Fall Webworm (Hyphantria cunea) has two overlapping generations per season in Ohio. The "fall" in the webworm's common name is based on large second-generation nests normally appearing late in the season. However, we are receiving reports that where localized webworm populations are high, the caterpillars are already producing some truly spectacular nests sometimes enveloping entire trees.
Fall webworm caterpillars feed in groups beneath their protective webbing. They only eat leaves that are enveloped by their silk. Early instars feed as skeletonizers on the upper or lower leaf surfaces consuming everything except the opposite leaf epidermis and the leaf veins. Skeletonized leaves turn brown making their nests even more obvious. Later instars eat everything except the largest leaf veins.
As caterpillars grow in size, they expand their nest by casting silk over an ever-increasing number of leaves to accommodate their expanding appetites. Nests may also grow in size between the first and second generations.
First-generation female moths often lay their eggs on or near the nests from which they developed. The resulting second-generation caterpillars then expand the nests constructed by first-generation caterpillars.
However, the nests can be a mixed bag because caterpillar development is not synchronized. There may still be some late instar first generation caterpillars in nests that are being expanded by early instar second generation caterpillars.
This native moth has a very wide host range. The caterpillars have been recorded on over 400 species of trees and shrubs including fruit trees. However, black walnut (Juglans nigra) seems to be a favored host, particularly for the red-headed biotype.
The Importance of Biotypes
The ultimate nest size depends on the webworm biotype. Fall webworms come in two distinct forms, known as biotypes: black-head and red-headed. They are so-named because of the color of their head capsules.
Caterpillars of both biotypes are very hairy but differ in body coloration, nesting behavior, dates for spring adult emergence, and to some extent, host preferences. Indeed, some entomologists are proposing that these biotypes should be considered different species or subspecies.
The two biotypes are more than an entomological curiosity. Black-headed fall webworm nests appear to include caterpillars from only a few egg masses. They tend to produce small, compact nests that envelop only a dozen or so leaves. However, several of these small communal nests may be found on the same branch and it's common to see the nests sprinkled randomly throughout tree canopies when moth populations are high.
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 the leaves on entire branches or even entire trees. This biotype is the more damaging of the two.
Historically, the red-headed biotype was confined to the northeast and eastern parts of Ohio and black-headed caterpillars were found elsewhere in the state. However, this appears to be changing.
Since 2016, I have been finding red-headed caterpillars in southwest Ohio. In fact, this biotype is the dominant form this season in my part of the state. Curtis Young (OSU Extension, Van Wert County) is also reporting a rise in the red-headed biotype in the northwest part of the state.
We're interested in your own observations regarding the occurrence of red-headed versus black-headed in your part of the county. You can send me a message by clicking on my name at the top of this Alert to get my e-mail address.
The insecticide option is seldom justified for these native moth caterpillars. The injury to the overall health of established host trees is usually insignificant. Most of the damage is done by the second generation; however, their late-season defoliation occurs after trees have acquired enough carbohydrates to support next season's leaf expansion. The exceptions may be newly planted trees if they are heavily infested, and fruit trees which require a continual flow of carbohydrate to support fruit expansion.
The second reason to avoid using insecticides is their potential negative impact on bio‑allies that help keep population densities of this moth in check. Fall webworms are native to North America and there are over 50 species of parasitoids and 36 species of predators known to make a living on fall webworms. They are the primary reason year-to-year fall webworm populations can rise and fall dramatically.
If nests are few in number and easily accessible, you can take advantage of the aggregation behavior of the caterpillars to apply a highly effective digital Integrated Pest Management (IPM) tactic. Although this approach is not for the faint of heart, it will help to preserve the bio-allies. Thus far, no populations have become resistant to this IPM tactic.
The caterpillars tend to spend the day congregated within an inner-nest of spent leaves.
1. Use your hand (gloves are optional) to sweep the assembled caterpillars out of the nest.
2. You can then use your hand to squeeze the life out of the caterpillars or place the collection on a hard surface and do the "caterpillar stomp."
Photo Demonstration: Fall Webworm Digital IPM Tactic