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August 7, 2018

Gardening: Stopping the nasty tomato hornworm

Photo/Steve Hall
This tomato hornworm caterpillar has white pupa from which adult wasps emerge in a few days.
By Ann Atanasio
Fauquier County Master Gardener

With vegetable growing season well underway, your plants look healthy and productive. You are pleased. Until one day when you enter your garden and observe massive defoliation of your tomato plants.

Closer examination reveals a large green caterpillar, 5 inches long, with white and black markings and eight V-shaped stripes on its body. On its posterior abdominal segment is a threatening looking, black horn-like protrusion.

Meet the tomato hornworm (Manduca quinquemaculata), probably the largest caterpillar that will ever invade your garden. It is an attractive green, and its overall appearance is not unlike Absolem, the caterpillar in Alice in Wonderland.

But for me, this one garden pest, more than any other, makes my flesh crawl. And I know from conversations with other gardeners, that many share my aversion. I can’t explain why. The caterpillar is harmless to humans, and the posterior horn isn’t a stinger. Maybe the caterpillar looks less like Absolem and more like Jabba the Hutt.

Whatever the reason that makes its appearance repugnant, the tomato hornworm is a voracious adversary and must be dealt with. And the best strategy for avoiding hornworm invasions is to understand their life cycle and how they come to feast on your tomatoes.

First, where do they come from? In late spring, adult moths emerge from the soil. The moths are a mottled grey/brown color with orange or brownish spots on their bodies. They have a wingspan of 4 to 5 inches, and their rear wings bear white, zigzag markings. The moths are known as sphinx or 5-spotted hawkmoths and are very fast fliers. They also have the ability to hover, earning them the nickname, “hummingbird moth.” Paradoxically, these moths are among the more important moth pollinators that inhabit our yards and gardens.

After adult moths emerge from the soil, they mate and the females deposit their light green, spherical eggs on the underside of leaves. In five days, the larvae hatch and begin feeding on the foliage of their host plants. These newly emerged caterpillars will reach full growth in three to four weeks.

As the larvae mature, the caterpillars drop off the host plant and burrow into the soil, where they transform to the pupae stage. Adult moths will emerge from the soil in two to four weeks to mate and lay eggs. The species may produce multiple generations per year, with the latest generation overwintering in the garden soil.

One of the greatest difficulties in controlling the tomato hornworm is that it blends in seamlessly with its host plant canopy and is capable of inflicting extensive damage before being discovered. For this reason, early detection is critical.

Develop the habit of routinely monitoring your garden foliage for light green eggs, small caterpillars or dark green or black droppings that appear as spots. Also be vigilant for stems with missing leaves, large holes in leaves, severe defoliation or scarring of fruit. Although tomato plants are the preferred hosts, these caterpillars also will feed on the peppers, eggplants and potatoes growing in your garden. So carefully examine those plants as well.

If you discover tomato hornworms, handpicking is the easiest way to get rid of them. The caterpillars aren’t dangerous and cannot sting. After removing them from the plant, you can squash them underfoot or drop them into a bucket of soapy water to kill them.

If you discover tomato hornworms that exhibit white protrusions similar in appearance to grains of rice, do not kill them. These protrusions are wasp larvae that will feed on and very quickly kill the host caterpillar. You can leave the host hornworm in place, or you may remove it from your vegetable plant and place it away from your garden so that it cannot inflict further damage.

The wasp larvae will continue to feed, killing their host. Eventually they’ll produce future generations of wasps that will aid in protecting your garden from the next generation of tomato hornworms.

To reduce the likelihood of tomato hornworm infestations in future years, roto-till your garden after harvesting. Turning over your garden soil at the end of the growing season has been shown to be 90-percent effective in destroying pupae that are overwintering in the soil because they burrow shallowly.

Another strategy to protect your garden is to plant parsley, basil, dill, yarrow and/or mustard near your tomato plants. Those herbs will attract wasps that feed on their nectar. The wasps’ larvae will kill the hornworms.

If pest levels cannot be controlled by hand-picking alone, contact your local extension office. An Extension Master Gardener volunteer will be able to assist you with information about combating tomato hornworms by treating with synthetic or organic pesticides that are labeled for use on garden vegetables.

You can reach the Master Gardener Help Desk at 540-341-7950, extension 1, or .(JavaScript must be enabled to view this email address).
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