Many beneficial insects enjoy feasting on young tomato hornworm caterpillars, including yellow jackets, lady beetles and green lacewing larvae. By attracting these helpful insects and implementing additional controls, such as handpicking and row covers, pesky tomato hornworms could soon be a thing of your gardening past.
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
The most common insect pests of tomatoes, tomato hornworms (Manduca quinquemaculata) and closely related tobacco hornworms (M. sexta) are frequently found feeding on leaves and stems of tomatoes, peppers and other solanaceous crops. Uncontrolled, they will eat holes in tomato fruits, too. Organic tomato hornworm controls include handpicking or applying a Bt insecticide.
Native to North America, tomato hornworms can be found in most climates where tomatoes will grow. Tobacco hornworms are more common in the southeast and southwest, but rarely seen in northern regions.
Large green caterpillars with white stripes on their sides are hornworms. There is also a rarely-seen brown form of the tomato hornworm. Hornworms are the larvae of a large mottled tan sphinx moth that flies at night. Although most commonly seen on tomatoes and tobacco, hornworms also can feed on peppers and other tomato cousins.
Gardeners first notice missing tomato leaves, often on the tips of branches. Tracing the pattern of damage is often assisted by a trail of frass, or caterpillar excrement. The hornworm is often found camouflaging itself along a leaf vein or stem. Very young hornworms do little damage, but the larger they get, the more they eat. Most of a hornworm’s leaf consumption occurs during its last week of feeding.
Tobacco and tomato hornworms overwinter as pupae buried in the soil. When adults emerge in late spring or early summer, they begin laying eggs on host plants. Eggs are deposited singly on both sides of leaves, and are often scattered in the midsection of tomato plants. Eggs hatch in about a week and the tiny larvae begin feeding. The larval stage lasts three to four weeks, with the caterpillars steadily growing larger. Mature caterpillars drop to the ground and wander a bit before going underground to pupate. In most climates there is one generation each year, but warm climate gardeners may face a second generation in late summer.
Soft-bodied tomato hornworms have many natural enemies, especially when they are young. Yellow jackets and other wasps gather newly hatched hornworms, and eggs and baby hornworms are often preyed upon by lady beetle and green lacewing larvae. In addition, a small braconid wasp lays its eggs inside tomato and tobacco hornworms. The braconid larvae feed inside the hornworms and then from rice-like white cocoons attached to the hornworm’s body. Hornworms found bearing these cocoons should be left alone, because the next generation of braconids will parasitize other hornworms.
Most gardeners get good organic control of tomato hornworms by handpicking them. Until you learn what to look for, spraying the plants with water can increase visual contrast and make handpicking easier. When numerous small hornworms are found, you can use Bt (Bacillus thuringiensis) or spinosad to kill them.
Gardeners who grow plenty of flowers and herbs to attract beneficial insects report having fewer problems with tomato hornworms. Allowing wasps to nest near the garden also results in enhanced hornworm control.
The fragrant annual flower called flowering tobacco or nicotiana can be used as a trap crop for tobacco hornworms. Grow the plants as far as you can from your tomatoes. In midsummer, after the first flush of flowers fades, cut plants back by at least half their size. In doing so, you may encounter (and dispose of) numerous tobacco hornworms. The nicotiana plants will grow back and resume blooming.
Following seasons with serious hornworm outbreaks, pull up tomatoes and peppers in the fall and cultivate the soil to expose pupae. No cultivation is necessary if you have chickens or other poultry willing to peck through the soil.
More information on organic tomato hornworms control is available from the University of Minnesota, Florida State University and the University of Maryland.