Identifying and Controlling Tomato Hornworms

hornworm eating tomatoes

The Spruce / K. Dave 

The tomato hornworm, Manduca quinquemaculata, is much dreaded by vegetable gardeners because they can devastate tomatoes and other members of the nightshade (Solanaceae) family, such as eggplants and peppers. Fortunately, once you identify these pests, they're fairly easy to eradicate.

The tomato hornworm and tobacco hornworm, Manduca sexta, are often confused with each other. They are very similar in appearance and both attack members of the Solanaceae family. The tomato hornworm is 3 to 4 inches long at full size (likely to be the biggest caterpillar we see in our gardens) and green in color, with white V-shaped marks along its sides. A black "horn" projects from the rear of the caterpillar. Tobacco hornworms, on the other hand, have diagonal white stripes and a red "horn." Both are treated in the same way when they attack your garden.

Life Cycle

The tomato hornworm represents the larval stage of the hawk or sphinx moth, also known as the hummingbird moth. These moths overwinter in the soil as dark brown pupae, then emerge and mate in late spring. They lay their eggs, which are round and greenish-white, on the undersides of leaves. The eggs hatch in four to five days and the hornworm emerges. It spends the next four weeks growing to full size, after which it will make its way into the soil to pupate.

Signs of Tomato Hornworms

Tomato hornworms are voracious pests, munching entire leaves, small stems, and even parts of the immature fruit. While most commonly associated with infested tomatoes, hornworms are also common pests of eggplants, peppers, and potatoes. Most likely, you'll notice the damage before you notice the hornworms because their color helps them blend in so well with the plant foliage. You can also look for their black frass (droppings) on the foliage and around the base of the plant.

Effect on Garden Plants

Undetected, a tomato hornworm can do a fair amount of damage to its host plant. They have hearty appetites and can defoliate a plant in a matter of days. If they are detected and removed early on, however, the plant will recover just fine.

Organic Control for Tomato Hornworm

Because the hornworm is so large, the easiest and most effective way to get rid of it is to pick it off of plants as soon as you detect it. You can then either crush it or toss it into a bowl of soapy water.

A bad infestation can be treated by applying BT (Bacillus thuringiensis). BT is a natural bacteria that is often found in soil. When hornworms eat the bacteria it paralyzes the caterpillar's digestive system; they soon stop feeding and finally die off. BT has no harmful impact on human beings and is a natural substance. BT application is most effective when the larvae are small. If worms are a problem year after year, try rototilling the soil either in late fall or in spring before you plant; this will either bury the pupae or destroy them.

However, if you see a hornworm covered with white egg sacs, leave it alone. The egg sacs are those of a parasitic wasp called the braconid wasp, a beneficial insect. Let the eggs hatch, and you'll have an army of wasps ready to defend your garden against all types of pests. These wasps are not a threat to humans. 

Another organic method for killing tomato hornworms is to mix liquid soap and water together and spray it onto the plant foliage, then sprinkle cayenne pepper over the foliage and fruit. The soap solution will kill the worms, while the cayenne repels them if the soap washes off. This treatment will need to be repeated after each heavy rain. 

Chemical Control for Tomato Hornworms

The synthetic chemical Carbaryl will kill tomato hornworms along with many other pests. However, it is highly toxic and is discouraged for use on edibles. Wherever possible, rely on organic methods to control tomato hornworms, as well as other garden pests.

Article Sources
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  1. Tomato Hornworm. University of Maryland Extension