Yesterday, I came across a small Fall Webworm (Hyphantria cunea, family Erebidae) nest on a Callery pear (Pyrus calleryana) in southwest Ohio. The webworms were the black-headed biotype, but more on that later.
I was thrilled to find the nest. Although fall webworm is a native species, the silk-nesting caterpillars have been virtual no-shows over much of Ohio for the past few years. Indeed, my Alert last year was titled, “Fall Webworms are Conspicuous by Their Absence.” You can read the Alert by clicking on this hotlink:
https://bygl.osu.edu/index.php/node/2241
Is fall webworm making a comeback? Of course, Aristotle’s cautionary assertion may apply on several fronts. Finding a single fall webworm nest does not mean more will soon appear. My brief springtime elation may be followed by a season of disappointment. On the other hand, finding anything that’s eating Callery pear is a source of happiness.
A Fall Webworm Primer
Fall webworm moth caterpillars feed on the leaves enveloped by their silk nest unless they run out of leaf-food. Early instar caterpillars feed as leaf skeletonizers with later instars consuming all leaf tissue except for the petioles and coarse veins.
As the caterpillars develop, they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites. However, the caterpillars may leave their nests in search of food if they defoliate their tree host before they complete their development.
Hairs on other caterpillars are sometimes used as defensive tools. However, the hairs on fall webworms are primarily used to help them remain suspended inside their silk nests. You can see this in the following picture. Note that the hairs fold back as the caterpillar appears to "swim" through the nest.
The common name "fall webworm" is based on when we typically see the largest nests. Female moths tend to lay their eggs on or near the nests from which they developed with webworm nests becoming larger with each generation. I’ve often wondered if fall webworm silk includes an oviposition stimulant as has been documented with mimosa webworm (Homadaula anisocentra). However, I’ve found no published research describing such an investigation.
We typically see two generations in Ohio: sometimes three. Thus, the nests reach their zenith in the fall (both astronomical and meteorological) which accounts for the common name.
Biotype Befuddlement
Two forms of the caterpillars are recognized based on the color of the caterpillars as well as the color of their head capsules and the small bumps from which the hairs arise, called “tubercles,” that run in longitudinal lines along the top of the caterpillar’s body. The two forms are variously referred to as “races,” “biotypes,” or “color-forms.”
The caterpillars referred to as the “red-headed biotype” have red to reddish-orange head capsules and tubercles that range from orangish-yellow to dark red. The caterpillars are most often tawny-colored.
The “black-headed biotype” has black head capsules and black tubercles. The caterpillars are pale yellow to yellowish-green; however, they sometimes appear black with starkly white hairs.
The two biotypes were once considered separate species with the black-headed biotype called H. textor and the redheads H. cunea. They are now considered the same species, H. cunea, with the biotypes representing variability within the species. Indeed, in 2016, I photographed a group of fall webworm caterpillars that showed features characteristic of both biotypes.
Ohio is home to both the red-headed and black-headed biotypes. Historically, northeastern Ohio was the dominion of red-headed fall webworms. The central and southwestern parts of the state were where we found black-headed webworms. Indeed, I never found red-headed webworms in Greater Cincinnati before 2016.
The distribution of two types of webworm caterpillars is significant because many of the differences between the biotypes that supported the “two species view” remain evident. Those differences include body coloration, the time of the season when overwintered eggs hatch, nesting behavior, and to some extent, host preferences.
In other words, it’s not only important to report what we’re observing with fall webworms, but it’s equally if not more important to note which biotype is being observed. For example, 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, if not just a single egg mass. Consequently, they tend to produce small, wispy nests that envelop only a dozen or so leaves, but it is common for several of these small 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. They can produce some truly spectacular multilayered nests enveloping whole branches or even entire small trees. So the red-headed biotype is the more damaging of the two owing to the caterpillar's ability to produce massive nests.
Finally, there is a strong association between the biotype and the time of the year when we first notice fall webworm nests. Overwintered eggs of the red-headed biotype tend to hatch much later than the eggs of the black-headed biotype. The lag time may be as long as 4 weeks. Thus, finding a nest of the black-headed biotype yesterday could mean the redheads will soon follow.
On the other hand, our fall webworm season may fizzle before it gets started. The caterpillars must survive a bevy of predators, parasitoids, and pathogens (the 3-Ps). The Two-Spotted Stink Bug (Perillus bioculatus, family Hemiptera) nymph pictured below was lurking within the webworm nest I found yesterday.
Will fall webworms once again be conspicuous by their absence this season? Failure to launch would confirm Aristotle’s cautionary assertion and trounce my brief moment of happiness.