Horn worms and Hawk Moths

By Al Lowe
Contributor

Almost every year, in the early fall, I receive questions about huge caterpillars, often found on the tomatoes.
These caterpillars are very large indeed, about as big round as a man's finger, and perhaps 3 1/2 to 4 inches long. Most of them are smooth and light green in colour, but some are brownish. They have a 'horn' at one end, and for that reason are sometimes called horn worms.
The most common ones in Ontario are the Tomato Horn worm, and in the proper locations, the Tobacco Horn worm. They are not always found on tomatoes or tobacco, although those are preferred foods. They also live on other similar plants, such as potatoes, peppers and even some trees.
The adult of this caterpillar is a big, thick-bodied moth. Such moths are called Hawk Moths or Sphinx Moths and belong to the family Sphingidae. Many are very broad-winged moths with wingspans of 4 inches or more. You will find them at dusk, feeding on the nectar of flowers. They hover in front of the flowers like hummingbirds, unroll a log tube from under the front of the head and suck the nectar up. They prefer heavily scented flowers like nicotine and petunias. These big moths are usually mottled brown in colour and some of them have orange or red spots along the abdomen.
The moth lays eggs singly on the appropriate plant. These hatch out into little caterpillars which grow rapidly, going through several moults before becoming the big caterpillars which we see in the early fall. During this time, their whole lives are devoted to eating.
Like all living things, they have their enemies. Quite a number of other insects, mostly wasps, prey on them, tearing them apart and eating the bodies. Another type of wasp lays its eggs right in the body of the caterpillar. The larvae of this wasp live inside the living caterpillar. And, of course, they are the favourite food of many birds because they are so big.
If the caterpillar survives all of this, and the insecticides, if falls to the ground and forms a pupa. In our climate, the insect winters over as a pupa in the ground. But where there are longer summers, there is usually a second generation produced during the summer months.
The Tomato Worm is not really very common in Northern Ontario, but it seems to appear in fairly large numbers every few years. In the southern parts of Ontario, the green worms are very common and are likely to be found in every backyard tomato patch. The worms, incidentally, when they are disturbed, raise themselves up and make a loud clicking sound, which is intended to warn its enemies off.
So, for those of you who are curious about those big and unusual caterpillars or about those big moths you see around your flowers at dusk, that's about it.