Fall Webworms (Hyphantria cunea) have at least two generations in Ohio and overwintered eggs that produce the first generation are now hatching in the southwest part of the state. I took the above image yesterday of a first generation nest on dogwood with 1st instar "black-headed" caterpillars constructing their characteristic silk nest and feeding upon the leaves enveloped within.
It's a Generational Thing
This weekend marks the "unofficial" start to summer. However, summer does not officially arrive in Ohio until the Summer Solstice on June 21st at 12:24 am. So why are fall webworms that are appearing in the spring called "fall" webworms?
First generation nests are seldom as numerous or as large in size as those produced by the second generation; the first generation nests normally involve only a few leaves. However, first generation female moths often 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 which accounts for the common name.
Feeding Behavior; Black-Headed vs. Red-Headed; and Nest Size
Fall webworm caterpillars feed as leaf skeletonizers and they only feed on the parts of leaves enveloped by their silk. 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 named for the color of their head capsules: 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.
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 and 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, last season I found red-headed biotypes in a county park in southwest Ohio.
A Word From Management
The caterpillars of both bio-types may be found on a wide variety of woody ornamentals as well as fruit trees. If first generation 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. Destroying the first generation nests will prevent second generation nests. 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.