Fall webworm, Hyphantria cunea, constructs silken tents in trees and shrubs, feeding on foliage within their tents. The tents may be eyesores in the home landscape, and large populations of fall webworm can cause significant defoliation. If the same tree is infested repeatedly, fall webworm may cause branch dieback.
Are you sure you have fall webworm, and not another caterpillar pest? You may confuse fall webworm with eastern tent caterpillars, or even gypsy moths. Unlike these other silk-spinning caterpillars, fall webworm appears in late summer or fall. Make sure you know the difference between these three caterpillar pests.
Adults lay eggs in masses on the leaves of host plants. When the larvae hatch, they immediately set to work. The caterpillars enclose the foliage in a silken tent, and feed communally within it. As the caterpillars grow, they simply expand the tent, which may eventually wrap an entire branch in silk.
In the north, where only one or two generations of fall webworm occur, you may not need to control them. Trees have already produced and stored the energy needed to survive the winter, and damage by fall webworm may be only aesthetic. In southern areas, fall webworm may emerge much earlier, and more generations will infest your trees. When multiple generations infest a tree, control may be required to maintain the tree's health.
Mechanical controls – Since the caterpillars remain within their silken tent, it's easy to remove infestations while the caterpillars are small. Prune out infested branches as soon as you notice a fall webworm tent. When the caterpillars reach their final instar, they leave the tent to feed; it will be too difficult to remove the infestation at that point.
Biological controls – The bacteria Bacillus thuringiensis is effective against fall webworm if applied correctly. Spray the leaves just outside the existing tent. When the caterpillars expand their tent, they will begin feeding on these treated leaves and die. Also, predators and parasites can be effective in keeping fall webworm under control. Allow birds, stink bugs, wasps, and other caterpillar predators to feed on the webworms.
Chemical controls - Some contact or systemic pesticides work on fall webworm. If you feel the infestation requires this drastic an intervention, contact a pest control specialist in your area.