The fall webworm, Hyphantria cunea, is a moth known principally for its caterpillar, which builds impressive silk tents that sometimes enclose entire branches of hardwood trees.
The tents appear in late summer or fall. Defoliation by fall webworm caterpillars does not usually cause damage to the tree, since they feed in late summer or fall, just before leaf drop. While the nests are unsightly, the caterpillars are not believed to harm otherwise healthy trees.
Fall webworm caterpillars will feed on any one of over 100 tree and shrub species. Preferred host plants include hickory, pecan, walnut, elm, alder, willow, mulberry, oak, sweetgum, and poplar.
The moth is native to North America, ranging from
One generation per year emerges in the northern part of this moth’s range. In the southern part of its range, there are two or more generations annually.
The caterpillars are highly variable in color, ranging from a pale yellow, to dark grey, with yellow spots and long or short bristles. There are two cream stripes along the sides.
After the caterpillar enters a cocoon, the pupa stage overwinters in the bark and leaf litter at the base of the trees. It is dark brown and about 10 mm long. The thin brown cocoon is made of silk covered with bits of twigs and dry leaves.
The adult is mostly white in the north, but in the south, it may be marked with black or brown spots on the forewings. It is quite 'hairy', and the front legs have bright yellow or orange patches.
Several methods of control can be applied to webworms. Small nests can be pruned out of small to medium trees. Try to detect the nests when only several leaves are involved. These small nests can be easily crushed. Do not burn the nests in trees as this may do additional damage to the tree.
The bacterial insecticide, Bacillus thuringensis (Bt), is quite effective against fall webworms if it is applied when the larvae are small. Thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested. Many standard chemical insecticides are also effective when applied to the foliage surrounding the nest.