Fall Webworm (Hyphantria cunea) has two generations per season in Ohio. The "fall" in the webworm's common name is based on the appearance of second generation nests late in the season. The first generation began to appear in southern Ohio in late May (see, Springtime Fall Webworms, May 28, 2017) and second generation caterpillars are now on the scene. Localized fall webworm populations are high throughout the state with nests becoming more evident as they undergo late-season expansion.
Biotype Matters: Beware of the Red-Heads
Fall webworms come in two distinct forms, known as biotypes, which are named for the color of the caterpillar's head capsule: black-headed and red-headed. 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. Indeed, some entomologists are proposing that these biotypes should be considered different species or perhaps subspecies.
In general, the red-headed biotype was confined to the northeast and eastern parts of Ohio and black-headed caterpillars were found elsewhere in the state. This is the reverse of the distribution of fall webworm biotypes in the U.S. with the red-headed biotype dominating the southern states. Last year, I found the red-headed biotype in Miami Whitewater Forest (Great Parks of Hamilton County) in southwest Ohio – I also found them this past weekend – and BYGLive! Walk-About participants observed this biotype in Mt. Airy Forest Arboretum on Monday.
The two biotypes are more than an entomological curiosity. All fall webworm caterpillars feed as leaf skeletonizers on the leaves they envelope within their silk nest. As caterpillars grow in size, they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites.
Also, female moths of both biotypes often lay their eggs on or near the nests from which they developed, thus second generation caterpillars expand the nests once occupied by the first generation. However, nest size ultimately depends on the webworm biotype.
Black-headed fall webworm nests appear to include caterpillars from only a few overwintered egg masses. They tend to produce small, wispy nests that envelop only a dozen or so leaves. However, it is not uncommon 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 the leaves on entire branches. This biotype is the more damaging of the two.
Fall webworm caterpillars have been recorded on over 400 species of trees and shrubs; they are particularly damaging on fruit trees. If nests are few in number and easily accessible, the best control approach is to use your five-fingered IPM tool to physically remove and destroy the nests and caterpillars. The digital control approach is highly effective and thus far, no populations have become resistant.
Insecticide applications should be used sparingly since insecticides may 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 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 compliment of hungry predators including predacious stink bugs. These and other beneficial insects are very effective in reducing year‑to‑year populations of this defoliator.