Various caterpillars attack trees in late summer and early fall. These include fall webworm, yellownecked caterpillar, walnut caterpillar, sumac caterpillar, and whitemarked tussock moth. The damage caused by these caterpillars to attacked trees is relatively slight because the leaves have already produced most of the food for the tree that they will produce. The loss of leaves at this time of year is not very important to the tree. The main tree health concern about late-season defoliation is that the tree will break lateral buds and replace the lost leaves.
Although refoliation is likely to occur earlier in the season, by September that becomes much less of a possibility. Aesthetic damage even becomes less important at this time of year because the public, including clientele, tends to spend much more time indoors after Labor Day, resulting in reduced concern about the appearance of the landscape.
For all of these reasons, treatment is usually not needed for late-season caterpillars. If control is needed, Bacillus thuringiensis ‘kurstaki’ (Dipel, Thuricide), spinosad (Conserve), carbaryl (Sevin), and various pyrethroid insecticides are effective.