The second half of May is peak season for one of the Vineyard’s most interesting bees, Colletes thoracicus, sometimes called the red-backed cellophane bee. Not exactly rare on the Island, the species certainly isn’t common, either. And my own interactions with this bee have left me with more questions than answers.
Fairly large as bees go, C. thoracicus can reach about 15 millimeters in length in the case of large females. Males are somewhat smaller, much more slender, and as is the rule with bees, endowed with noticeably longer antennae than their female counterparts. Females, when fresh, sport distinctly reddish hair on the thorax, giving this species its common name. On males, the red coloration is generally much less pronounced, though still evident enough to distinguish male thoracicus from males of any of our eight other Colletes species.
While it has presumably occupied the Island for centuries or millennia, the red-backed cellophane bee was not documented here until 2011, when a number of specimens were collected at Wasque during a major study of Vineyard bees coordinated by Paul Goldstein. My own first local encounter of this bee came just last year, when I found smallish colonies in Correllus State Forest and at Long Point Wildlife Refuge.
This season has seen a modest flowering of additional locations for this species. My colleague at BiodiversityWorks, Silas Beers, recently photographed a female near Lucy Vincent Beach. Penny Uhlendorf and I have both found C. thoracicus near Lake Tashmoo, and in mid-May, I found a nice aggregation of nests of this species on the sandy hillside overlooking Sunset Lake in Oak Bluffs.
Perhaps this record reflects new colonies, established in the recent past following an unusually successful year for this bee. But this bee seems to breed reliably at the same sites over long periods of time, and my guess is that these recent discoveries simply reflect breeding populations that were previously overlooked.
In any case, the breeding biology of this bee is interesting. Like all Colletes species, thoracicus is considered a solitary bee, in that its females each build, provision, and lay eggs in their own nests. But unusually for a solitary bee, this and a few other Colletes species tend to build their nests gregariously, with dozens, hundreds, or even many thousands of nests packed close together where suitable conditions prevail. If you think you’ve found a single, isolated nest of Colletes thoracicus, think again; I guarantee that if you poke around, you’ll find other nests, probably quite a few of them.
Like other members of its genus, female red-backed cellophane bees have special glands that produce a sticky substance with which the bees line the inside of their nest burrows. The resulting dried film is thought to protect the nest from extremes of wetness, and it is this film that accounts for the “cellophane” part of the common names in the genus.
Most Colletes species have one or more associated nest parasites — specialized bees that don’t nest on their own, but rather lay their eggs in the nests of other species. C. thoracicus, oddly, appears to lack such an associate, or at least one has not yet been discovered. This does not mean, however, that the red-backed cellophane bee leads a wholly carefree existence. I consistently find flies that I think are in the genus Leucophora hanging around C. thoracicus nests, and this fly genus produces larvae that are also nest parasites, living in bee nests and feeding either on stored pollen, or on the bee larvae themselves.
It’s not surprising, then, that female C. thoracicus (and indeed those of other Colletes species) spend a great deal of their time plugging the entrances to their nest burrows. The nests themselves look like large, wide-bore ant nests, with a small volcano of fresh sand surrounding an entrance hole. In a high percentage of those holes, you’ll find the face of a female bee, looking for and ready to repel would-be intruders.
Nest defense, though, comes at the expense of foraging time; female bees need both to support themselves and to provision their nests. So in some mysterious, presumably instinctive manner, Colletes thoracicus females must constantly juggle the need to stay at home with the need to go shopping. When they do forage, I have very little idea what flowers they visit. C. thoracicus is considered a generalist that visits the flowers of many woody plant species. But I don’t recall ever seeing a female on a flower. And while I’ve often seen pollen-laden females returning to their nests, I’ve never been able to discern the source of that pollen.
Like other Colletes species, C. thoracicus has a relatively short window of adult activity, and by the time you read this, the season may be over for this bee. Nests will still be visible as patches of fresh, pale sand. But the entrance holes will have been clogged by wind- or rain-driven sand. Inside, bee larvae, hopefully free of Leucophora maggots, are cheerfully feeding on stored pollen of some kind, and awaiting their maturation and emergence next May.
I can relate to the mama bees, juggling time!
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