Fall webworm
Click for larger image Webbing on branch tips caused by the fall webworm (Lepidoptera)

The fall webworm, Hyphantria cunea, is a general feeder on nearly all trees except conifers. While this native North American insect attacks over 100 different tree species, it tends to prefer mulberry, walnut, hickory, elm, sweetgum, poplar, willow, oak, linden, ash, apple, and other fruit trees. The insect makes webs at branch tips and is harmful mainly to the beauty of the host. It is considered to be more of a nuisance than a threat to the health of the tree.

Symptoms and Diagnosis

A distinctive web of silk, containing many caterpillars, is constructed around leaves at branch ends. Each "nest" may contain hundreds of larvae that feed together for a while. By late summer, the unsightly nest may measure three feet across and, in addition to the growing larvae, contains excrement, dried leaf fragments, and cast skins. An unusual characteristic of fall webworm caterpillars is that if alarmed, all the caterpillars in a nest make jerking movements in unison. It is thought that this is a potential defensive mechanism to startle and deter predators.

Another tent-forming caterpillar is the eastern tent caterpillar, Malacosoma americanum. However, eastern tent caterpillars make webbed silk nests in a fork of a branch or tree trunk and leave the nest to feed. Fall webworm caterpillars have nests at branch tips and feed inside the webbing.

Life Cycle

In late spring or early summer, adults emerge from overwintering pupal cases and lay hair-covered masses of several hundred eggs on the underside of leaves. The larvae that emerge can be either yellowish green with a black head or tan with a red head. Both color forms have many long, gray hairs and pairs of wart-like black spots running down their backs. Once feeding begins, the larvae congregate in masses and produce the silky web that surrounds the entire colony. Larvae feed inside the web and expand it as they grow. Larvae stay in the colony until their last molt after which they may be found crawling anywhere on the host plant. The larvae crawl to a protected place to spin a flimsy cocoon and pupate. The adults emerging from pupation have two color forms: either all white or white with black spots. There are two generations a year.

Integrated Pest Management Strategies

1. Live with the problem and let nature take its course. Hosts are seldom seriously harmed because defoliation usually occurs later in summer rather than during a period of active growth and not enough terminal growth is consumed to affect tree growth. In addition, more than 75 natural enemies parasitize and prey on the fall webworm.

2. Prune out webs. Branches that have active webs ("nests") may be cut out and destroyed. Webs are always on branch ends and are easier to remove when they are small. Pole pruners are helpful for reaching into trees.

3. Apply insecticidal sprays. If chemical control is truly necessary, treatment is recommended when webs first appear. This is because the smaller caterpillars are more susceptible to insecticides and, secondly, the webbed nests are somewhat waterproof and can be difficult to penetrate with sprays. The microbial insecticide Bt (Bacillus thuringiensis) is available as Dipel or Thuricide and can be used on the small caterpillars. Other pesticides registered for use include acephate (Orthene), carbaryl (Sevin), pyrethrins and spinosad.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches. For an organic approach to Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt), pyrethrin and spinosad products.

More images:

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Webbing at end of branches on a sweet gum (Liquidambar styraciflua) caused by fall webworm (Lepidoptera)
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Webbing of the fall webworm (Lepidoptera) at end of branches on a sweet gum (Liquidambar styraciflua)
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Skeletonized leaves and the caterpillars of fall webworm (Lepidoptera)
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Fall webworm (Lepidoptera) on crabapple (Malus)