Fall webworm

Photo of hyphantria nest

This caterpillar, Hyphantria cunea, constructs web nests in trees late in summer and, although the webs are unsightly, little if any damage is done to the trees. The large silk webs enclosing tips of branches are sure signs of fall webworms. These buff- colored caterpillars are covered with long white to yellowish tan hairs. It is common across southeastern Minnesota in most years and was particularly noticeable this season. This defoliator is native to North America and ranging from Canada to Mexico. The fall webworm is known to feed on more that 100 species of forest and shade trees. It is also one of the few American insect defoliators that have been introduced into Europe and Asia. There is even a report of it from New Zealand.

Depending upon location in the US, there can be from one to four generations per year. Commonly, there is only one generation here in Minnesota as the nests are only seen in the late season. Because of this, little real damage is done to the infested branches and trees. Shade trees can be heavily defoliated and the presence of the large, unsightly webs can make them aesthetically unattractive. Webworm nests can be pruned out of small to medium trees, insecticide sprays (Bt) can be used or you can opt to do nothing as there are many species of parasites and predators that are known to be active of the fall webworms.

Webworms are most often seen inside the light gray, silken webs in late summer. Webworms enclose leaves and small branches in their nests, unlike the eastern tent caterpillars that make a smaller nest in the crotch of branches and occur in the spring. Webworms remain inside the webbing and, if food runs out, new foliage is encased. This insect overwinters in the pupal stage in the leaf litter.