Ways to keep hornworms from horning in on tomato plants

Q For several years, tomato worms have appeared on my tomato plants. I pick them off when I discover them, but often they are only noticed after many leaves have been eaten. Are they coming from the potting soil or are the eggs in the garden soil? Is there some precaution I can take before they emerge?

- M.F., Kennebunk, Maine

A It sounds like hornworms, says Mike Hoffmann, associate professor of entomology at Cornell University in Ithaca, N.Y. The leaf-eating worm can decimate foliage but, he says, a tomato plant can tolerate the loss of 20 percent of its leaves.

Professor Hoffmann recommends a combination of picking the hornworms off and using a product containing Baccilus thuringiensis (Bt) - a mild bacteria that is effective against hornworms but does not harm the plant. Look for such products at a local nursery or home and garden center.

It is possible to foil the pest early on.

Hornworms are larvae of an adult moth, which is widespread across the United States, according to Gene Miyao, a University of California Extension farm adviser. He says that the moth lays its eggs on the leaves of the tomato plant. Mr. Miyao said the eggs are green and about the size of a pencil lead so they could be hard to detect. The larva will burrow in the ground and pupate during the winter and emerge during spring. Tilling the soil could, he says, help slow its progress.

Mesh draped over the plants can also prevent the moth from laying eggs on leaves, according to Mr. Miyao. He adds that applying BT to the plant a few times during the growing cycle should be sufficient to control hornworms.

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