Several caterpillar species are becoming obvious in trees in Illinois. Fall webworm, with its large silk tents over tree foliage, is common. Brown foliage is being caused on honeylocust by mimosa webworm. Yellownecked caterpillar and walnut caterpillar are numerous on many tree species.
The loss of leaves at this time of year does not harm healthy trees. The leaves have already produced food for the plant, and it is late enough that the loss of leaves should not cause releafing. However, for aesthetic reasons, these caterpillars can be controlled with several labeled insecticides. Realize that large caterpillars may not be killed.