White-marked tussock moth caterpillars should be hatching on oak, crab apple, walnut, pecan, hickory, and other trees throughout the state. These are probably the prettiest caterpillars in Illinois, with long tufts of hairs front and rear. They have a broad yellow stripe down the back, enclosing a narrower black stripe. Within the black stripe, are short tufts of white hair and red spots, and, along the sides, there are bright blue stripes. Mature caterpillars are about 1-1/2 inches long. They can cause considerable defoliation and will have a second generation later in the summer.
The first generation of mimosa webworm should also be appearing throughout the state. They feed on honey locust throughout the state and also on silk tree or mimosa in southern Illinois. Severely cold temperatures in the northern half of Illinois should have reduced their numbers, but this insect was numerous last summer in southern Illinois, so watch for it there. Very early damage makes leaflets glisten and shine in the light. Later, the damage turns brown. In the first generation, the caterpillars usually only web together and damage three to four leaflets, but the moths tend to lay their eggs back into the damage for the second generation. If there are many damaged areas in the tree, treatment is warranted to prevent the heavy damage likely to be caused by the second generation.
Fall webworm has two generations per year in the southern half of Illinois. Be on the lookout for the first generation, particularly in the southern quarter of Illinois. These tussock moth and webworm caterpillars can be controlled with Bacillus thuringiensis kurstaki as well as other labeled insecticides.