With more than 180 species in nearly 30 genera, the bee fauna of Martha’s Vineyard presents an amazing diversity of appearance, life history, and ecology. It also presents the observer with a wide range of difficulty: Some groups of bees are hard to find, fiendishly difficult to identify, or both, while other groups are common and have a distinctive structure that makes them fairly easy to recognize.
The so-called leafcutter bees, in the genus Megachile, rank among the latter. We have about 10 representatives of this genus here, and to one degree or another they share the same body structure: medium-sized for the most part, with a stout body and a broad, flat abdomen that is often held with the tip raised slightly above the horizontal. Seen under a microscope, most Megachile species also share characteristic broad, heavily toothed mandibles. I still get fooled now and again, but by and large, this is one of our easiest bee genera to recognize.
Those mandibles are integral to the life histories of these bees and also closely linked to both the common and scientific names of the genus. “Megachile” is taken from Classical Greek and means something like “large lipped.” And these bees are called leafcutters because the females of most species, employing their robust mandibles, snip out sections of leaves for use in their nests.
The damage done to leaves is distinctive: circular holes perhaps a half-inch in diameter, cut from the outer edges of a broad leaf. The excised disks are brought back to the female bee’s nest, which may be a hole in wood or in the ground and is often a re-used hole rather than one excavated by the bee. Within her nest tunnel, each female bee leaves a series of eggs, each accompanied by a wad of pollen to support the larva when it hatches. And the eggs are in a series of cells, starting at the bottom of the burrow and working outward, each cell separated from adjacent ones by leaf disks.
The reason for this laborious process is not entirely clear, though it has been suggested that by including pieces of green leaf in the nest, the female bee helps ensure that the humidity within the nest stays high enough to keep the eggs and larvae from drying out. But in any case, this nest design is fundamental to the genus, with the big mandibles clearly a trait that evolved specifically for leaf-snipping.
Nest structure is not the only unique feature of Megachile. Many other types of bees transport pollen back to their nests on specialized leg hairs, augmented in some genera by similar hairs on the thorax. But in Megachile, the rule is to carry pollen on hairs on the underside of the abdomen. These hairs are often distinctively colored: white in many cases, but reddish or golden in others. Even when not brightly colored, these belly hairs often take on the color of their pollen load, which is often yellow. So a flash of color on the underside of the abdomen is often a good indicator that you’re looking at a female Megachile.
Males, having no need to provision nests, lack those specialized hairs, though they’re often quite fuzzy overall. In many species of Megachile, males have enlarged front legs which may bear a sort of mop of elongated hairs, along with less visible scent glands. As part of the mating process, males, according to an information page on Bugguide.net, “partially cover the female’s eyes with the hairy legs and the odor glands are placed close to the female’s antennae.”
If the lifestyle of a Megachile sounds odd, you should consider that there is even a closely related genus that has evolved to parasitize leafcutter bee nests! Members of the genus Coelioxys (about four species occur on the Vineyard) resemble their cousins in Megachile but lack much of the body hair (since they have no need to carry their own pollen) and have distinctive pointed tips to their abdomens. The sharp abdomen of a “cuckoo leaf-cutter bee” is in fact a tool: Female Coelioxys use the point to break into Megachile nests, laying their own eggs, which will then develop on the pollen the female Megachile stored for her offspring.
While most of our leafcutter and cuckoo leafcutter bees are native species, we have one non-native Megachile, and it is a mighty impressive insect. Originally an Asian species, the sculptured resin bee, Megachile sculpturalis, was first detected in the United States in the 1990s. From an initial foothold in North Carolina, the species has expanded into most of the Eastern half of the continent. Among our largest bees at about an inch in length, they vary the usual Megachile behavior by using sap or resin, rather than leaf disks, to construct their nests.
Leafcutter bees are quite common across the Island and reach their peak diversity and numbers in August. So now is the time to look for these unusual bees visiting flowers, or for the distinctive round holes the females cut in leaves.