Overwintered fall webworm (Hyphantria cunea) eggs are hatching and first-generation nests are appearing in southwest Ohio. Look for these hairy caterpillars inside small silk nests enveloping just a few leaves. The nests will rapidly expand over the next few weeks to include more leaves and become more evident.
Fall webworm caterpillars only feed on the leaves enveloped by their silk nest. Early instar caterpillars feed primarily as leaf skeletonizers with later instars consuming all leaf tissue except for the petioles and coarse veins. As caterpillars grow in size, they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites.
The caterpillars may be found on a wide variety of woody ornamental trees and shrubs as well as fruit trees. Some online references list over 90 tree species as fall webworm hosts.
Fall webworm is a native moth that ranges throughout North America from southern Canada through Mexico and into Central America. It was accidently introduced into Europe and Asia where it became a serious pest of fruit trees in China.
The number of fall webworms generations depends on their geographical location. We typically see two generations in Ohio; sometimes three. Further north, there is only one generation. There may be as many as four generations in the southern U.S.
What's in a Name?
The common name "fall webworm" is based on when we typically see the largest nests. First-generation caterpillars immediately begin to construct silk nests as soon as they hatch from overwintered eggs.
The female moths that eventually arise from these nests tend to lay their eggs on or near the nests from which they developed. Thus, second-generation caterpillars expand the nests once occupied by first-generation caterpillars. The second-generation nests typically reach their maximum size in the fall (both astronomical and meteorological) which accounts for the common name.
Know Your Biotype
Given its wide geographical range, it's not surprising that there is variability in the appearance of both the adults and caterpillars. For example, caterpillars are divided into twobiotypeswhich are named for the color of their head capsules: the red-headed biotype and black-headed biotype.
Caterpillars of both biotypes are very hairy but differ in body coloration, dates for overwintered egg hatch, nesting behavior, and to some extent, host preferences. Hairs on caterpillars are sometimes used as defensive tools. However, fall webworms the hairs on fall webworms are primarily used to help them remain inside their silk nests. You can see this within the following picture. Note that the hairs fold back as the caterpillar appears to "swim" through the nest.
The overwintered eggs of the black-headed biotype tend to hatch earlier than the eggs of the red-headed webworms. In fact, I visited a local park this past Friday where both biotypes occur and only found nests being constructed by first-instar black-headed webworms.
Both biotypes produce communal nests occupied by caterpillars from multiple nearby egg masses. However, black-headed fall webworm nests appear to include caterpillars from only a few egg masses. They tend to produce small, wispy nests that envelop only a dozen or so leaves, but it is common for several of these small communal nests to be found on the same branch.
Red-headed fall caterpillars are far more cooperative; their communal nests may include caterpillars from a large number of egg masses. Thus, they can produce some truly spectacular multilayered nests enveloping whole branches or even entire small trees.
The red-headed biotype is the more damaging of the two owing to the caterpillar's ability to produce massive nests. Historically, this biotype was most commonly found in northern Ohio with the black-headed biotype dominating the middle and southern parts of the state. However, I've been finding recurring pockets of the red-headed biotype in southwest Ohio since 2016.
Another important difference between the two biotypes will be observed at the end of caterpillar development prior to pupation. Red-headed caterpillars remain in their silk nests throughout their development. They don't leave their nests until they are ready to pupate and even then, they don't crawl very far. They pupate inside thin cocoons in bark crevices or in the leaf litter beneath their tree. Final instar black-headed fall webworms often leave their nests to go on a wide-ranging crawl-about prior to pupation. They may be found in unusual places far from their nests.
Engineering the Fall of Webworms
Fall webworms typically cause little harm to the overall health of established healthy trees. However, newly planted trees may be at risk, particularly from the red-headed biotype, and heavy defoliation by both biotypes can affect fruit sizing on fruit trees.
However, insecticide applications are problematic. Most are stomach poisons and penetrating the dense silk nests to deposit the insecticide onto the enveloped leaves is a challenge. Of course, applications may also kill bio‑allies that help keep population densities in check. Fall webworms are native to North America and there are over 50 species of parasitoids and at least 36 species of predators known to make a living on fall webworms.
Indeed, this past Friday, I found two types of predators in a fall webworm nest: a ground beetle (family Carabidae) and a two-spotted stink bug (Perillus bioculatus, family Hemiptera). Last season, I came across a large fall webworm nest with no caterpillars, but it was full of fat ground beetle larvae!
Physically destroying first-generation nests of both biotypes will prevent or at least reduce the development of the larger, more destructive second-generation nests. If first-generation nests are few in number and easily accessible, the most effective control option is to apply digital management. Simply remove the silk nests and caterpillars by hand; gloves are optional. Thus far, no populations have become resistant to this handy pest management tactic.