Like all insects, the tomato hornworm goes through four developmental stages—egg, larva, pupa (left) and adult, which is the five-spotted hawk moth.
It's the larval stage of the tomato hornworm that does all the damage. The pale green caterpillar—with a black horn on its rear end—voraciously gobbles up the leaves and stems of tomato plants. At first quite small, the caterpillar goes through several instar stages and, in a month's time or less, measures some 3-1/2 to 4 inches long.
Monitor your tomato plants for small green caterpillars during July and August. Hornworms can be hard to spot because they're so well camouflaged, especially when small. Hand-picking or an application of Bt (Bacillus thuringiensis), the nontoxic product that's also used to kill mosquito larvae, are both effective controls.
The tomato hornworm's worst enemy in the garden is a small parasitic wasp, which lays its eggs inside the caterpillar. The wasp larvae then feed on the hornworm. If you see a hornworm with white projections on its body, leave it alone and don't kill it; Mother Nature will do it for you.