Hornworms are among the largest caterpillars found in Colorado, with some species reaching lengths of up to 7.5 cm (3 inches). Many hornworms have a spine or horn at the rear, while some species have markings that resemble eye spots instead of a horn. There are over 30 species of hornworms in Colorado, but only two species are considered pests of vegetable gardens in the state: the tomato hornworm (Manduca quinquemaculata) and tobacco hornworm (Manduca sexta). Tobacco hornworms are generally more common, but both species can occur in the same location. The caterpillars feed on leaves and fruit and can rapidly defoliate plants. Tomatoes are very susceptible to feeding injury, but hornworms can also infest peppers and potatoes.
The tomato hornworm is characterized by a dark green horn with black sides. They have white stripes along the sides of their body that form a “V” pattern. Most have bodies that are varying shades of green, and while darker forms do exist, they are less common. Tobacco hornworms are similar in appearance with two key differences: the horn has red sides, and the white lines on the body appear as diagonal dashes.
The adults of both species are relatively large moths and are often mistaken for hummingbirds as they hover over flowers. Tomato hornworm moths have brown and gray wings on the forewings, and smaller hindwings with a zigzag pattern. They can have a 13 cm (5 inch) wingspan and have a brown and white abdomen with five yellow spots lining each side. Tobacco hornworm moths have narrow gray, brown, and white wings with a wingspan of up to 10 cm (4 inches), and six yellow spots lining each side of the abdomen.
The eggs are spherical and can be white or light green with a diameter of 1.5 mm (1/20 inch). Pupae are dark brown have a protrusion that resembles a handle, which contains the developing mouthparts of the adult.
- Hornworms are caterpillars of large moths that typically have a pronounced horn at the rear.
- Two species of hornworm are of significance in vegetable gardens. They are the tomato hornworm (Manduca quinquemaculata) and tobacco hornworm (Manduca sexta).
- Vegetables susceptible to feeding injury include tomatoes, peppers, and potatoes.
- Removing caterpillars by hand is an effective but challenging management tactic due to their cryptic coloration. Physical removal should be done at dusk or dawn when caterpillars are more likely to be actively feeding. Other controls include applications of certain garden insecticides or Bacillus thuringiensis.
Caterpillar of tobacco hornworm. Note the red horn and the series of diagonal white lines running down the body, which is a distinguishing feature of this species. Of the two pestiferous hornworm species in Colorado, tobacco hornworm is more common in vegetable gardens. Image credit: R.J. Reynolds Tobacco Company, R.J. Reynolds Tobacco Company, Bugwood.org
Tomato hornworm feeding on garden tomato. Note the pointed horn at the rear and “V” shaped white lines running along the body, which is a distinguishing feature of this species. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Darker form of the tomato hornworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult of tobacco hornworm. Note the row of six yellow spots on each side of the abdomen. Image credit: Nancy Hinkle, University of Georgia, Bugwood.org
Adult tomato hornworm. Note the row of five yellow spots on each side of the abdomen. Image credit: Forest and Kim Starr, Starr Environmental, Bugwood.org
Pupa of Sphinx chersis, which is in the same family (Sphingidae) as the tomato hornworm and tobacco hornworm. Note the protrusion that resembles a handle, which is also observed on tomato hornworm and tobacco hornworm pupae. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Adults can fly relatively long distances and can migrate into Colorado from southern areas. Female moths lay eggs on the upper surface of leaves. After hatching, young caterpillars will feed on the plant for a month or longer. The caterpillars are voracious feeders and consume more plant tissue as they grow. Upon reaching maturity, caterpillars will leave the plant and pupate in the soil. There can be two generations during warm summers in southern areas, with a second generation of caterpillars present in July and August.
Caterpillars can be physically removed from plants by hand picking. They can be difficult to spot given their coloration, but they tend to feed near dusk and dawn on the exterior of plant structures.
Tomato hornworms can be managed with certain insecticides. Applications of Bacillus thuringiensis can also be effective in managing hornworm larvae.
Volesky, N. and M. Murray. 2019. Tomato hornworm and tobacco hornworm. Utah State university – Extension. Available https://extension.usu.edu/pests/research/tomato-tobacco-hornworms