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Tomato Hornworm

By Carol Allen

This is the time of year when avid gardeners wish they had low-maintenance (whatever that is!) gardens. The heat has us sneaking out in the early hours to do what we must—harvesting, weeding, or watering—and beating a retreat when the sun starts to get high in the sky. In spite of the draining heat, one of the highlights of mid-summer is watching the pollinators visit our gardens, including the occasional hummingbird moth. While we welcome the hummingbird moth, you may not be as happy to have a visit from one of its close relatives: the larvae of the five-spotted hawk moth, aka tomato hornworm. Most members of the family Sphingidae (hummingbird moths and sphinx moths) are night-flying moths and , because they are large moths, they have very large, hungry larvae. As a group, their larvae sport a fearsome-looking projection on their rear ends, but it is all for show. It is just one of nature’s devices allowing soft, defenseless creatures to look scarier than they are. Fortunately, most members of that family feed on plants we do not consider as precious as our tomatoes. They are solitary, so their presence goes unnoticed. Sphinx moth larvae are typically host-specific feeders and if you see one off its host plant, it may be mature enough to go to ground and pupate. Many of these species do pupate in the ground, so be considerate of their life cycle and don’t be too tidy in the garden. Fluffy skiffs of leaves at the base of plants and in the nooks and crannies of the garden can provide good sites for these dramatic creatures to pupate. The Maryland Biodiversity Project lists 40 species of hawk moth as being found in the state. Most feed on trees, but some notable species feed on azaleas, and hydrangeas, and a very common one feeds on Virginia creeper. That makes keeping some of it around a very good idea. It is the tomato hornworm that usually gets our blood pressure up. These sphinx moth caterpillars can be up to 4 inches long before they pupate and can be found not only on tomatoes, but also eggplant, peppers, and moonflowers. They can be large in size, so they have a correspondingly big appetite and can defoliate plants rapidly. They are easy enough to spot and hand-pick, then drown in a cup or jar of soapy water. However, spare it if you see one with the white pupal cases of a parasite attached to it. Two different parasitic wasps help control tomato hornworm; a species of Trichogramma and Cotesia congregata, a braconid wasp. These wasps go pretty much undetected in a garden because they are the size of a fruit fly. They do not harm humans and are not interested in anything other than hornworms. If you find a tomato hornworm covered by little white pupal cases, let it be. Those caterpillars stop feeding and soon die, and the parasitic wasps hatch to go on to find another hornworm to parasitize. Ideally you planted enough tomatoes to go around. o

Carol Allen describes herself as a committable plant-a-holic. She has more than 25 years’ experience in the horticulture industry, with a special interest in plant pests and diseases; is a Licensed Pesticide Applicator in the state of Maryland; and is an ISACertified Arborist. She can be contacted at carolallen@erols.com.

Top photo, a tomato hornworm, larva of Five-spotted hawk moth, by Amanda Hill, CC0, via Wikimedia Commons.

Bottom photo, a tomato hornworm with parasitic wasp eggs by Max Wahrhaftig, CC 3.0, via Wikimedia Commons.