Master Gardener ScoopAugust 17, 2016

By Will Summers,

Master Gardener

It’s midsummer and time to start looking out for those wispy, cloudy networks full of caterpillars. Infestation usually starts in early August, but it seems that it gets earlier each year. This year we found our first nest in the second week of July.

Fall webworms, Hyphantria cunea, encase the rapidly growing tip of the outer branches of trees about 5 to 20 feet above ground. The web is expanded as the population expands, and most of the foliage is consumed within the protection of the web or immediately adjacent. Webs are constructed of thick, sticky fibers. The fibers form a dense network that protects feeding webworm caterpillars within.

These “worms” are not really worms but caterpillars. They range in size from very small up to two inches long with dense bristles. They are light green to yellow with fine stripes and small darker black or brownish spots. The adult becomes a large, attractive, wooly, white moth about an inch long. Look for the adults around your porch lights after dark.

There may be two or three generations of webworms in summer. The female hatches from a thickly-woven cocoon in the soil in mid-June and begins laying several hundred yellowish-green eggs on the underside of leaves located on the outer branches. Caterpillars hatch in about seven days and immediately begin weaving the web nest as they skeletonize leaves. The larvae mature in about six weeks, and then fall to the ground and weave a thick brown cocoon in or on the soil. The web nest is visible as trees lose their leaves in the fall and larger nests may persist through the winter.

Fall webworms attack almost any species of deciduous tree but are most damaging on fruit and nut trees. They seem to avoid conifers. The attack is usually only cosmetic. Treatment is seldom needed beyond simply pruning affected branches. Webs are usually located at the ends of branches one or two feet in diameter. Fall webworms seldom leave the protection of the web. They expand the web as they feed which may eventually encapsulate the entire plant in rare situations.

There are several natural enemies for fall webworms including parasitic wasps, stink bugs and spiders. Unfortunately, the dense networks of sticky, finely woven webs protect caterpillars from parasites, predators and even rain. Often, I find wasps, biting flies and spiders trapped in the network.

Well-established webs may reach three feet in diameter. Insecticides have little effect on this insect due to its protective web. Sprays cannot penetrate deep enough into the web to have an effect. However, biological controls using Bacillus thurengensis, (variety: kurstaki) sold as Dipel or Turicide is an organic bacterial insecticide and has reduced their numbers.

I find the best method is to prune out any affected branches and securely dispose of the webbed stems in a closely-tied plastic bag. Also, simply just tearing the webbing out of the trees while collecting as many caterpillars as possible will severely damage the colony. Fall webworms are harmless, but don’t be concerned with the jerky movements. When disturbed, they seem to be choreographed to jerk and posture themselves in unison.

Fall webworms fall into a category of moths such as bagworms and tent caterpillars that build elaborate defenses. These insects are voracious feeders and frequently defoliate large portions of woody trees or shrubs. Most damage is usually only cosmetic – damaging only the appearance. If found early enough, start by hand removing when first observed. However, repeated defoliation by these insects can weaken affected plants exposing them to more fatal diseases or infestations.

Please take this opportunity to learn more about these unique creatures. Consider educating your children of the unique ability of these insects to care for themselves. For more information on any insect pests, please consult your local University of Illinois Master Gardener or your local Extension Office.

Please look for the Washington County Master Gardener’s Plant Swap as part of the Nashville Fall Festival on Saturday morning, Septermber 24th, outside the Nashville Public Library. Bring your favorite plant to trade and take home a new one.