Tomato hornworms eat large, ragged holes in tomato leaves, easily consuming whole leaves. Hornworms also eat stems and green fruit, and if they arrive early enough in the season seedlings and young plants.
Tomato hornworms feed not only on tomatoes but also peppers, eggplants, and potatoes—any member of the nightshade family.
The best way to control tomato hornworms is to handpick them from tomato-family plants and crush them. Parasitic wasps released in the garden can also control hornworms.
The tomato hornworm (Manduca quinquemaculata) is a green caterpillar with seven or eight white stripes and a black horn projecting from it rear end. It is the best known tomato pest and the largest; the hornworm can grow from three to four to five inches long. The tomato hornworm is the larval stage of the five-spotted hawk moth; it too is large with a wingspan of nearly five inches. (A similar species is the tobacco hornworm (Manduca sexta), also green with white stripes, but a red horn jutting from its rear.)
Lifecycle of the tomato hornworm. Hawk moths emerge from soilborne pupae in early summer at start of the tomato gardening season. Adult moths lay eggs on the underside of leaves, and eggs hatch in a week. The larvae—the tomato hornworm–feed for about four weeks then pupate in the soil usually until the following summer. But in long summer regions, there can be two, three, or four generations in a year.
Adult moths. The hawk moth is a mottled grayish or brownish moth with a large, furry body. White zigzags appear on the rear wings and orange or brownish spots on the sides of the abdomen. The hawk moth has long, narrow gray wings and a wingspan up to 4- to 5-inches. The moth flies mostly at twilight and can hover hummingbird-like in the air to sip nectar from deep-throated flowers. The hawk moth lays eggs on the underside of tomato-family crops and weeds.
Eggs. Hawk moths lay single greenish-yellow eggs on the underside of leaves. The eggs hatch small green caterpillars in about a week.
Larvae. The tomato hornworm is the larval stage of the hawk moth. It is a green caterpillar up to 5-inches long with V-shaped white marks down the sides of the body and a black horn at the tail end. (The related tobacco hornworm is similar but has seven white stripes down the side and a red horn.) The hornworm chews on leaves leaving only the midribs of leaves behind and sometime chews into green fruit. The hornworm reaches full size after three or four weeks of feeding.
In autumn, hornworms pupate forming a hard shell. The pupae rest in the soil over the winter three or four inches below the surface.
Hornworms with white rice-like projections sticking out of their backs have been attacked by parasitic wasps. Parasitic wasps—commonly braconid wasps–lay white eggs on the backs of hornworms; the larvae hatch and feed on the internal organs of the hornworm.
Tomato hornworm controls:
• Garden cultivation. Early in the garden season, turn the soil where tomato family plants grew the season before. Cultivation will expose hornworm pupae to killing cold and birds.
• Handpicking. Handpick caterpillars from foliage. Check tomato-family plants frequently for larvae and handpick them. (Hornworms may look mean, but they will not harm humans.) Remove hornworms from plants and crush them or drown them in soapy water. Do not handpick and destroy parasitized caterpillars; they will hatch beneficial parasitic braconid wasps.
• Parasitic wasps. Attract or release parasitic bracnoid wasps into the garden. Braconid wasps lay eggs on caterpillars, often forming small white cocoons on the skin. When the wasp eggs hatch, the larvae feed on hornworms. Trichogramma wasps parasitize hornworms in the egg stage.
• Spray. Spray small caterpillar with Bt, Bacillus thuringiensis, a pathogenic bacterium (safe to man). Spinosad (a bacterium-based insecticide) can also control hornworms.
• Repellant plants. Plant borage, marigold, or opal basil in the garden. These plant repel hawk moths and hornworms.