Why be Concerned with The Fall Webworm in the Spring

web worm

The fall webworm is a misnomer. These caterpillars are called fall webworms as their nests are more evident in the late summer or fall. However, there are two generations of these caterpillars, one which starts in the late spring-early summer in smaller numbers and the other in the fall.

Adult moths lay their eggs on the undersurface of leaves in the late summer, early fall, the masses overwinter and then the caterpillars hatch about a week later and immediately spin a web over the foliage they are eating. As the caterpillar numbers increase so does the web. If the population is large, several branches or even an entire small tree can be encased in webbing. The caterpillars mature in about six weeks and drop to the ground to pupate and then emerge as moths.

Webworms only eat within their webs, while tent caterpillars eat in the open and return to their webs at night or on cold, wet days. They have an interesting distinctive jerking movement, moving in unison if their nest is disturbed. Fully-grown caterpillars are about one inch in length. Spring webworms are tan, with two creme-color stripes. Fall webworms are white, often flecked with dark gray.

Their lives as larvae are usually about six weeks, but long after they have left, the webs remain. If the web is white, it is new. If it is tan or brown, there are no larvae there. Webs can last into the winter before falling out of the tree during wet snow or a windstorm. Trees they prefer include sweetgum, crabapples, persimmons, black walnut, pecan, hickory, fruit trees, river birch, and occasionally even elms, maples, and willows.

There are differences between the Eastern Tent Caterpillar and the Fall Webworm:

  • The fall webworm is a moth in the family Erebidae known principally for its larval stage, which creates the characteristic webbed nests on the tree limbs of a wide variety of hardwoods in the late summer and fall. It is considered a pest but although unsightly, does not harm otherwise healthy trees.
  • Eastern tent caterpillars put their nests in the crotch of a tree, while fall webworm nests are found at the ends of branches.
  • Fall webworms do not leave the loosely woven webs that also enclose foliage, eastern tent caterpillars leave the nest daily to feed, spinning a thread behind them to help others find food.
  • While webworms are yellow-white, with two rows of black spots and hairy; tent caterpillars are black with a gold or white stripe down their back.

Native insects are normally effective as pest control, including parasitic flies, stink bugs, birds, and social wasps as some of the webworm predators. These webworm predators also help reduce the amount of damage to the trees with nests.

Preventing Webworms:

  • Spraying with a lime-sulfur insecticide is the better way of controlling webworms.
  • Prune the limbs where the nests are.
  • There is no need to burn the webs, as that often results in damaging the trees you are trying to save.

If the webs are too high up in the tree, consider calling Precision Tree Care for help. Because the fall webworm does its most significant damage during the hottest and driest part of the season when most trees have stopped growing anyway, their damage is primarily considered of only aesthetic concern. The nests are more unsightly than damaging unless the tree is young. According to fs.usda.gov, stressed or recently planted, smaller trees can be substantially weakened by intense webworm defoliation, potentially rendering them more vulnerable to opportunistic pathogens and/or insects.