Watch for Large Hungry Hornworms

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Scout your tomato plants now for hornworms. Late July and early August are usually the time when we see tomato hornworms because they have grown considerably since they hatched from tiny eggs. Larvae blend in with the foliage and are not easy to detect.

The large caterpillars typically appear in small numbers and cause damage with voracious feeding. They can quickly devoir all the foliage on a tomato plant and even nibble on the fruit and stems.

The search for the caterpillars is sometimes time consuming but defoliated stalks and dark-green droppings (fecal pellets) are easy to locate. Horn worms can also be found on other members of the solenaceous crop species including tobacco, pepper, potato, and related solanaceous weeds.

Hornworm caterpillars belong to the moth family Sphingidae. They get their name from the horn-like projection at the end of the last segment. They are known for the uncommonly large size of many of the species. The larva of the giant sphinx may exceed six inches in length. Some, like the tomato and tobacco hornworms, are common pests of vegetable gardens. Others are rarely noticed except as adults.

Hornworm caterpillars turn into sphinx or hawk moths, a remarkable group of moths that often fly during both day and nighttime hours. With their sharp wings and hovering flight, hawk moths are frequently mistaken for small hummingbirds.

Adult moths lay their greenish-yellow, spherical eggs on the undersides of leaves. The caterpillars that hatch feed on leaves for 3-4 weeks, molt five times, and increase their feeding as they grow. They may measure up to four inches in length and 1/2 inch in width when full grown.

The two-main species of hornworms found in our area include the tobacco hornworm (Manduca sexta) and the tomato hornworm (Manduca quinquiemaculata). These species are biologically similar but are easily distinguished from one another as larvae. Both species are green with a distinct horn on the top of the tail end. Larval tobacco hornworms have 7 diagonal stripes on each side and a red posterior horn, while larval tomato hornworms have 8 chevrons on each side and a bluish black horn. There can be two or three hornworm generations in North Carolina, depending upon temperature.

Many native biological control agents feed on hornworms in North Carolina, and these predators and parasites play an active role in reducing the damage hornworms cause. The most obvious hornworm parasitoids are Braconid wasps which lay their eggs inside of hornworm larvae. As the caterpillars mature, so do the wasp larvae. The wasp larvae emerge from the hornworm and pupate in white cocoons on their backs. If you find a hornworm with cocoons, it is best left undisturbed to allow the cocoons to develop and increase the predator population. A parasitized hornworm eats about 1/5 that of a non-parasitized worm.

The mature caterpillar burrows into the soil to complete its life cycle. It forms a dark brown pupae that is two-inches long. A special protective sheath for the mouthparts looks like a handle-like case on the exterior. The pupae overwinters a few inches deep in the soil.