The tomato hornworm (Manduca quinquemaculata) is a common garden pest that can quickly ruin your tomato crop by defoliating plants and eating into the fruit. Hornworms, larvae of a hawk moth, are light to medium green with a row of black spots along the sides, white chevron-like markings across the back, and a horn-like protrusion on their rump. They can grow to 5 inches long, which makes them a giant among the bug pests afflicting tomatoes. Control involves multiple tactics.
Thoroughly turn the soil in the tomato patch with a garden roto-tiller at the beginning of the gardening season. Tomato hornworms spend the winter underground as larvae in a cocoon. Turning the soil destroys overwintering larvae.
Inspect tomato plants daily for hornworms from early July through August, looking for dark green droppings on the top of the leaves. Adult moths emerge in late spring and lay their oval, green-yellow eggs on the underside of the leaves. The eggs hatch in about a week. The caterpillars reach maturity in about a month, drop off plants and burrow into the soil to pupate for the next generation. Closely inspect the stems and the undersides of leaves. The green caterpillars hide by clinging to stems and the undersides of leaves, where they blend in with the foliage.
Crush eggs and hand-pick caterpillars off the stems and leaves. (The caterpillars do not bite or sting.) Drop the caterpillars in a container of soapy water to kill them.
Spray plants with the organic insecticide bacillus thuringiensis (or Bt) if you have a heavy infestation or large tomato patch. This bacteria causes a stomach disease that paralyzes the caterpillars’ digestive system but is harmless to people and pets. Dissolve 1 to 3 tsp. of the Bt powder in a gallon of water and apply to leaves with a pressure-type garden sprayer. Reapply weekly.
Things You Will Need
- Garden roto-tiller
- Container with soapy water
- Bacillus thuringiensis insecticide
- Pressure-type garden sprayer
- Till the soil again at the end of the growing season to kill larvae produced over the summer.
- Wasps are beneficial as a natural hornworm predator because they use hornworms as hosts for their larvae or as a direct food source. Unusually heavy wasp activity in your tomato patch may signal a major hornworm outbreak.
- In warm climates with long growing seasons, watch for a second generation of hungry hornworms to appear about three weeks after the first generation matures.
- BananaStock/BananaStock/Getty Images