Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 530-A   May 18, 1974
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

In the spring of 1956 we noticed that trees in the Palos forest preserves, especially elm and hackberry, were infested with countless numbers of slender caterpillars, about three-quarters of an inch in length, feeding on their new leaves. By June most of the elms, including many magnificent specimens, were completely bare. Picnickers were pestered by these worms crawling on them in a peculiar looping fashion when, each suspended by a long silken thread, they dropped toward the ground. Those were moth larvae called Cankerworms or measuring worms, inchworms, and loopers.

During that summer the trees clothed themselves with a second growth of leaves. The same thing happened in 1957. Then the cankerworms virtually disappeared until 1971. In '72 and '73 there was a similar infestation along Salt Creek woods and several other forest preserves. Cankerworms stripped the foliage from elms, apple trees, lindens and some hawthorns. On the linden or basswood trees nothing remained of their large leaves but skeletons -- the midrib and the larger veins.

Cankerworms have been known as pests since colonial days and have periodically defoliated many kinds of forest, shade and fruit trees. Such outbreaks seem to occur in cycles but, after being present in enormous numbers for a few years, the insects disappear almost completely. Cold stormy weather immediately after the caterpillars have hatched may be one factor. Diseases may be another. Natural enemies such as birds, spiders and predatory insects destroy vast numbers of adults, eggs and larvae.

There are two common native species, the principal difference being in when the moths appear and lay their eggs. The moths, active mostly at night, are brownish gray and the males have a wingspread of about an inch. The females are wingless. The caterpillars or larvae may be pale green, gray, brown or nearly black, with whitish lines. The Fall Cankerworm occurs throughout eastern United States and southern Canada. as well as in several midwest and western states including California. The Spring Cankerworm seems to be more eastern in its distribution.

Caterpillars of the fall cankerworm have three pairs of true legs in front and three pairs of fleshy prolegs near the hind end. When fully grown they crawl or spin their way to the ground where they build cocoons in the soil and transform into pupae. The moths emerge in late autumn, mate, and the females crawl up trees where, on twigs and branches, each lays a compact layer of from 100 to 400 tiny eggs from which the larvae hatch in early spring. Caterpillars of the spring cankerworm have only two pairs of prolegs and merely excavate a cell in the soil where they pupate. The moths emerge very early in spring. The females climb trees and each lays about 50 eggs under or in crevices of bark.

Many kinds of birds -- especially the warbler, chickadees, thrushes and vireos -- feed on cankerworms. In the 1850's, English sparrows were foolishly imported from England "to destroy cankerworms. .

Banding tree trunks with sticky material which the female moths cannot cross is not effective unless properly done and maintained. Spraying trees with lead arsenate, soon after the eggs hatch, controls cankerworms but the most modern method is to apply a safe insecticide with spraying machines or from aircraft.

Twelve inchworms make a dozen but never a foot.

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