Fall Webworms

Carol Morton
Adams County Master Gardener

Fall is fast approaching with cooler night temperatures and the constant chirping of crickets and humming of cicada. Pumpkins are turning orange and cornstalks with drooping and drying leaves are preparing for the coming October harvest and Halloween activities.

There are other signs of fall this year and they are the many tents of the fall webworm, Hyphantria cunea (Drury). The fall webworm is a native of North America and Mexico and attacks at least 88 species of trees, and shrubs, ornamentals and annual plants. It is distributed throughout most of the United States and Canada. These are not to be confused with the Gypsy Moth larvae that appear in May-June or the Fall orange-striped Oakworm.

Fall webworms weave their webs on the ends of the branches to feed on foliage and are protected by the silken tent. The young scrape and eat upper leaf surfaces, leaving brown husks behind. As they grow, they eat everything except the thickest veins. The tent becomes enlarged, as the webworms grow larger.

There are two races of a single species one blackheaded and the other, redheaded. They differ in their markings of both larvae and adults as well as in their food and biology. Depending upon the climate one to four generations of fall webworm occur per year; two generally occur in our area.

The Eastern tent caterpillar differs from the webworm in timing and appearance of its web. Eastern tent caterpillars hatch earlier than webworm as their favorite cherries and crabapples begin to leaf out and end their single-generation season as 1 -inch, egg-laying adults in June. The tent caterpillars build in the crotch of trees and usually go no further.

There are other characteristics that separate the webworm and tent caterpillars. Both have long, soft hairs flaring from their sides, but differ in color. Fall webworms are yellow-white to yellow-green, marked from head to end with a double row of black dots. Eastern tent caterpillars are black, with a central white strip on their backs and flanks lined with blue dots.

Another difference is in their eating habits. Webworms eat within their tent while the tent caterpillars eat in the open, returning to their tent for protection mainly at nights and on cool, wet days.

The webworms drop into the mulch or soil beneath the plant to pupate over winter. The adult white moth emerges in May to July depending on the climate and lay masses of white eggs usually on the underside of leaves. In a few days hatching occurs and the larvae begin to spin silken webs over the foliage, covering the area that has been devoured. Usually the larvae of the first generation are active in June and July and the second generation in August and September. The nests produced by the second generation are usually more numerous than the first.

The fall webworm tents are ugly but they seldom kill mature trees and plants. Most plants will tolerate leaf loss after midsummer and will come back the following year. There are several methods for control of the webworms. (1) A number of native insects and animals prey on webworms. The webs can be opened with a stick, rake or stream of water to allow the natural predators inside. (2) Remove tents by pruning the end of the branch and destroy. (3) Though seldom required, several insecticides can be used. 

Microbial Bt will attack small caterpillars or Diazinon, Sevin or Orthene for larger tents. Sprays will probably do more harm to the beneficial insects than to the webworms. Never set fire to the tents that are in the tree. This may damage the tree and start a fire under dry conditions.

The best control is to remove and destroy the tents as soon as you notice them. Inspection of your landscape throughout the summer for insects and diseases will provide an opportunity for early controls.

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