Should You Kill Tomato Hornworms? Why Experts Say No

Tomato hornworms, Manduca quinquemaculata, are large green caterpillars that devour tomato plants along with other nightshades including peppers, potatoes, and eggplant.

They can also be a little intimidating, as garden pests go, due to their large size. A tomato hornworm can grow up to 5 inches long and as fat as your index finger. They have V-shaped, yellowish-white markings and a black ‘horn’ or spur on their posterior segment.

These caterpillars feed for three to four weeks after hatching, molting five times and growing larger every time they shed their skin—with an appetite that increases along with their size. A mature caterpillar can easily kill a large tomato plant in a couple of days.

These destructive pests do have some enemies. Here, we’ll look at the beneficial insect that can eradicate hornworms and other pests from your garden.

Should I Kill Tomato Hornworms?

By the time you can easily spot a tomato hornworm, it’s usually feeding near the top of your tomato plant with visible damage.

If the caterpillar is large, actively feeding, and you see lots of vines stripped of their leaves or tomatoes with damaged skin that looks like it’s been nibbled you need to get rid of the caterpillar before it fully ruins your crop.

If the caterpillar is carrying what looks like grains of white rice on its back, it is probably almost or already dead.

It has been inoculated by a parasitic braconid wasp that laid its eggs underneath the skin of the hornworm.

The wasp larvae actually feed on the caterpillar starting a process of desiccation that causes it to dry up and die. By the time the wasp larvae spin their white rice-like cocoons, the caterpillar has suffered irreversible damage and can no longer damage your tomato crop.

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How Can Tomato Hornworms Help Your Garden?

Hornworms serve as host caterpillars for female braconid wasps which seek them out as a food source for their developing wasp larvae.

Female wasps can lay up to 200 eggs a day as long as a host insect is available. By not killing hornworms, you provide an opportunity for more beneficial wasps to help control the pest population in your garden.

These small, slender-bodied wasps are considered an important biological control method for many gardening pests including aphids, beetle larvae, sawflies, and other unwanted caterpillars. They represent a non-toxic method for maintaining an ecological balance in the garden environment.

Unlike chemical applications, they do not adversely affect other beneficial insects and are not harmful to pets or humans. Braconid wasps also are efficient pollinators visiting both ornamental and vegetable flowers transferring pollen for fertilization.

Once a hornworm has been injected with braconid wasp eggs, its life cycle starts to deteriorate. If you find a hornworm in your tomato patch covered with wasp cocoons, leave it alone.

Allow the life cycle of the wasp to complete and other hornworms and garden pests will become prey to this beneficial wasp.

How to Attract Braconid Wasps

Create a habitat for braconid wasps by providing a water source and potential nesting sites. Besides parasitizing hornworms, they also lay eggs in garden and yard waste such as leaf piles and tall grass. Adults feed on nectar and pollen from a number of herbs and native flowers. Here are some plants you can grow to attract braconid wasps.

FAQ

• Lady beetles and green lacewings feed on small hornworms and hornworm eggs. Paper wasps and other parasitic wasps in the Trichogammid species also eat hornworms.

• Plant mums, marigolds, nasturtiums, and petunias to keep hornworms away. They also avoid a number of herbs including basil, dill, lavender, catnip, and rosemary.

• Hornworms are easiest to spot in late afternoon and early evening feeding at the top of your tomato plants. In the heat of the day they stay in the lower foliage and are harder to spot.