Tomato hornworms are a common garden pest, known for consuming the foliage of solanaceous plants like tomatoes, peppers, and eggplants. These caterpillars, recognized by their bright green coloration, can quickly defoliate plants if left unchecked. Despite their size, they often blend seamlessly with plant leaves, making early detection challenging.
The Hornworm’s Transformation
The tomato hornworm (Manduca quinquemaculata) undergoes a complete metamorphosis, transitioning from a larva to a pupa, and finally into a large moth. Once the caterpillar reaches maturity, typically after three to four weeks of feeding, it burrows into the soil, usually four to six inches deep, to begin its pupal stage. This pupa is a hard, reddish-brown casing, approximately two inches long, with a distinct loop resembling a pitcher handle at one end.
The pupal stage can last about two weeks during the growing season, with moths emerging to begin a second generation. If pupation occurs in late summer or early fall, the pupae can remain in the soil through winter, emerging as adult moths the following spring. The adult form is the Five-spotted Hawk Moth, a large, heavy-bodied insect with a wingspan of four to five inches. These moths are typically mottled gray-brown with five pairs of yellow spots along their abdomen, and they are nocturnal, often mistaken for hummingbirds as they hover to feed on flower nectar.
Identifying Hornworms and Their Damage
Identifying tomato hornworms often begins with observing plant damage. These caterpillars, which can grow up to four inches long, are known for rapid defoliation, consuming entire leaves and occasionally chewing holes in the fruit. They often start feeding on the upper parts of plants, causing significant damage quickly.
Despite their size, hornworms are highly camouflaged due to their bright green bodies, which blend in with plant foliage. A key visual identifier for the tomato hornworm (Manduca quinquemaculata) is its eight white, V-shaped markings along each side of its body and a black horn at its rear. Another strong indicator of their presence is the discovery of large, dark green or black droppings, known as frass, on leaves below the feeding site or on the ground. Regular inspection of plants, particularly in the evening or early morning when hornworms are most active, can aid in their detection.
Managing Hornworm Infestations
Managing tomato hornworm infestations involves several approaches. One direct and effective method is manual removal, where gardeners hand-pick the caterpillars from plants. These hornworms can then be dropped into a container of soapy water to eliminate them.
Encouraging natural predators also contributes to management efforts. Braconid wasps (Cotesia congregata) are beneficial insects that parasitize hornworms by laying their eggs inside the caterpillars. The wasp larvae feed internally before emerging to spin distinctive white, rice-like cocoons on the hornworm’s exterior, killing the hornworm. Leaving these parasitized hornworms in the garden allows the wasps to complete their life cycle and continue to control hornworm populations.
Cultural practices are also beneficial, such as tilling the soil in the fall or spring to disrupt and destroy overwintering pupae. Crop rotation can further reduce infestations by preventing hornworms from establishing in the same location year after year. For more extensive infestations, organic pesticides containing Bacillus thuringiensis (Bt) var. kurstaki can be applied. This naturally occurring bacterium is effective when ingested by young caterpillars, paralyzing their digestive systems, and is considered safe for humans, animals, and most beneficial insects.