Most of the leaf-chewing larvae of butterflies and moths can be grouped together and referred to as caterpillars. They are soft-bodied except for a hard head capsule and have three pairs of thoracic legs and two to four pairs of fleshy abdominal prolegs. Caterpillars come in various sizes, shapes and colors. They may chew holes in leaves or chew inward from the leaf edge. Some caterpillars are hairy, while others are smooth-bodied and mimic twigs. Most have a protective coloration that helps them blend into the background of twigs and leaves.
Several species of caterpillars can feed on shade trees and ornamentals in the north central states, but few commonly cause significant levels of defoliation. Most caterpillars occur chronically at non-damaging levels and are damaging only rarely. Problems one season often are not followed by problems the next year.
Because the great majority of feeding by a caterpillar is done during the latest larval stages, defoliation may appear to occur very suddenly. Often problems are not detected until feeding is nearly over and treatments are no longer of benefit. To detect potential problems early, watch for the small larvae and the early phases of leaf or needle feeding. In some situations, the pelleted excrement (frass) of the young caterpillars on the ground may tip you off to an infestation developing within a tree.
For most caterpillars in shade trees and ornamentals, Orthene and Sevin are standard insecticides for control. Also, Bacillus thuringiensis (Dipel, Thuricide), a microbial insecticide that is exceptionally safe to use, is effective against most caterpillars if you apply it so that the insects ingest it.
Apply insecticides against caterpillars that make protective structures (fall webworms, leafrollers, etc.) shortly after the insect eggs hatch. Treatments made later will generally fail to reach the insect.