Among the 204 bee species known from Martha’s Vineyard, there are many rare or specialized species that you’re unlikely to encounter unless you make a point of searching. But there are also many bees that are widespread and abundant on the Island. If you spend any time at all looking at flowers, you’ve likely encountered dozens of these, whether you know it or not.
People probably underestimate the diversity of our common bees because so many of them look about alike: small insects, somewhat less than a centimeter long, with black bodies and white stripes of hair along the edges of their abdominal segments. Scores of Vineyard species meet this general description; one of my favorites, and one of the few that can be readily identified in the field, is the ligated furrow bee, Halictus ligatus.
I’ll treat this bee as a single species, but biologists more or less agree that what we call Halictus ligatus is actually a complex of multiple similar but distinct species. More so than most bees, it could be confused, with H. ligatus features with strong white hair bands on the abdomen, especially in females. (These are probably the “ligations” of the specific and common names.) Structurally, females of this species have large, blocklike heads, noticeably wider than the bee’s thorax, and the combination of strong hair bands and head shape is often enough to identify this bee.
Hard to see in the field, but often visible in photographs, is an even more diagnostic feature: H. ligatus females have odd, thornlike protuberances pointing backward from the lower edges of each cheek. I have no idea what function, if any, this anatomical oddity serves, but it comes in mighty handy from an ID perspective.
Males of this species are easy to confuse with those of our other three Halictus species. All our Halictus males are slender bees with long antennae and extensive yellow on their legs, not looking much at all like the females of their species. But if you reach the point of calling a bee a male Halictus, odds are very good that it is H. ligatus, by far the most common member of the genus. Across Halictus, females seem to vastly outnumber males on the Vineyard, especially earlier in the season.
Part of the reason for the abundance of this important bee is surely its ecumenical taste in dining: H. ligatus forages on an extraordinarily wide range of flowers. Local favorites include many species with composite flowers, such as sunflowers, coreopsis, black-eyed Susan, and, later in the season, the Vineyard’s many species of native asters. But there isn’t much that I’d be surprised to find H. ligatus checking out. Abundance and generalist foraging habitats make H. ligatus a potent pollinator in both agricultural and wild settings.
Bees in general range from strongly social to completely solitary in their breeding habits. At one extreme, think of the highly social honeybee, with vast hives of sterile female workers governed by one or at most a handful of fertile queens. At the other extreme, females of many of our mining bees (the genus Andrena) build, maintain, and provision their nests in complete isolation from other females of their species. Between those extremes, one finds virtually infinite degrees of gregariousness and cooperation. Halictus ligatus falls fairly close to the honeybee end of the spectrum, exhibiting complex and surprisingly adaptable sociality.
A few fertilized females survive the winter in underground burrows, emerging in late spring to start new colonies. Often, burrows from the year before are renovated for use, so nest aggregations of Halictus ligatus tend to persist in the same places for many years. After laying in a stock of pollen from early-season flowers, these foundresses begin to lay eggs to produce the next generation. And here is where it gets interesting.
The number of potentially fertile females produced has been shown to vary with the availability of floral resources; likewise, the social structure of the colony responds to prevailing conditions. With ample pollen supplies, more members of the colony may produce eggs, adding to the diversity and productivity of the colony. But if times are hard, through drought or excessive rain, a combination of physical aggression and hormonal suppression reduces the number of egg-laying females to a minimum, and shunts more colony members into maintaining the nest or collecting food for the offspring of the few fully fertile queens. The colony, that is to say, can sacrifice diversity to increase the odds of producing at least some viable offspring.
This ability to adjust a colony’s social structure on the fly is one of the more elegant bits of bee biology that I’ve come across. Social flexibility and the knack for utilizing nearly any type of flower add up to enormous success for Halictus ligatus, and make it one of the Vineyard’s most widespread and ecologically important insects.