Inchworms are not actually worms, but moth larvae--better known as caterpillars. Inchworms are indeed pests in the garden because they eat plant leaves, especially vegetable plants. They also infest trees and eat their leaves, sometimes causing serious and irreversible damage. Many types of caterpillars are referred to as inchworms, including the cankerworm. Inchworms typically appear in either the spring or fall.
Encourage wasps and birds in your garden because they prey upon inchworms. Grow plants such as Queen Anne's lace, sunflowers and parsley to attract wasps. Install a bird feeder in your garden or hang birdhouses in nearby trees or on poles.
Wrap a sticky band around your nearby trees. Moths climb up trees to mate and lay eggs--which will hatch into inchworms--so a sticky band will catch and kill them on their way. These bands are available at most garden centers. These are best applied in either early fall or early spring, about a month or two before you typically see inchworms in the garden.
Apply a pesticide that contains the active bacterial ingredient Bacillus thuringiensis (Bt), var. kurstaki to your garden. Other varieties of Bt do not control caterpillars. Follow the label instructions for proper application and frequency. It is best to use Bt soon after the inchworms hatch, usually in the spring or fall. Inchworms are small enough, however, that Bt will be effective even as they age.