Tree Facts: Fall webworm management

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The unsightly, light gray webs on the trees occurring in late summer and early fall are caused by the fall webworm. This pest is native to North America and is common from Canada into Mexico. It is one of the few American insect pests that have been introduced into Europe and Asia. It is common all across the Dakotas though it seems to have major outbreaks every few years.

Fall webworm larvae have been known to feed on over 85 species of trees in the United States. In the Dakotas chokecherry, poplar, cottonwood, willow, elm, fruit trees are preferred hosts but and other hardwoods are occasionally attacked.

This pest eats leaves and the nests usually only cover small portions of trees. Consequently, little real damage is done to most trees. However, the nests do look very unsightly and fall webworms can cause significant defoliation.

• Description and Life Cycle – The insects produce one generation per year. Adults come out in late June to mid-July. Females lay large egg masses on the undersides of leaves. The eggs hatch in about a week and the small mass of caterpillars web over single leaves and feed by skeletonizing. Caterpillars feed through late summer into early fall. The large silk webs enclosing tips of branches are filled with fall webworms. When food runs out the webworms expanded the webbing to encase more leaves. The larvae mature in about six weeks and are about one inch long, pure white with long white to yellowish tan hairs and have a few black spots. Soon after maturing they drop to the ground to pupate.

• Control Hints – Though the webs look bad, damage to most trees is insignificant. This pest tends to go through periodic population explosions. Severe outbreaks occur every four to seven years, last for two to three years and then natural control agents reduce their numbers.

• Mechanical Control – Removal of Nests – Small nests can be pruned out of small to medium trees. Look for nests early and try to catch them when only several leaves are involved. These small nests can be easily crushed. Do not burn or torch the nests in trees as this may do additional damage to the tree.

• Biological Control – Be patient and give the over 80 species of parasites and predators that are enemies of the fall webworm a chance to control them. Social wasps, birds, predatory stink bugs and parasitic flies and wasps are the most important. Do not use insecticide sprays until it is evident that the parasites and predators are not getting the job done.

• Chemical Control – Standard insecticide sprays with acephate, carbaryl, malathion or permethrin active ingredients can be effective against fall webworms. The nest can be blown out of the tree with a strong jet of insecticide mix but this usually wastes insecticide. Another way is to locate nests early and merely wet the nest and cover nearby foliage with insecticide. As the larvae walk on the nest surface or incorporate new foliage, they will ingest the poison. Systemic insecticides may be best for use where nests occur in tall trees which are difficult to spray with ground equipment. These trees can often be treated with translocated systemics that are applied to the ground for root uptake or injected.

My sources for this news release were the Ohio State University Extension and the North Dakota State Forest Service. If you would like more information about “Fall Webworm Management,” contact Bob Drown @605-244-5222 Extension 109 or by e-mail at robert.drown@sd.nacdnet.net.