The tomato hornworm, Manduca quinquemaculata, is much dreaded by vegetable gardeners because it can devastate tomatoes and other members of the nightshade (Solanaceae) family. Fortunately, once you identify these pests, they're fairly easy to eradicate. If it wasn't for the damage they cause, these insects would be fascinating to study. The worms are large and brilliant green in color, with a formidable-looking but harmless horn on the back. The adult is a large, interesting hawk moth with a wingspan up to 5 inches.
But in the hornworm phase, they are voracious pests, munching entire leaves, small stems, and even parts of the immature fruit. While most commonly associated with infested tomatoes, hornworms are also common pests of eggplants, peppers, and potatoes. Most likely, you'll notice the damage before you notice the hornworms because their color helps them blend in so well with the plant foliage. You can also look for their black frass (droppings) on the foliage and around the base of the plant. Undetected, a tomato hornworm can do a fair amount of damage to its host plant. They have hearty appetites and can defoliate a plant in a matter of days. However, if they are detected and removed early on, the plant will recover just fine.
The tomato hornworm represents the larval stage of the five-spotted hawk moth, sometimes known as the hummingbird moth. These moths overwinter in the soil as dark brown pupae, then emerge and mate in late spring. They lay their round, greenish-white eggs on the undersides of leaves. The eggs hatch in four to five days and the hornworm emerges. It spends the next four weeks growing to full size, after which it will make its way into the soil to pupate prior to emerging as a new moth.
6 Ways to Get Rid of Tomato Hornworms
Pick Off the Worms by Hand
Because the hornworm is so large, the easiest and most effective way to get rid of it is to pick them off the plants as soon as you detect them. You can then either crush them or toss them into a bowl of soapy water. While the worms look a bit fierce, the horn on the back is not sharp enough to pierce the skin. If you're squeamish, though, you can wear a glove or use tongs to pick off the worms.
Use Bt (Bacillus thuringiensis)
A bad infestation of hornworms can be treated by applying Bt (Bacillus thuringiensis), a natural bacteria that is often found in soil. When hornworms eat the bacteria, it paralyzes the caterpillar's digestive system; they soon stop feeding and finally die off. Bt is a natural substance that has no harmful impact on human beings and most animals. Application is most effective when the larvae are small. There are many strains of Bt available; consult your local garden center for recommendations on what strain is currently most effective on hornworms.
Till the Soil Annually
If worms are a problem year after year, try roto-tilling the soil either in late fall or in spring before you plant; this will either bury the pupae or destroy them before they can hatch into egg-laying moths.
Encourage Natural Predators
Tomato hornworms have a number of natural predators. If you live in an area that allows chickens, turning a few birds loose in your tomato patch will likely cure your hornworm problems, as chickens love to eat hornworms and caterpillars. Many other birds also eat hornworms, such as crows and owls, though some of these birds are also known to eat tomato fruit.
An even more effective predator is any one of the several types of parasitic braconid wasps that attach eggs to the backs of tomato hornworms. When the eggs hatch, the larvae consume the hornworm as food, eventually killing it. If you discover a hornworm covered with tiny white egg sacs, leave it alone, as this indicates you have a healthy population of parasitic wasps present in your garden to control the hornworms. Let the eggs hatch, and you'll have an army of wasps ready to defend your garden against all types of pests. These wasps are not a threat to humans.
A number of plants are known to draw parasitic wasps to a garden, including dill, clover, sweet alyssum, and fennel.
Make Your Own Repellant
Another organic method for killing tomato hornworms is to mix liquid soap and water together and spray it onto the plant foliage, then sprinkle cayenne pepper over the foliage and fruit. The soap solution will kill the worms, while the cayenne repels them if the soap washes off. This treatment will need to be repeated after each heavy rain.
Use Chemical Pesticide
The synthetic chemical carbaryl (found in the Sevin brand) will kill tomato hornworms along with many other insects. However, carbaryls are highly toxic and their use is discouraged on edibles. Wherever possible, rely on organic methods to control tomato hornworms and other garden pests.
What Causes Tomato Hornworms?
Tomato hornworms are often found in areas where there are vegetables from the nightshade family (tomatoes, potatoes, eggplant, etc.) growing. The adult moths have a preference for the nectar of certain plants with large, fragrant white flowers, such as Datura meteloides (moonflower), Oenothera caespitosa (evening primrose), and Mirabilis multiflora (four o' clocks). A garden where there are nearby plants catering to both phases of the insect is very likely to experience hornworms.
How to Prevent Tomato Hornworms
Regular tilling of the soil and crop rotation is important for preventing hornworms, as this either destroys the pupae in the soil or prevents the emerging moths from having convenient plants on which to lay eggs when they hatch into moths.
Tomato Hornworms vs. Tobacco Hornworms
The tomato hornworm and tobacco hornworm, Manduca sexta, are often confused with one another. They are very similar in appearance and both worms attack members of the Solanaceae plant family. The tomato hornworm is 3 to 4 inches long at full size (likely to be the biggest caterpillar we see in our gardens) and green in color, with eight diagonal white stripes along its sides. A black horn projects from the rear of the caterpillar. Tobacco hornworms, on the other hand, have seven diagonal white stripes and a red horn. The adult moth is known as the Carolina sphinx moth or tobacco hawk moth
Both worms are managed in the same way as garden pests.
Do Tomato Hornworms Bite or Sting?
Although they look somewhat ferocious, tomato hornworms are basically harmless to humans. The horn-like projection is sometimes described as a "hood ornament" and is not sharp enough to pierce the skin. If you handle a hornworm, it may wrap around your finger and cling to your skin—a sensation that is odd and sometimes uncomfortable, but not dangerous.
How Long Do Hornworms Live?
Small hornworms emerge from eggs when they hatch in late spring or early summer, then go through five molts as they grow into full-sized adults. The entire process lasts three or four weeks, then the worms drop off the plant to burrow into the soil and create pupae that will hatch into moths. In warmer climates, there may be a first generation that completes its cycle before the second generation overwinters in the soil.
Are There Companion Plants that Repel Hornworms?
There are various flowers that attract parasitic wasps, which can control hornworms by feeding on them. But many gardeners swear that marigolds actually repel the worms and moths by emitting a strong odor that the insect cannot bear. It's worth planting some marigolds around your tomatoes, potatoes, eggplants, and peppers (all the nightshade species) to see if this method works. Marigolds have a reputation for also repelling cabbage worms, thrips, whiteflies, and others. It's best to choose a marigold variety that has a noticeable strong scent, not hybrids, which are often rather scentless.