Fall Webworm (Hyphantria cunea) has two overlapping generations per season in Ohio. The "fall" in the webworm's common name is based on the moth caterpillars becoming more apparent late in the season due to larger second-generation nests than those produced by the first generation.
However, “large” and “apparent” are understatements for describing the fall webworm outbreak in Yellow Springs, Greene County, OH. Thanks to some dramatic images taken by Rachel Hoverman and sent by Curtis Young (both with OSU Extension, Van Wert County), my wife and I took a Sunday drive yesterday to the village located just north of Xenia on State Route 68.
Fall webworms are taking center stage in the village best known for its namesake spring, Antioch College, excellent local eateries, and the Little Miami Scenic Trail. Black walnut (Juglans nigra) trees are the most common host for the outbreak with large mature trees completely wrapped in webbing and stripped of their foliage.
Every nest that I opened only contained the “red-headed biotype.” I believe this accounted for the spectacularly large nests, but more about that later.
Images of entire trees encased in fall webworm silk can be found on the Web; however, I’ve never observed this first-hand. Silk lines ran down main stems connecting one nest to the next which is rare for fall webworm. The webworms were behaving more like what we see with eastern tent caterpillars (Malacosoma americanum) during one of their outbreaks in the spring.
I’ve also never seen red-headed biotype caterpillars feeding on leaves outside of their protective silk fortifications. Mature trees shrouded in silk and wandering starving caterpillars searching for more foliage-food are on display in Yellow Springs and speak to the massive, localized webworm population.
It’s worth a trip to Yellow Springs. You can spend the day webworm sightseeing and end the day at a fine restaurant. Just mind the frass (insect excrement) if you sit outdoors.
Fall webworm caterpillars usually feed in groups beneath their protective webbing. 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.
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 be late instar first-generation caterpillars co-mingling with second-generation caterpillars as the nests are being expanded. On the other hand, I only observed late instars in every nest that I opened in Yellow Springs.
The moth caterpillars normally only eat leaves that are enveloped by their silk. However, it’s obvious in Yellow Springs that the caterpillars will dispense with maintaining a silk barrier between themselves and their numerous enemies if they are desperate for food. I observed this on a redbud growing within the dripline of a defoliated black walnut. The starving webworms produced sparse webbing as they moved onto the redbud.
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. On the other hand, as noted above, it was obvious in Yellow Springs that the caterpillars will readily expand their palate once they defoliate black walnut. Look closely at the image below and you'll see a defoliated black walnut behind the infested street trees.
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 although the head capsules of late instar red-headed webworms may darken before pupation.
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. Obviously, the red-headed biotype is dominating the Yellow Springs region.
Fall webworms are native to Ohio. They co-evolved with their native tree hosts. Under normal circumstances, the webworms do not cause significant harm to the overall health of their native hosts. The most significant defoliation occurs late the season after trees have produced and stored enough carbohydrates to support leaf development next spring.
Of course, this perspective applies to mature, established trees. It may be a different matter for young trees, newly planted trees, or fruit trees. A continuous supply of carbohydrates is important to fruit development and sizing.
Regardless, it’s too late to apply meaningful management options to curb the outbreak in Yellow Springs. However, help is most certainly on the way. There is a high probability that natural events will cause the dramatic populations to “crash.” Foxes and rabbits provide a good example.
As a native insect, fall webworms have certainly drawn the attention of the “3-Ps”: predators, parasitoids, and pathogens. There are over 50 species of parasitoids and 36 species of predators known to make a living on fall webworms. I posted an Alert in 2019 describing the complete destruction of a fall webworm nest by ground beetle larvae (family Carabidae). The 3-Ps are the primary reason year-to-year fall webworm populations can rise and fall dramatically.
Currently, Yellow Springs appears to be the primary focus for this webworm outbreak in southwest Ohio. We observed few nests as we drove into the village from the southwest. The number and size of the nests declined as we drove south on S.R. 68.
Whether or not this means the outbreak will progress outward next season from the Yellow Springs focal point remains to be seen. However, the current dramatic outbreak suggests that those living in the region should monitor their trees for the development of first-generation nests early in the season. Eliminating first-generation nests on small trees will help to reduce the development of the more expansive and damaging second-generation nests.