Barrens buck moths

A unique Vineyard treasure.

An adult female buck moth tries to lay eggs on a scrub oak twig. - Matt Pelican

From the naturalist’s perspective, the Vineyard is an odd and endlessly surprising place. Between the young geological age of the Vineyard, the ocean barrier that surrounds us, and the human history of burning, cutting, and grazing the land, a highly distinctive mix of species has come to occupy the Island.

Some species that seem like they “ought” to occur here don’t. But more than making up for those are the Vineyard’s populations of regional rarities: species that are uncommon or absent on the southern New England mainland but can be found here, sometimes in abundance.

A prime example was on display in scrub oak habitat last Sunday, with its unseasonably mild air and extensive sun. Barrens buck moths, Hemileuca maia, were out in force — a large and strikingly patterned moth that, while fairly common on the Vineyard, is rare enough regionally to be listed as a Species of Special Concern under the Massachusetts Endangered Species Act.

Occupying much of the eastern United States, the buck moth can be downright abundant farther south, indeed sometimes so numerous that it can be a bit of a nuisance. But we are near the northern extremity of this moth’s range, where conditions (likely the harshness of our winters) can be marginal for this species. Moreover, there is good reason to believe that our buck moths, which associate strongly with sandplain habitat in general and low scrub oak in particular, may be distinct enough to merit at least subspecies status. So the Vineyard population of this moth is an important one in biological terms.

Hemileuca maia is a gorgeous insect, with black wings crossed by a bold white band. The upper legs are orange (this is often clearly visible on flying buck moths), and on males, the tip of the abdomen is orange as well. A white collar wraps around the black thorax. These are big moths, with a hefty female having a wingspan approaching three inches. Hemileuca is in the same family as the large silk moths, like the luna, imperial, Io, and Polyphemus moths — all impressive, attractive insects.

Unlike most moths, Hemileuca is active during the day, and fall is the season for finding adults, which fly strongly over their favored habitat in search of mates or suitable places to lay eggs. On Sunday, I saw more than 30, some darting along as if they just remembered an important appointment, others searching purposefully among the low branches of recently mowed scrub oak.

Of these, only one was kind enough to land, and it proved to be a female setting up to lay eggs. Clinging to a scrub oak twig about a foot off the ground, she laboriously bent her abdomen up until its tip touched the branch, and squeezed out a tiny, perfect white sphere! Unfortunately, the egg failed to stick to the branch, falling instead back onto the moth’s fuzzy abdomen. Perhaps it takes a few tries to produce appropriately sticky eggs?

When things work correctly, buck moths lay eggs in a distinctive pattern, with several dozen eggs packed tightly into a band wrapped around the twig. The eggs will stay there over the winter, hatching out into tiny, dark caterpillars in May or June, when the scrub oak is leafing out. Young caterpillars are highly gregarious, forming tight groups whether they are resting or feeding on scrub oak leaves. As they mature, the caterpillars grow steadily less social; mature caterpillars are solitary, though they may still tend to be concentrated in the same general area as their siblings, simply because they don’t disperse very far from their starting point.

The caterpillars are festooned with branched spines, and these spines mean business: if touched, they can pierce skin and inject an irritating chemical that stings like crazy. (The closely related Io moth, even more common on the Vineyard, also has caterpillars with stinging spines, and it’s not a bad idea to learn to recognize that species, too.) It is this trait, useful for the caterpillar of course but not so good from the human perspective, that can make buck moths problematic when they are abundant.

Still, our buck moth caterpillars stick close to their scrub oak food source; if you’re not in an area rich in that plant, you’re unlikely to encounter a buck moth caterpillar, and it’s not much of a burden to remember not to handle large, spiny caterpillars feeding on scrub oak! With just a bit of caution on the human end, problems with buck moths can be avoided. And to my mind, anyway, Islanders should be proud of our healthy population of this remarkable insect.