Something is eating your tomato plants, stripping leaves until only stems remain. You find “frass” (larval droppings) on neighboring leaves and on the ground. The culprit? It’s 4 inches long, green as grass, sporting eight V-shaped white marks down each side and a blue-black “horn” on one end: the tomato hornworm, the larval stage of the sphinx moth, also known as a hummingbird or hawk moth.
Tomato hornworms (Manduca quinquemaculata)—and their orange-horned cousin the tobacco hornworm (Manduca sexta)—can strip leaves and blooms in record time and munch on fruits, as well. These big babies feed only on solanaceous plants, aka the nightshade family. Although tomatoes are their main target, peppers, eggplants, potatoes, and related weeds, such as henbane, jimson weed and nightshade, can also fall prey to them. The horns of these pests intimidate predators, but aren’t dangerous.
Hornworms can produce between two and four generations per growing season, depending on the region. Two to eight days after the moth deposits her spherical, greenish-white eggs primarily on the lower surface of foliage, 1/2-inch-long larvae hatch out. For approximately the next 20 days, your garden is at risk of their voracious appetites.
Here are six ways you can keep tomato hornworms from decimating your garden.
First line of defense is tilling—spring, fall and every few weeks in between. The University of Florida says the average life cycle of the hornworm is 30 to 50 days, unless the pupae enter diapause (dormancy). Mature larvae drop to the ground, burrowing to a depth of roughly 1/2 inch, and then enter the pupae stage.
- Advantage: destroys up to 90-percent of the 45-60mm pupae, thus effective in population control
- Disadvantage: doesn’t prevent larvae damage
2. Hand Picking
Unlike many garden pests, a tomato hornworm’s size makes it possible to go on a search-and-destroy mission. The Colorado State Extension office advises that larvae are more easily seen during shadier periods near dusk and dawn, when they tend to feed on the exterior parts of plants.
- Advantage: prevents larvae from defoliating your plants and from reproducing
- Cryptic coloration makes hornworms hard to locate.
- Destroying the caterpillars is distasteful to some gardeners.
3. Light Traps
According to the University of Florida, light traps for adult moths can be used in the garden for added prevention. The moths are attracted to the light and then captured, preventing them from laying eggs.
- Advantage: reduces the egg-laying population
- Disadvantage: minimally effective
4. Predators and Parasites
Lady beetles and green lacewings prey on hornworm eggs and early larval stages, and Japanese paper wasps (Polistes spp.) will attack and feed on all sizes of hornworm larvae. Encouraging wasp colonies near your garden may protect your tomatoes.
Several small parasitic wasps—Trichogramma spp, Cotesia congregata and Hyposoter exigua—will lay eggs and pupate on hornworms. If you capture a caterpillar with what looks like rice grains on its back, sequester it in a screen-lidded container with a few leaves. As the wasps emerge from their little white cocoons, they kill their host and escape the jar, seeking out other hornworms to parasitize.
To increase effectiveness, purchase additional beneficial insects and/or plant host plants.
- Advantage: effective, natural, no-hassle pest control
- Lacewings and lady beetles aren’t effective as the caterpillars gain size.
- With parasitic wasps, a certain amount of defoliation still takes place until parasitism takes effect.
- Paper wasps sting people.
5. Bacterial Insecticides
Bacterial insecticides include Bacillus thuringiensis of the Kurstaki strain (BtK—e.g. Dipel, Thuricide). Applied to foliage, they are most effective when hornworm larvae are small. Caterpillars lose the ability to feed soon after munching leaves sprayed with BtK, according to Gardens Alive, a retailer of eco-friendly garden products. (Follow label instructions for application.) If hornworms have been a problem in previous years, you can spray prophylactically every two weeks to prevent damage to plants.
- You don’t have to find the caterpillars—just spray the host plants well.
- BtK is effective against all caterpillars, but will not harm beneficial insects.
- Disadvantages: Overspray onto butterfly host plants can kill butterfly caterpillars.
6. Chemical Insecticides
Chemical insecticides, carbaryl, permethrin, bifenthrin, and spinosad, kill larvae on contact, though should be used with caution if you have animals near the garden or are concerned about keeping beneficial insect populations intact.
- effective on all stages of hornworms
- readily available
- kills beneficial insects
- toxic to mammals