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Evidence of Tobacco and Tomato Hornworm on Tomatoes

We can see evidence of hornworms in our tomato fields during the growing season. Sometimes, the only clue is droppings on a few leaves of the plants.

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Updated:

November 10, 2023

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Figure 1. Tobacco hornworm. Notice the red horn and black margins on the white stripes. Photo: Tom Butzler, Penn State

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Figure 2. The dark brown/black droppings are large enough to be seen in plain sight. Photo Tom Butzler, Penn State

Figure 3. (Left) A closer inspection reveals the fecal matter in flower- or star-shaped segments. (Right) For detective work in the field or garden, if the pellets are green, they are fresh, and the hornworm recently deposited them. Blackened droppings mean they are somewhat older, and the hornworm could have traveled a bit away. Photos: Tom Butzler, Penn State

Figure 4. Hornworms have chewing mouthparts and feast on leaf blades, starting at the top of plants and moving down. Photo: Tom Butzler, Penn State

Figure 5. Hornworms will also feed on the fruit, flowers, and stems. You can see three tobacco hornworms in the picture. Notice how easily they blend in with the plant. Photo: Tom Butzler, Penn State

Tobacco and tomato hornworms in Pennsylvania can be a problem in tomatoes from July through September. They are usually a minor pest in field-grown tomatoes; however, scientists at Purdue University found that they can be a cause for concern in high tunnels.

Tobacco and tomato hornworms are two closely related species that look very similar to each other but are not hard to distinguish. Tobacco hornworms have red horns— the small protrusion on the final abdominal segment of the caterpillar (Figure 1), while tomato hornworms have dark blue horns. The markings on the hornworms also differ, with tobacco hornworms having black margins on white stripes and tomato hornworms having green margins on white stripes. North Carolina State University Extension has an excellent video showing these differences: Identifying Hornworms | NC State Extension (ncsu.edu).

Management is the same for both pests. After heavy feeding, the hornworms fall to the soil, burrowing a few inches beneath the surface to pupate. However, tilling the soil in the spring and fall can expose hornworm pupae to the elements, including wind, weather, and natural predators. This interruption in their life cycle is crucial because it prevents the emergence of hawk moths (the adult stage) in the spring, which typically lay their eggs on tomato leaves to initiate the hornworm life cycle once more. Having a crop rotation plan that includes crops that are not hosts of hornworms can help in management. As can managing solanaceous weeds (nightshades) that serve as hosts for hornworms. Handpicking hornworms from plants and destroying them can be effective in small areas; however, please leave any parasitized by braconid wasps.

Eggs and early instar stages are vulnerable to attack by lady beetles, minute pirate bugs, green lacewing larvae, and spined soldier bugs. Braconid wasps are also natural enemies of hornworms. Wasps lay their eggs in hornworm larvae. When these eggs hatch, larvae feed on the insides of the hornworms and then pupate in cocoons. White cocoons appear on the outside of the hornworms' bodies. Wasp cocoons are easily seen on parasitized hornworms. When the wasps emerge, hornworms will die, and adult wasps will seek out other hornworms to repeat the cycle.

Figure 6. The tobacco hornworm in the center of the photo is parasitized with braconid wasps. A braconid wasp pupa is inside each white cocoon. Photo: Tom Butzler, Penn State

When more than one hornworm larvae are found per 10 plants in affected areas of the field, chemical control may be warranted. Several materials are effective at killing hornworms. The key to success is to apply them when larvae are less than ¾ inch long.

The following biorational options are from Penn State's Vegetable IPM with an Emphasis on Biocontrol manual (available for purchase). The products in parenthesis are OMRI-approved, but if you are an organic farmer, check with your certifier to ensure they can still be used.