First-generation nests of the deceptively named fall webworm (Hyphantria cunea) are now becoming evident in Ohio. This native moth has two generations per season in Ohio with the first-generation appearing once the overwintered eggs hatch.
The first-generation caterpillars immediately begin to construct silk nests. 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.
Fall webworm 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.
Biotypes and Nest Size
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.
However, nest size ultimately depends on the webworm biotype. Fall webworms have two distinct biotypes; black-headed and red-headed, which are named for the color of their head capsules. Caterpillars of both types are very hairy but differ in body coloration, nesting behavior, dates for spring adult emergence, and to some extent, host preferences.
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. This biotype is commonly found in the eastern part of Ohio. The black-headed biotype is common in the central and western parts of the state. However, I have found recurring pockets of the red-headed biotype in southwest Ohio since 2016.
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.
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.
Insecticide applications should be used sparingly to avoid killing 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, it is not unusual to find fall webworm nests surrounded by a complement of hungry predators including predacious stink bugs. These and other beneficial insects are very effective in reducing year‑to‑year populations of this defoliator.