Hornworms are the bane of every tomato gardener's existence. Not only can they be imposing to observe, but these large, horned insects can severely damage and even destroy an entire tomato crop in a matter of days if they go undetected. Fortunately, most infestations can be controlled with a combination of naturally occurring predators and hand removal.
Tomato hornworms are 3- to 5-inch-long caterpillars native to the United States that are the larval form of the hawk moth, also known as the sphinx moth. Their light green coloration and V-shaped white and black markings help them hide deep within the foliage of tomato plants, as well as on the leaves of other members of the solanace family such as peppers, eggplant and potatoes. Because tomato hornworms are heavy feeders, they are capable of quickly devouring leaves and, in some instances, the fruit.
Life Cycle and Growth Stages
Mature hawk moths lay a series of eggs on both the top and undersides of leaves in the spring. The oval-shaped eggs can be either pale yellow or green and hatch in about one week. Newly hatched hornworms are light yellow and will undergo several transformations, called "instars," during which they will turn from yellow to white, then become green and develop black and white V-shaped markings along the body as they reach maturity. This process takes three to four weeks, after which the fully grown hornworm larvae will leave the tomato plant and burrow in the ground to pupate. The pupae will later emerge from the ground as moths and seek out tomato plants to lay eggs on so that their life cycle can be repeated.
Damage to Plants
Hornworm larvae feed on the leaves and fruit of tomato plants, which can quickly cause extensive damage and plant mortality. Sometimes they will eat holes in the leaves, other times they will eat the leaves all the way down to the stem.The larvae are close in color to the tomato plant leaves, which makes it fairly easy for them to blend in with the foliage and go undetected until you see obvious signs of severe plant damage. Therefore, it is important to regularly check not only the tops of leaves but also the undersides as well as stems. Dawn and dusk are ideal times to look for hornworms since they dislike direct sun and may be found on the exterior leaves of the plants. Often the first sign of a hornworm infestation is a series of small, dark droppings on the leaves.
Fortunately, hornworms have some natural predators. Parasitic wasps will lay their eggs on the mature hornworm's back and when the eggs hatch they will feed on the hornworm. Should you find a hornworm in your garden with white eggs on its back, it's best to leave it alone since the wasps will ultimately hatch and go after additional hornworms. Other insects such as ladybugs and green lacewings feed on hornworm eggs and young larvae.
Gardeners can quickly, safely and easily remove hornworms by hand or by brushing them off the plant with a stick before discarding them. If you do not want to touch or handle the hornworm directly, remove them by cutting off a populated stem. After the hornworms have been removed from your plants, check your tomato crop regularly for any signs or indications of a repeat infestation.