Toast and jam. Macaroni and cheese. Burgers and fries. Some pairs go together so well that either item seems woefully abandoned on its own.
October brings just such a pairing in the natural world: white-rayed aster flowers and the aster mining bee, Andrena asteris. Both are prominent on the Vineyard’s landscape, at least if you know how to look. And it’s downright surprising to find one without the other.
As far as the flowers go, two species are especially prevalent on Martha’s Vineyard: heath aster, Symphyotrichum ericoides; and frost aster, S. pilosum. These two common wildflowers are not especially closely related, assigned by botanists to different subgenera. But they resemble each other in appearance, have similar ecological preferences, and often occur in the same places, growing intermingled in suitable habitat.
Tending to be compact in shape, heath aster has smaller flowers, barely a half-inch across, including the rays. Healthy plants carry these in expansive, dense clusters, like white clouds. The leaves of this species are tiny and needle-like. Frost aster is a taller, lankier plant overall; its flowers, carried more sparsely, are larger, with longer rays and longer, softer, though still rather thin leaves. Examined closely, the stems on this plant bear fine, dense hairs (“pilosum” means “hairy”), and this feature is a handy way to distinguish the two species with certainty.
Both of these asters tend to occur in the type of areas that humans refer to as “waste places” — roadsides, abandoned pastures, edges of agricultural fields. “Waste” is a misnomer, though; these marginal areas can be hotbeds of biological activity.
Regularly attracting aster mining bees, robust populations of both asters now illuminate my front yard in early October. All these plants, as far as I know, originated from a single stunted example of each species that I found growing in our gravel driveway. As I began converting our pitifully unsuccessful lawn into a successful meadow, I simply sprinkled seeds from each plant onto bare soil, and the hardy nature of these asters did the rest.
The association of the aster mining bee with white-rayed asters is remarkably strong. When I began studying bees, I assumed that this species was uncommon at best on Martha’s Vineyard; the magisterial study of Vineyard bees coordinated by Paul Goldstein back in 2010–11 turned up only a single individual aster mining bee, and I felt like I was making a real discovery when I began to find this species with some regularity.
But that study relied heavily on passive trapping, using colorful bowls filled with a weak soap solution to lure in and capture bees for subsequent examination. The method works well for generalist bees, with the colored bowls resembling flowers closely enough to tempt investigation. But specialist bees such as Andrena asteris are not so easily fooled; they have a clear sense of their preferred pollen sources, probably detecting them both by sight and by scent, and simply ignore any other attractant. So most specialist bees were underrepresented in the Goldstein study.
I now regard the aster mining bee as one of the most common late-season solitary bee species on the Island. Dozens can be found at one time around rich populations of either of these white asters, and if you hang out and watch for a while, even isolated single plants will often attract a bee or two.
Conspicuously hairy and about a half-inch long, Andrena asteris is a distinctive species once you get the knack of looking for them. The hair on the thorax is copious and pale brown; the face on these bees is very flat, a feature that, surprisingly, can be quite evident even in the field. On females, the upper legs bear long, bushy hairs, modified to carry large quantities of pollen. If a bee has been foraging for a while, often spinning around on individual flowers to systematically extract all their pollen, those hairs carry massive clumps of yellow.
Perhaps the most impressive display of the white aster/aster mining bee partnership that I’ve found was at the parking loop at the Gay Head Cliffs in Aquinnah. White asters abound on a steep, low, south-facing bank on the northern side of the parking loop, just east of the shops; with full exposure to the sun, the bank can be toasty warm even on a chilly October day. And Andrena asteris can be found there by the dozen, often in the company of many members of less specialized species.
On the lawn inside the parking loop, these bees nest in large numbers, each female’s nest appearing as a pencil-thin hole in the ground surrounded by a tumulus of sand. It’s an ideal arrangement: perfect soil for nesting, right across a narrow driveway from a robust food source.
Once in a while, I find Andrena asteris on a blue-rayed aster, or even on goldenrod; all are in the same taxonomic family, and the physical structure of their pollen grains is similar. But these are clearly second choices for the aster mining bee; for all practical purposes, this common bee relies entirely on two white-rayed aster species, in return doing the bulk of the pollination those flowers require. A perfect pair.
