Overwintered fall webworm (Hyphantria cunea) eggs have hatched in southwest Ohio and first-generation nests are becoming noticeable. 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 far more evident.
Fall webworm caterpillars 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 accidentally introduced into Europe and Asia where it became a serious pest of fruit trees in China.
The number of fall webworms generations depends on 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 arise from first-generation nests tend to lay their eggs on or near the nests from which they developed. I’ve often wondered if fall webworm silk includes an oviposition stimulant as has been documented with mimosa webworm (Homadaula anisocentra)
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 two biotypes which 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 other caterpillars are sometimes used as defensive tools. However, the hairs on fall webworms are primarily used to help them remain suspended inside their silk nests. You can see this in the following picture. Note that the hairs fold back as the caterpillar appears to "swim" through the nest.
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 before 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 leaf litter beneath their tree. Final instar black-headed fall webworms often leave their nests to go on a wide-ranging crawl-about before pupation. They may be found in unusual places far from their nests.
Bringing About 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.
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, insecticides 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, I have commonly found two types of predators in fall webworm nests: an unidentified ground beetle (family Carabidae) and the two-spotted stink bug (Perillus bioculatus, family Hemiptera). In 2019, 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 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.