Fall webworm continues to be numerous, particularly in the southern half of the state. In that region (where there are two generations of this insect), moths continue to fly, eggs continue to be laid, and new tents are being formed. Mating adult moths were found as recently as August 29 in Union County in southern Illinois. With these insects overwintering as pupae in soil and debris, additional damage will continue.
It is very possible that we may be seeing at least a partial third generation in southern Illinois as a result of the early spring with continued warm weather. A partial generation is a method by which a species will try to increase its numbers while not putting all of its eggs in the same basket. Entomologically, a partial generation occurs when some of the individuals enter an overwintering stage while others develop a new generation. In this case in southern Illinois, probably some of the pupating fall webworms are staying as pupae in the soil and will overwinter there; others are emerging as adult moths, mating, and laying eggs. These eggs will hatch into caterpillars that will be able to reach pupation before freezing weather or leaf drop if we have a long fall. If not, they will die, but those that are remaining as pupae will survive for next year.
Damage by these insects this late in the growing season will not be important to the health of trees. Leaves eaten now will soon be dropping anyway and it is late enough in the season that attacked trees are unlikely to produce new leaves to replace eaten ones. The silk tents may produce aesthetic damage that can be controlled through pruning or insecticide application.