GREENER PASTURES

By Peter Warren, Extension Agent
Published: July 31, 2008

About this time of year many of us are finding insects eating in our vegetable gardens; most are welcome because they are eating each other and some not so much because they are after our prized crops. Some of the most recognized are the hornworms as they strip the leaves and fruit from our tomato plants.
Hornworms, the larvae of sphinx or hawk moths, are among the largest and most familiar caterpillars found in Virginia, some reaching lengths of three inches or more. Almost all have a dorsal “horn” on the eighth abdominal segment or back end, and hence their common name. The most widely recognized hornworms are those that feed on tomatoes - the tomato hornworm and the tobacco hornworm. Although these two insects are considered garden pests, the majority of the hornworm species found in Virginia is rarely observed and do not cause significant injury to plants. The large frass or poop of hornworms often reveals their presence in a tree or garden.
Hornworms are commonly encountered as they wander over the ground in search of pupation sites. Full-grown hornworm larvae migrate from their host plant and dig in loose soil where they pupate. Pupation occurs a few inches below the soil surface in a small chamber of packed earth. Pupae are typically brown, two inches or more in length, and many have a pronounced “snout” off the head end. Sometimes we find pupa while preparing our gardens for the next crop.
Adult stages of hornworms are heavy-bodied, strong flying insects known as sphinx of hawk months. Many of these, including the tomato and tobacco hornworm moths, fly at night and are rarely observed except occasionally at porch lights. However, some day flying species are popularly known as “hummingbird moths.” These moths have a superficial resemblance to hummingbirds or bumblebees in flight while they feed from deep-lobed flowers. The species that fit this description include the snowberry clearwing (Hemaris diffinis), the hummingbird clearwing (Hemaris thsybe), and the whitelined sphinx (Hyles lineata).
Tomato hornworms
The most familiar hornworms, those associated with tomatoes and related plants, are a complex of two species, the tomato hornworm (Manduca quinquemaculata), and the tobacco hornworm (Manduca sexta). Both species are pests in vegetable gardens. The caterpillars chew leaves, and plants can be rapidly defoliated. Fruits may also be chewed. Tomatoes are particularly susceptible to injury; but other related plants, such as peppers and potatoes, are occasionally infested.
The tomato hornworm has a dark green “horn” with black sides, while that of the tobacco hornworm is red. The caterpillars also are differentiated by the white striping along their sides. These form a series of V’s with tomato hornworms, while there are diagonal dashes on the tobacco hornworm. Occasional dark forms of the tomato hornworm larvae occur. The caterpillars of these are much darker than the normal green, although the adult moths differ little in appearance.
Many of the tomato and tobacco hornworms overwinter as pupae in the soil in the vicinity of previously infested gardens, typically emerging in late May and June. However, the adult moths are strong fliers that may travel long distances. Female moths lay large pearl-colored eggs on the upper surface of leaves. The young caterpillars hatch and feed on the plant for a month or more. They have tremendous appetites and consume large amounts of leaves as they grow older and larger. After feeding, they wander away from the plant and pupate in the soil.
Adult moths are identified by examining the hind wing which has two separate wavy bands next to the border on the tomato hornworm that are fused with the tobacco hornworm. The white marks on the abdomen of the tomato hornworm are also more angularly marked.
Control in gardens
Tomato hornworms are easily controlled by garden insecticides (e.g., carbaryl, permethrin, spinosad). Depending on the size of your crop, examine the foliage with patience (for they are difficult to see) and tossing them aside when you find them can be effective. Larvae tend to feed on the exterior parts of plants during shadier periods, near dusk and dawn, when they may also be more readily observed and destroyed. A biological control organism that is also highly effective and sold commonly is Bacillus thuringiensis (e.g., Dipel, Thuricide). Best of all there is a natural enemy of hornworms; a braconid wasp (Cotesia congreagata) lays dozens of eggs inside the larvae. The immature wasps eat their way out of the hornworms and pupate by spinning a cocoon on the outside of the caterpillar body. If you find hornworms covered with these tiny cocoons you should leave them be so the wasps can complete their life cycle and live to sting another hornworm.