How to identify fall webworms

fall webworms

The tiny fall webworm, which is the Hyphantria cunea species from the Arctiidae family of tiger moths, makes its presence known from early summer through the first few weeks of fall. This amazing little creature, not more than an inch long, can turn most deciduous trees into a mass of intricate webs almost overnight. The webworm is actually the larva stage of a medium sized stout bodied moth that has a wingspan of close to two inches.

The moth, in most instances, is white with a sprinkling of dark or pale orange spots on its body. It can be observed from early spring through the later summer months depositing its eggs on the host plant preferred by its species. This may be fruit trees, nut trees, flowering trees, ornamental trees, maple trees, sweetgum trees, chokecherry trees, willow trees, cottonwood trees, elm trees, shrubs, or even some vines.

Amazingly, just as the pesky little tent caterpillars are completing the building of their nest, the fall webworm tent-like structures begin to appear. If this occurs seemingly overnight, this is because the fall webworm does most of its work during the evening hours. Even though the nest of the tent caterpillar and fall webworm is similar, the distinct difference is that the fall webworm builds at the tips of branches while the tent caterpillar prefers to make its home in the forks between branches.

Like the tent caterpillar, the fall webworm is considered a serious pest. From the moment they hatch these tiny worms begin spinning their silken webs to protect them from predators while they feed hungrily on the leaves of their host. Interestingly, the fall webworm feeds entirely on the body of each leaf leaving behind the veined areas. As the food within an area is quickly depleted, they extend their web to the next area of growth and continue to eat. Left unchecked, some webs have been observed covering the entire outer branches of one tree.

To observe the fall webworm, gently tear a small hole in the webbing. Unlike the tent caterpillars, fall webworms rarely leave their nest to feed preferring to build their nest around their food. But, occasionally on a very warm day, they can be observed moving on the outside of the web. The fall webworm has a blackhead area with a body that is sparsely covered with long hairs. In most cases, the body will be a pale greenish-yellow with a black stripe running the length of the body. The inside of the nest will be littered with a multitude of cast-off skins from the five molts each caterpillar experiences before it’s final molt takes it into adulthood.

Although the nest provides the webworm with quite a lot of protection from predators, they are much more vulnerable than tent caterpillars.
Some of the main predators of the fall webworm are birds, parasitic wasp, and hornets. Birds, such as the yellow warbler, are known to repeatedly return to the fall webworm nest once it is discovered until they have emptied it of prey.

Much like some birds, the hornet will discover the nest of the fall webworm and continue returning. In most cases, they will attack a caterpillar that is close to the surface of the nest, but some hornets enter the nest to attack their victim. Parasitic wasps enter the fall webworm nest, laying their eggs inside the larva. Once the eggs hatch they wasp larva feed on the caterpillar until it dies.

The last generation of fall webworms will crawl to the ground beneath their food tree, hibernating in a cocoon in the loose bark. The adult moths emerge during the last part of spring and the fertilized females deposit their eggs on the leaves of their species food source.

It is interesting to note that the web nest of the fall webworm tends to protect the caterpillars from most pesticides that are used to destroy them. In most cases it is suggest that each web be torn open before applying pesticides.

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