The Tomato Hornworm

Tomato hornworms are one of the garden’s largest caterpillars at nearly 3 to 4 inches long and about as big around as your little finger. They are green with diagonal white stripes with a black or reddish horn-like protrusion projecting from its rear end, hence hornworm.

If you haven’t been out into your garden lately because of the heat or perhaps you were on vacation, and you return to discover that your tomato plants’ stems and leaves and maybe even some of the fruit are chewed pretty badly, then chances are you have one or more tomato hornworms. (Manduca quinquemaculata) You may even find some on your peppers or potatoes. They are very difficult to see because their protective colorings blend in so well with the plant. Tomato hornworms are one of the garden’s largest caterpillars nearly 3 to 4 inches long and about as big around as your little finger. They are green with diagonal white stripes with a black or reddish horn-like protrusion projecting from its rear end, hence hornworm. Don’t worry it will NOT sting you! Their eggs are laid on the underside of the leaves by large adult moths that are a mottled brown, sometimes referred to as the “sphinx” or “hawk” moth. Much of the damage from the tomato hornworm is during midsummer and usually on the top half of the tomato plant. If you see large black or brown droppings on the plant’s leaves or on the ground around the plant, look hard because there is a tomato hornworm on that plant somewhere. They are a little easier to spot if you see one with small white larvae attached to its back. They look like rice grains but they are actually and usually the larvae of the Braconid Wasp which will eventually kill the hornworm. The wasps are not a threat to humans. Once the parasitic wasp cocoons emerge they will eat the internal organs of the hornworm, literally eating the caterpillar from the inside out! They desiccate or dry-up the hornworm. So you can leave these hornworms on the plant if you choose because they will eventually die. But the easiest way to remove them is to simply pick them off by hand. If you are squeamish about squishing them just drop them in soapy water. If you are okay with using insecticides to prevent the hornworm, choose one with the ingredient Bt. To prevent a return of these garden pests next year, till the soil well at the end of the season and again in the spring which will destroy their larvae that winters in the soil, and then emerges again in the summer to begin the cycle all over again. Remember, look carefully. They are hard to find!

Curt Houtz, Penn State Master Gardener