Everybody has received the basic message about native bees: they are ecologically vital, and populations of at least some species have declined markedly, often without an obvious explanation. These are good reasons for learning about these insects, which as a group turn out to be incredibly variable in almost every way.
For one thing, there are probably more bee species than the casual observer expects. A two-year study about a decade ago, coordinated by entomologist Paul Goldstein and with field work carried out largely by volunteers, documented more than 180 species of bees on Martha’s Vineyard. The actual diversity here is probably a bit higher than that since surveys invariably miss at least a few species.
Casual observers typically overlook a lot of this diversity, tending to lump related bees under one rubric. “Sweat bee,” for example, refers not to one type of bee but rather to a whole family (Halictidae) with at least 62 representatives on the Island. Similarly if less dramatically, the name “bumble bee” subsumes eight current Vineyard species plus two more with historical records here.
This is not even to consider other groups, less familiar to most of us, some with bizarre names like leaf-cutter bee (somewhere around 10 species in the genus Megachile) or mining bees (30-plus Vineyard species in the vast genus Andrena). For many bees, males and females look different enough that they could easily be mistaken for different species.
Each member of this panoply of bee species has its own specific story. One way they specialize is in seasonality. Some of our ground-nesting bees, for instance, are active as adults only very early in the season. In a few extreme cases, the window of adult activity essentially starts and ends within the month of April. At the other end of the season, some species can’t be found as adults until August or even September. When you look will dramatically influence what bees you’ll find.
Even within a species, seasonality is often evident. With bumble bees, for example, the rule is that adult females are present for much of the season, but that males appear only in a narrower window later in the year. And even the females show seasonal variation, with fertile queens present at the start and end of the season, and infertile workers present in the middle.
Bees vary widely in their nesting habits, with some species burrowing into the ground, others nesting in hollow stems or similar cavities, and a few, such as the notorious carpenter bee Xylocopa virginica, chewing their homes out of solid wood. These nesting requirements, of course, largely dictate where you’ll find particular bees; some ground-nesting bees, for example, are incredibly finicky about the soil type they need to nest.
Meanwhile, some bees don’t bother building nests at all, instead laying their eggs in the nests of other bee species (often but not always close relatives). Such “cuckoo bees,” astonishingly, constitute about one-quarter of the Island’s bee fauna, according to the 2010-2011 Goldstein study. These nest parasites, having no need to transport pollen, have evolved to lose most of their body hair; as a result, they can look quite un-beelike, and some may be mistaken for wasps.
While some bees are ecumenical about the flowers they will visit for nectar and pollen, a surprising percentage are specialists. Melissodes agilis, for example, a type of “long-horn bee” (named for the prodigious antennae possessed by males), shows a distinct fondness for foraging on sunflowers. Andrena hirticincta, a late-season, ground-nesting bee, is rarely found foraging on anything other than goldenrod. These preferences, of course, relate to seasonality. In the case of a goldenrod specialist, does the bee fly late in the season because that’s when its preferred flower blooms? Or did a late-season bee develop a taste for a flower that is plentiful during its flight period?
Finally, bee species differ in their degree of sociality. At one extreme, the non-native honey bee is highly social, forming huge colonies in which queens, workers, and males perform highly differentiated tasks for the hive. At the other extreme, many of our native bees are solitary, with a single female building her own nest and performing all the necessary construction, egg laying, and provisioning for young entirely by herself.
But the picture is not quite that clear. Among many bees considered solitary, there are gradations of social or quasi-social nesting behavior. While the rule may be independent females for a species, under some conditions, more than one female may use the same nest burrow, or they may dig separate burrows that connect underground. (Knowing this, it’s easy to see how the nest parasitism of cuckoo bees evolved.)
In evolutionary terms, you can think of bees as simply highly evolved wasps that have adopted a vegetarian lifestyle and developed hairy bodies to help carry pollen (except for cuckoo bees that have lost that hairiness). But this description, while accurate, is reductive: bees are astonishingly diverse and have evolved to occupy a huge range of ecological niches. Give these remarkable insects some attention as the season winds down.