by Shelby Baker
Chewed fruit, nubby stems, and spotty leaves can be a gardener’s nightmare. So much time and effort in growing tomatoes, peppers, eggplants can all be lost over the course of a few short weeks. The scene of the crime is an innocent, leafy green garden. The culprit—a large, meaty, crawling monster known as a tomato hornworm (Manduca quinquemaculata).
Identifying The Problem
Hornworms are the caterpillars of moths, the voracious devours of gardens, the scourge of the nightshade family. The laid hornworm eggs hatch within a week from the undersides of foliage. In four to six weeks, the larva and its appetite rapidly grow before cocooning in the soil overwinter or for two to three weeks if the weather is warm.
From there, the moths emerge, and the cycle begins again. The adult moth form of the tomato hornworm turns into a pretty big-bodied moth that is often known as a hawk moth or a sphinx moth. Hawkmoths have a wingspan that ranges from two to eight inches.
Named for a small protrusion found on their rear that looks like a horn, hornworms can reach anywhere from four to six inches long and are as fat as a finger. The caterpillars are a light green with black and white markings along the sides of their bodies, denoting the type of hornworm they are. And they love to eat tomato, pepper, eggplant, and potato plants, all members of the nightshade family. They can also be attracted by common weeds that act as an alternate hosts, including jimsonweed, horsenettle and nightshade, and then find their way to your garden after consuming those. They are unfortunately a quite common garden pest in the home garden.
Tomato hornworms are found most commonly in the northern United States and have white V-shaped markings down their sides. Tobacco hornworms, on the other hand, are more commonly found in the South and have white slashes along their sides. Their territories, menus, and vast appetites plague gardeners without discrimination. Thankfully, their removal and prevention are the same, too.
By the time an infestation is discovered, severe damage has usually already been done. Signs of the hornworm’s work include dark green droppings on leaves, missing leaves, and wilted leaves hanging down on the verge of giving up. In the worst cases, the plants experience severe defoliation in the form of devoured flowers, large holes in leaves, and scarring on the surfaces of the produce. Keep an eye out for white cocoons nearby in the soil as well.
Often, the hornworm damage to a garden goes noticed until midsummer, after the caterpillars have already invaded the garden and started to seriously defoliate plants. There they remain from late summer until the harvest—or until measures are taken to get rid of the garden pests.
Controlling Hornworm Infestations
Luckily, or unluckily for the squeamish, the best and most through method of control for a small garden is to hand-pick the hornworm caterpillars. With time and devotion, they can be found and drowned in liberally soapy water or squashed underfoot. Though not dangerous, hornworm can eject a dark brown or black liquid when startled and picked up, so be sure to wear gloves when weeding them out. Check daily for best results.
Due to their natural camouflage, spotting the worms can feel like squinting at a page out of “Where’s Waldo.” To aid in finding them among the leafy foliage, fill a spray bottle with liberally soapy water or a commercial insecticidal soap, and then spray the plant. This treatment will cause the pests to writhe and move, making them easier to see. Removing them will also be easier at dusk, dawn, and nighttime, when they emerge to feed.
If physical removal by hand isn’t enough to control the infestation, or if the affected area is large, consider botanical Bt (Bacillus thuringiensis spp). An organic pesticide approach to widespread annihilation of the pest, botanical Bt is not a chemical. It is a biological control that acts as a stomach poison and causes the hornworms to lose their appetites without harming plants, animals, or more beneficial insects. The organic pesticide spinosad is also effective against hornworms.
Natural enemies of the hornworm can be used against them as well. If you spot a hornworm with white, rice-like protrusions on its body, leave it alone. Those little grains are temporary homes for braconid wasp larvae that attack the caterpillars. While attached, the wasps feed on the hornworm and finish it off after hatching.
Introducing wasps into the garden can keep population numbers down, but they aren’t the only predatory insects to plague hornworms. Consider introducing lady beetles and green lacewings, beneficial gardening insects, to the garden. They won’t attack the older hornworm larvae stages but will absolutely prey on the eggs and younger caterpillars.
Tomato Hornworm Prevention
The first step in preventing a hornworm outbreak is to till the soil at the beginning and end of each gardening season. An adult female moth can lay up to 2,000 eggs in the soil, and tilling can eliminate 90 percent of possible larvae.
Crop rotation can also help with reducing hornworm invasion. Although not the best prevention method, moving the crops each season can remove them from the eggs in the soil and make finding the crops a little harder for the hornworms.
Using black plastic mulch will prevent the pupae from creeping their way to the surface. By keeping the moths in the ground, the cycle is interrupted, as the moths have nowhere to go and will eventually die before laying new eggs.
Distract the caterpillars with one of their favorite treats: dill. Learn more in our article: Keep Tomato Hornworms Away With a Sacrificial Dill Host Plant. Hornworms love dill, and the dill may be worth the sacrifice to some gardeners to save their tomato plants. Planting additional crops around tomatoes can also help to deter the pests. Basil and marigold, when planted intermittently with nightshade crops, can reduce hornworm numbers.
Though a little more expensive, pop-up nets can isolate your plants from moth and hornworm interference. When setting up the pop-up nets, it’s important that measures have already been taken to prevent and kill off the eggs in the soil. Otherwise the caterpillars and moths will be trapped with the plants.
Use a combination of monitoring, control and prevention methods to save your plants from hornworm harm. A life cycle for the pest lasts 30 to 50 days, but those few weeks can wreak havoc on a garden. If you spot one of these green monsters, take immediate action, and check the rest of the plants for further signs of infestation. When facing the dreaded hornworm, the best defense is a strong offense.
Want to learn more about preventing and treating tomato hornworms?
The Old Farmer’s Almanac covers Tomato Hornworms
University of Minnesota covers Tomato Hornworms in Home Gardens
The Veggie Gardener covers Getting Rid of Hornworms
Plant Natural Research Center covers Tomato Hornworm Control
Garden Tech covers How to End Hornworm Havoc