Hyphantria cunea, also referred to as fall webworm, is a native insect of North America. Fall webworm passes through four stages during its life cycle, with the larval and adult stages representing its most notable forms. The larvae appear from late summer to early fall, and feed on the leaves of various trees and shrubs. As they feed, the larvae cause minor to significant defoliation on host plants. Fall webworm is a relatively innocuous pest. Host plants generally recover from defoliation within a single growing season.
Distribution & Habitat
Fall webworm is indigenous to North America. It may be observed from southern Canada to northern Mexico. Outbreaks of fall webworm occur frequently in the United States. Fall webworm is one of the few insects to have been introduced to other continents from North America. Over the years, it has come to occupy most of Europe, ranging from France to the Caspian Sea. It has also penetrated into Central and East Asia, appearing in Turkmenistan, Uzbekistan, Krygyzstan, southeastern Kazakhstan, China, southern Mongolia, Korea, southeastern Russia, and Japan.
Fall webworm feeds on over 636 species of plants, including alder, ash, cherry, cottonwood, elm, maple, persimmon, sweetgum, walnut, willow, and birch. In the eastern United States, the insect’s preferred hosts are pecan, walnut, American elm, hickory, crabapple, and maple. In the western United States, fall webworm commonly infests alder, willow, cottonwood, and fruit trees.
Adult females lay flat egg masses on the underside of leaves. Egg masses are white, and covered with hairs. Each egg mass contains several hundred light yellow or green eggs, from which the larvae hatch. When the larvae first emerge, they are pale yellow, with distinct black dots on each abdominal segment. Once they have matured, the larvae vary in their coloring and markings. Much of their appearance is related to their geographical location. As such, fall webworm is often separated into two unique races.
Larvae from the northern race have black heads with pale yellow or green bodies, and long white hairs that arise from red or black tubercles. Northern larvae also feature a black stripe that runs along their abdominal segments. Larvae of the southern race are tan and yellow with red or orange heads, and brown hairs that arise from reddish-brown tubercles. Pupae are enveloped in a thin brown cocoon composed of silk and soil particles. Adult moths vary in coloration, ranging from pure white to white with black spots on the forewings. Adults also have soft brown hairs on their abdomen, and bright yellow or orange patches on their front legs.
Adult females produce one to two generations each year. Females deposit their egg masses on the underside of leaves. The larvae hatch a week later, appearing from mid June to early July. Upon hatching, the larvae navigate to the tips of branches, and commence feeding on nearby foliage. When young, the larvae only consume the leaf epidermis. As they develop, they begin feeding on the midribs and veins.
During this period, the larvae begin constructing a small silken web, which they gradually enlarge, enshrouding increasing amounts of foliage in a loosely spun tent of silk. The larvae feed within these webs, incorporating leaf fragments, cast skins, and droppings into its contents. If disturbed during their feeding, the larvae will twitch and wave their bodies synchronously in an effort to elude potential predators. The larvae feed together until their final instar, whereupon they feed independently. Feeding persists until mid-September, when the larvae reach maturity. Once mature, the larvae vacate the host plant, and descend to the ground to pupate. The larvae pupate in nearby cracks and crevices, or within the soil. During the pupal stage, they overwinter, concealed within a light silken cocoon. They emerge as adults the following spring.
Effects on Plants
The larvae envelop branches in large webs, and consume the foliage. When present in forested settings, they generally cause minor defoliation. Dense populations can cause extensive defoliation, rendering host plants more susceptible to disease pathogens, and additional infestations. The webs that the larvae construct are not injurious to plants, but their unsightly appearance can reduce the host plant’s ornamental value.
Prevention & Management
- Most plants generally recover from substantial defoliation, especially if it occurs late in the growing season.
- Fall webworm has several natural predators that help limit its populations. Birds, insect predators, and parasitoids prey on the eggs and larvae.
- Webbed branches may be pruned and destroyed. Pruning is not advised if it would interfere with the aesthetic shape of the plant.
- Larvae can be controlled using a registered insecticide. Applications should be performed in July, when larvae are small. Larvae become more resistant to insecticides as they mature.
- Avoid burning any webs or larvae. Fire can damage, or kill the host plant.
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