The Purdue Plant and Pest Diagnostic Laboratory

Tomato Hornworm

Question: Do tomato hornworms turn into a pretty butterfly? I won't feel so bad squishing them if they don't.

Also, I had some moonflower plants (the poisonous kind that are related to the wild variety, which grow in pig/cow pastures). Anyway, I had some kind of green hornworm eat every leaf off every plant. How can a hornworm eat a poisonous plant and not die?

Answer: Tomato and tobacco hornworms are large caterpillars, up to four inches in length. The prominent "horn" on the rear of both gives them their name. They can quickly defoliate tomatoes, potatoes, eggplants and peppers. Occasionally, they may also feed on green fruit. You're likely to notice the damage before you see the culprit. Hornworms are often difficult to see because of their protective coloring and their tendency to feed on the interior of the plant where they are protected from the sun and predators.  

The tobacco hornworm larva is generally green with seven diagonal white lines on the sides and a curved red horn. The tomato hornworms have eight V-shaped marks on each side, and their horn is straighter and blue-black in color. The adult of the tobacco hornworm is the sphinx moth. The five-spotted hawk moth is the adult of the tomato hornworm. Both moths are stout-bodied, grayish-colored insects with a wing spread of 4 to 5 inches. They're not exactly pretty butterflies but are large and striking. A quick search on the Web proved there are many excited butterfly/moth fanciers that think the hawk moths are fascinating!

If you're interested in tomatoes and not in moths, you can control them with the following methods:

Handpicking: The large size of hornworms makes it easy to get hold of them. Once removed from the plant, they can be destroyed by squishing them, snipping them in half with shears or dropping them into a bucket of water.

Tilling: Turning up the soil after harvest will destroy any pupae that may be there.

Biological controls: Bacillus thuringiensis, or BT (e.g., Dipel, Thuricide), is also considered effective, especially on smaller larvae. Follow label directions.

Natural enemies, such as the parasitic wasp that lays its eggs on the hornworm's back, are common.  If you come across a hornworm with something that looks like fuzzy pieces of rice on it, leave them in the garden so the emerging wasps can parasitize other hornworms. The larvae feed on the hornworm.

The answer to your question about why hornworms can eat poisonous plants is just that those plants aren't toxic to them. Some birds, animals and insects can eat many plants that would be toxic to humans.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service