Garden Series: Hornworms

Garden Series- Hornworm

If you have grown tomatoes before, you have likely noticed stems that were stripped bare or even large stems clipped off over night. The culprit can be hard to find due to their ability to blend in with the plant, but if you search hard enough, you may find that your plant has fallen victim to a hornworm.

You’ve no doubt heard of tomato hornworms, however the hornworm most commonly seen in this area is actually the tobacco hornworm. Both are widely distributed throughout the United States, but the tobacco hornworm is more dominant in the southern states. However, the distributions of the two do overlap. Since we see them on tomatoes so often, tobacco hornworms have taken on the generic name of “tomato hornworm.” Tomato hornworms and tobacco hornworms are very large worms that commonly feed on tomato plants. They can also be found on other nightshades such as peppers and eggplants. Additionally, they are very similar in appearance, so much so that they are often thought to be the same species. Both are large, green caterpillars with white markings and have horns on the last abdominal segment, hence the name “hornworm”. The horn is harmless, contrary to myths that it can sting, puncture, or otherwise damage. The white markings and the color of the horn differentiates the two. The tobacco hornworm, Manduca sexta, has diagonal white stripes and a red horn, while the tomato hornworm, Manduca quinquemaculata, has sideways white V-shaped markings and a black colored horn. Both caterpillars later become large sphinx moths that are also very similar in appearance. The tobacco hornworm adult moth has six orange spots along the sides of its body (“sexta” means “six” in latin), while the tomato hornworm adult moth has five orange spots.

These caterpillars are capable of heavy damage on tomato plants. The good news is they are fairly easy to control, and have many natural enemies such as birds and wasps, some of which parasitize hornworms. Hornworms can be picked off by hand as you see them, or sprays can be used to control them. Products containing carbaryl, cyfluthrin, bifenthrin, permethrin, or spinosad are recommended by LSU for use on hornworms. Also recommended are products containing “Bt” (Bacillus thuringiensis), a bacterium found in the soil that destroys the stomach lining of worms, causing them to stop feeding and starve. I have used Bt for two years now and it works very well. The only drawback I have found is that it has to be reapplied often. However, Bt is very affordable and, depending on the formulation, can be used up to or on the day of harvest (consult the label for exact pre-harvest interval). It is important to note that Bt only works on worms. Other pests such as beetles or aphids will not be effected. . As with all pesticides, pay close attention to the label, especially the pre-harvest interval (the amount of time you must wait after spraying before harvesting). Also, keep pollinators in mind if you choose to spray. For summer crops, pay special attention to any instructions about temperature requirements.

For more information contact Randall Mallette, County Agent, at the local LSU AgCenter Extension Office 318-357-2224. You can also visit us on the web at www.lsuagcenter.com or at 624 Second St, Natchitoches.