Wild Side: Colletes, the cellophane bees

From early spring to the winter, there’s a Colletes species flying.

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My current favorite bee genus? Why, thank you for asking: Colletes!

Colletes isn’t the most diverse bee genus, and its members are not the prettiest of bees. But this genus, which is fairly well represented on Martha’s Vineyard, exhibits a lot of fascinating biology, and poses identification challenges that seem to be at just the right level of difficulty for my current skills. I could study these bees all day!

There are roughly 100 Colletes species in North America, 10 of which occur on the Vineyard (so this genus represents about 5 percent of the Island’s bee species). One of those 10, Colletes nudus, is listed for Dukes County in the definitive state checklist. I’ve never seen this species, and was unable to track down any specific records. C. nudus has been well-documented on Tuckernuck Island, however, and any bee that occurs there likely occurs on the Vineyard as well.

The remaining nine species are firmly documented here, but none is truly common, and a few appear to be quite rare. So you can add the challenge of simply finding these bees to the list of reasons I find Colletes fascinating.

Sometimes known as “cellophane bees,” members of this genus line their underground nest tunnels with dried body secretions that resemble cellophane. The purpose may be to regulate humidity within the nest; perhaps the lining also discourages nest parasites or predators. More usefully for purposes of field ID, Colletes bees are medium-size, generally quite fuzzy bees with distinctive, heart-shaped faces. Their large eyes, viewed from the front, are closer together at the bottom than at the top.

A particular wing vein, straight in all other bees but curved or even S-shaped in Colletes, is a reliable mark often visible in photos or, occasionally, even in the field. Like many bees, Colletes has bands of hairs on the edges of its abdominal segments; in Colletes, the hair of these bands tends to be short, white, and dense, looking almost painted on to my eye — another useful field mark.

While recognizing Colletes as a genus gets fairly easy with practice, distinguishing the species is another matter entirely. While the overall appearance of our species is similar, clues including face shape, hair color, habitat association, and seasonality often help with ID. But even with a specimen under a dissecting scope, firmly identifying these bees can be challenging, depending on subtle and sometimes rather subjective criteria.

Members of this genus tend to have fairly short periods of adult activity, and each species has its typical window of operation. Taking the genus as a whole, though, activity by one Colletes species or another spans virtually the entire bee season on Martha’s Vineyard. C. inaequalis, perhaps our most common Colletes, is among the very first bees to be active here, with adults appearing around the start of April. C. validus, the locally scarce C. thoracicus, C. productus, and C. solidaginis, in that order, fill the time from early May into mid-August. And then C. simulans, C. compactus, C. speculiferus, and C. americanus close out the season, flying from late August until cold weather.

A few of our Colletes species, notably the early season inaequalis, are generalists, foraging on a wide range of flowers. But some degree of specialization is the rule for this genus. C. validus favors blueberry blossoms; C. productus associates very closely with maleberry (Lyonia); the late season Colletes species all prefer native composite plants, such as asters and goldenrod. These bees may have habitat preferences, as well: C. speculiferus, for example, is a bee of coastal dunes, rarely found far from the characteristic dune goldenrod species Solidago sempervirens.

Colletes bees are solitary nesters, with each female digging her own nest burrow in sandy soil. But females tend to nest gregariously, with anywhere from a couple to scores of nests near one another across a few hundred square feet. While I can’t prove it, I suspect that this gregariousness leads to some measure of cooperation among females; perhaps they sometimes provision each other’s nests, whether accidentally or as an evolved reproductive strategy.

Watching Colletes bees at their nests can be great fun. A pollen-laden female arrives and plunges into her nest burrow. A few minutes later, her face appears at the burrow’s entrance. Carefully, always alert for predators or potential nest parasites, she creeps all the way out, and then, with no warning whatsoever, she takes flight to go foraging again.

As do many other bees, female Colletes face a tension between nest defense and foraging. Survival of their young depends on adequate pollen supplies in each underground nest cell. But survival also depends on keeping parasitic bee species from entering the nest and laying eggs that will eventually outcompete the Colletes young. Both leaving home and staying to play defense, then, come with risks.

So far this year, I’ve seen many C. inaequalis on flowers, including dandelion and Japanese andromeda. With blueberry flowers just opening, C. validus will soon be on the wing. As the season unfolds, I’m hopeful I can improve my acquaintance with all of these fascinating bees.

 

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