At this time of year, several species of caterpillars become obvious, feeding on trees. Leaf damage at this time has little effect on tree health; tree leaves are most productive during the spring. By midsummer, the loss of many leaves does not seriously reduce food production for a tree. However, heavy defoliation well before fall may cause trees to sprout new leaves, which causes a loss of stored energy. It is unusual for defoliation by these caterpillars to be heavy enough to cause many new leaves to be produced.
Even though their threat to tree health is small, caterpillar-feeding damage is obvious and important aesthetically. Control is relatively simple, if deemed necessary. Many of these caterpillars feed in groups, where removal by hand or pruning is effective. They are all controlled with Bacillus thuringiensis kurstaki, sold as Dipel, Thuricide, and other brand names, as well as many other insecticides.
Fall webworm is probably the most obvious late-summer caterpillar. This gregarious caterpillar spins large, obvious silk tents over the ends of branches and feeds on the contained leaves. As these leaves are eaten, the webbing is expanded to cover more leaves. The yellow, hairy caterpillars have two races, a black-headed race with rows of black spots and a red-headed race without black spots. Many common trees are hosts- including walnut, maple, hickory, crabapple, and oak. An earlier generation occurs in the spring in the southern half of Illinois, but all of Illinois has this insect at this time of year. Eggs hatch over several weeks, so both older and young caterpillar groups may be present. The silk tent is waterproof and sprayproof, so high-pressure spray is needed to penetrate the tent.
Mimosa webworm has two generations per year, but the second generation becomes very obvious on honey locust and silk tree at this time of year. Multiple caterpillars live together within two to three compound leaves webbed together. As the caterpillars feed, these webbed leaves become brown, making the damage very obvious. The caterpillars themselves are slender, up to one inch long, and brown to greenish. Spray applications require high pressure to penetrate the webbed leaves.
Whitemarked tussock moth commonly attacks oak, crabapple, maple, and many other trees. The caterpillar is one of the most attractive insects in Illinois, with long hair tufts front and rear, and short tufts running down the back. The bright red head is complimented with red spots, along with black and yellow stripes down the back. They feed individually and are usually found that way. However, large numbers that strip branches of their leaves can occur.