant with gusto. And, the first tomato! He'd eaten exactly one half of my first tomato.

I broke off the branch and stuck it in the fence under the House Wren's house. I included a pencil in the photo I took of it there so that readers could see what a monster the caterpillar was. The worm was almost as big as the Wren, so I didn't know if the bird would take the bait. But the next day he was gone. Perhaps I have saved my second tomato, now about the size of my thumb.

Here's what I found about these fellas from Virginia Cooperative Extension:


Tomato Hornworms

The most familiar hornworms, those associated with tomatoes and related plants, are a complex of two species, the tomato hornworm (Manduca quinquemaculata), and the tobacco hornworm (Manduca sexta). Both species are pests in vegetable gardens. The caterpillars chew leaves, and plants can be rapidly defoliated. Fruits may also be chewed. Tomatoes are particularly susceptible to injury; but other related plants, such as peppers and potatoes, are occasionally infested.

The tomato hornworm has a dark green "horn" with black sides, while that of the tobacco hornworm is red. The caterpillars also are differentiated by the white striping along their sides. These form a series of V's with tomato hornworms, while there are diagonal dashes on the tobacco hornworm. Occasional dark forms of the tomato hornworm larvae occur. The caterpillars of these are much darker than the normal green, although the adult moths differ little in appearance.

Many of the tomato and tobacco hornworms overwinter as pupae in the soil in the vicinity of previously infested gardens, typically emerging in late May and June. However, the adult moths are strong fliers that may travel long distances. Female moths lay large pearl-colored eggs on the upper surface of leaves. The young caterpillars hatch and feed on the plant for a month or more. They have tremendous appetites and consume large amounts of leaves as they grow older and larger. After feeding, they wander away from the plant and pupate in the soil.

Adult moths are identified by examining the hind wing which has two separate wavy bands next to the border on the tomato hornworm that are fused with the tobacco hornworm. The white marks on the abdomen of the tomato hornworm are also more angularly marked.

Control in Gardens

Tomato hornworms are easily controlled by garden insecticides (e.g., carbaryl, permethrin, spinosad). Depending on the size of your crop, examination of the foliage with patience (for they are difficult to see) and tossing them aside when you find them can be effective. Larvae tend to feed on the exterior parts of plants during shadier periods, near dusk and dawn, when they may also be more readily observed and destroyed. A biological control organism that is also highly effective and sold commonly is Bacillus thuringiensis (e.g., Dipel, Thuricide). Best of all there is a natural enemy of the hornworms; a braconid wasp (Cotesia congregata) lays dozens of eggs inside the larvae. The immature wasps eat their way out of the hornworms and pupate by spinning a cocoon on outside of the caterpillar body. If you find hornworms covered with these tiny cocoons you should leave them be so the wasps can complete their life cycle and live to sting another hornworm.

I have six tomato plants, lots of pretty yellow flowers, and the one thumb-sized tomato remains. I'll use the "observe and destroy" method to rid the plants of any more pests. I hate to use the word pests because, after all, they are quite beautiful. What a striking shade of new leaf green!