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Controlling and Identifying Hornworms

 Judy McFarland, Solano County Master Gardener, wrote an article in the Summer 2010 issue of Seeds for Thought, about how to identify and control pesky hornworms.

The tomato hornworm is closely related to the tobacco hornworm and both attack tomato plants. It is the larval (caterpillar) stage of the hornworm life cycle that does the damage. They are characterized by a large horn on the posterior end of the body. A tomato hornworm has seven white stripes, while the tobacco hornworm has eight white V-shaped marks, on their sides. These larvae reach about four inches in length.

Tobacco Hornworm Egg
 

The adult form is a large moth that lays her eggs on the tomato leaves (one egg per leaf), like the photo above. The moth generally flies after dusk and is therefore rarely seen.

Hornworm droppings

As soon as you see defoliation on your tomato plant, check the leaves and ground for small black droppings, similar to those in the picture above. If you find these you have active hornworms! To get rid of them cut them with garden shears or step on them, or have a little fun and toss them on your roof for the birds to eat! Beware that if you get rid of hornworms once, they will most likely come back again as they generally live two life cycles in one tomato growing season.

If you are unable to get rid of them by hand, you can try to gain control with Bacillus thuringiensis (or Bt for short), which is a naturally occurring bacteria that causes disease in only the larva of moths and butterflies. That does mean if you use Bt, you will also put whatever butterflies come to your garden at risk.

Hornworm pupa

After tomato season, it is very helpful to dig up the soil around the plants to locate and dispose of any hornworm pupae which migrate underground only to reemerge the following summer as moths and start the cycle over again. The pupae are hard shelled, brown, and shiny and can be 2.5″ in length. They also have a curved appendage at one end that resembles that handle of a pitcher.

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