Thursday, June 9, 2016

Battling Tomato Hornworms

Doug from Mather writes in:
"As a first time tomato grower, I have two plants in pots (Patio & Bush Better Boy), and four in the ground (Roma, Sun Gold, Lemon Boy, & Black Krim). Something was eating the young tomatoes in the pots. Upon closer inspection I found three juicy, green caterpillars around the plants. I did some research and they seem to be tomato hornworm (Manduca quinquemaculata) - a common pest and best controlled by hand picking and dumping in soapy water, or allowing the beneficial wasps to do the job.
Any other suggestions controlling these tomato eaters?"

Doug, one option might be to stick that critter in the envelope that he’s resting on, and mail it to someone you don’t like. However, the envelope may require extra postage. 

This time of year, many backyard gardeners, in addition to Doug, are glaring at their partially eaten tomato plants, and muttering under their breath, "Just where do these blankety-blank tomato worms come from?"

Contrary to a popular urban legend, the larvae of the tomato hornworm do not lurk inside tomato seeds, a diabolical plot between seed growers and chemical manufacturers to increase profits. Nor are the worms drawn by the scent of your tomato plants from deep within your garden soil, emerging forth to wreak havoc.
The tomato and tobacco hornworm begin their life cycle as a small, singular, light green egg, about the size of a thick pinhead, laid in late spring and early summer on the underside of a tomato leaf.

That egg got there courtesy of a flying culprit, the sphinx moth. Both the tomato hornworm sphinx moth and the tobacco hornworm sphinx moth have similar features: about a four-inch-wide wingspan, gray body, brown wing streaks as well as yellow and white body markings.

The egg laid by the sphinx moth hatches within a week, and the emerging hornworm (technically, a caterpillar) begins eating. And eating. And growing. A full-grown hornworm, satiated by its tomato plant diet (supplemented with whatever else is handy, including potatoes, eggplants and peppers) can get up to four inches long.

If you miss catching the tomato hornworms, these critters will descend into the soil at the end of the season, wrapping themselves into a cocoon:

Disking or rototilling after harvest destroys their pupae in the soil and prevents the adult moths from developing and emerging from the soil the following spring. Again.

Hand snipping the tomato worms with scissors or pruners can be a satisfying evening chore. The trick, as seasoned gardeners know, is trying to find the hornworms in the first place. Tracing their black, pellet-shaped excrement from the ground back up the plant usually yields successful results. The best time to find them is in the cool of the morning or evening. Another popular tomato worm hangout: the tender, new growth at the top and sides of tomato plants.

If you prefer to douse tomato hornworms in chemicals, use one registered for use on this pest. Soaps and oils might slow them down but won’t kill them. What does work are stomach poisons that contain a bacterial insecticide, such Bacillus thuringiensis (Bt) or spinosad. They can be applied directly to the offending hornworms. However, this works best while the worms are still small. The bigger ones are more problematic, but there may be help already at work in your yard.

Encouraging birds to hang around your property is a good strategy. they enjoy these green treats. Dense, broadleaf evergreen shrubs are a favorite hangout of many birds. (More info to attract birds)

Besides birds, the tomato experts at UC Davis point out that there are a lot of garden good guys that can help you battle the hornworms. The UCD Integrated Pest Management website says: “Natural enemies normally keep tomato hornworm populations under control. Hornworm eggs are attacked by Trichogramma parasites (a small wasp); another small wasp, Hyposoter exiguae, attacks the larvae."
Hornworm parasitized by Trichogramma wasps. Let it be.

There are also several general predators to keep populations under control, including green lacewings, damsel bugs, assassin bugs, big-eyed bugs, minute pirate bugs, soldier beetles, ground beetles, and spiders.


  1. My chickens make very short work of these nasty little creatures. I consider the tomatoes that the girls eat an equitable trade :)

  2. My chickens make very short work of these nasty little creatures. I consider the tomatoes that the girls eat an equitable trade :)

  3. Astonishing, they really do camouflage so well with the leaves don't they?

  4. Box turtles think it's Thanksgiving when you give them a tomato hornworm. YUM delicious!

  5. I live in Chapin, SC and I've already pulled 3 decent sized hornworms off my tomato plants, and killed at least 3 smaller ones and it's not even July. Usually I don't start seeing them until August. Two of the larger specimen had wasp eggs attached to them, which I just learned about today. Never thought those with things they carried were wasp eggs. I saw some like that last year and had no idea what it was. The big one I catch too late. Literally ate a 3' plant down to about a foot. At first I thought a deer had gotten hold of it. Otherwise, been a good season. Not too hot, and plenty of rain. Most in q2 for over 3 years.