Backyard Gardener: The tomato hornworm – a big worm with a big appetite
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Hello Mid-Ohio Valley Farmers and Gardeners! We are experiencing one of the wettest Augusts in recent memory as rain totals this month are more than 8 inches in many areas of the valley. As the harvest of tomatoes, peppers, green beans, sweet corn and other vegetables continues, please contact the WVU Extension Office with questions on freezing, canning or other garden-related questions.
Keep in mind green beans are a low-acid food and must be processed in a pressure canner. The high temperatures of a pressure canner destroy Clostridium botulinum spores, preventing the formation of the botulism toxin which can cause illness or even death.
This week, I want to discuss two insects that can attack tomato plants, the tomato hornworm (Manduca quinquemaculata) and tobacco hornworm (Manduca sexta). These hornworms can quickly defoliate tomato plants in the home garden. They earned the name because of their prominent horn-like tail. Scout for these caterpillars at least twice a week on your tomato plants during the growing season.
Hornworm larvae primarily feed on tomato leaves but occasionally feed on green fruit. Hornworm larvae have a voracious appetite, devouring up to four times their weight in leaves and fruit each day. If left unchecked, they can defoliate a tomato plant. In addition, hornworm-damaged fruit often decays on the vine or will have dry feeding scars, rendering them unusable.
Hornworms feed only on plants in the nightshade family (family solanaceae), especially tomato and tobacco. In addition, they may also attack potato, eggplant, peppers and some weeds which serve as alternate hosts. These weeds include horsenettle and jimsonweed.
Caterpillars of both species are green with a distinct "horn" on the top of the tail end. Both hornworm caterpillars grow very large and can measure up to 4 inches in length.
The caterpillars of tomato and tobacco hornworms both have five pairs of prolegs. Young caterpillars are yellowish-white but the older ones are typically pale green (black and brown forms may occur also).
Tomato hornworms have eight chevrons (looks like a sideways V) on each side and a bluish black horn, while the tobacco hornworms have seven diagonal white stripes on each side and a red posterior horn. The pupae of both species are dark brown and have a handle-like tongue case on the exterior.
Hornworms overwinter as pupae in the soil. The adults begin to emerge in late May and continue into August. The adult is a large gray-brown "hawk" or "hummingbird" moth with a 4-5-inch wingspan. The front wings are much longer than the back wings, which have dark and white uneven bands. The moth's abdomen has five orange-yellow spots (the tobacco hornworm has six). The moths are most active at sunset and again just before dawn, feeding on nectar from flowers.
After mating, females lay small, pearl-like eggs individually on tomato foliage and the leaves of other hosts. The caterpillars hatch and usually pass through five larval stages (called instars) in about one month. The caterpillars feed until late summer or early fall and then pupate.
Fully grown larvae pupate in the soil. There are usually two generations produced each summer and the larvae of the second generation is the one overwintering in the soil.
Young caterpillars begin feeding on the leaves on the upper portions of plants. Often, they may blend in with the plant canopy and go unnoticed until they have done a significant amount of damage. Rather than chewing holes in the leaves, hornworms consume the entire leaf.
As they feed, they create dark green or black droppings. These BB-sized, cube-shape droppings on leaf surfaces are a clue indicating hornworm caterpillars are feeding above. These caterpillars have quite the appetite. Two mature caterpillars can defoliate a whole tomato or tobacco plant.
Control of tomato and tobacco hornworms includes several strategies. Keeping your garden as weed-free as possible will discourage egg-laying on solanaceous weed hosts. Handpicking hornworms from infested plants is the most effective means of managing them. After picking, drop them into soapy water to kill them.
Tomato hornworms are fairly easy to find because of their large size. Tilling the soil after harvest will destroy burrowing caterpillars and pupae. Bacillus thuringensis, or BT (tradenames Dipel, Thuricide), is a biological control that is very effective, especially on smaller larvae. It can also be used as preventative spray.
Products containing spinosad may also be used including Colorado Potato Beetle Beater, Entrust, Captain Jack's Deadbug Brew, Conserve, Monterey Garden Insect Spray and many others.
Mother Nature has provided us with spinosad, a naturally occurring by-product of the bacteria Saccharaopolyspora spinosa. Products containing spinosad are a natural alternative to control insects in the garden. It is effective against many chewing insects like thrips, beetles and caterpillars but spares many of our beneficial bugs.
There are many natural enemies of the tomato hornworm, including lady beetles and green lacewings. They prey upon the egg stage and on young caterpillars.
Tomato hornworms are also parasitized by a number of insects. One of the most common is a small braconid wasp (Cotesia congregates). Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate. The cocoons appear as white projections protruding from the hornworm's body.
If such projections are observed, the hornworms should be left in the garden to allow the adult wasps to emerge. These wasps kill the hornworms when they emerge from the cocoons and will seek out other hornworms to parasitize.
Contact me at the Wood County WVU Extension Office 304-424-1960 or my e-mail jj.barrett@mail.wvu.edu with questions. Good Luck and until next time, Happy Gardening!