Megachile
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many (>50) |
The genus Megachile is a cosmopolitan group of solitary bees, and is the only genus within the family Megachilidae which truly deserves to be called leafcutter bees, as certain species of only this genus build their nest cells from neatly cut pieces of petals or leaves, this behavior otherwise being unknown in the family (other genera, and other species within Megachile, chew leaves or petals into fragments, or use resins or other materials to build their nests). Nests are sometimes constructed within hollow twigs or other similarly constricted natural cavities, but often are in burrows in the ground. This is one of the largest known genera of bees, with well over 500 species in over 50 subgenera. North America has many native Megachile species, though the imported Alfalfa leafcutter bee (Megachile rotundata) is the only truly important species used for pollination.
Nests are typically composed of single long columns of cells, the cells being sequentially constructed from the deepest portion of the tunnel outwards. The female places a supply of food (pollen or a pollen/nectar mix) and an egg in each cell, then builds a cap and a wall that separates the completed cell from the next one. The larva hatches from the egg and consumes the food supply. After moulting a few times, it spins a cocoon and pupates, often after several months of hibernation as a prepupa. Then it emerges from the nest as an adult. Males, which are typically smaller and emerge in advance of females, die shortly after mating, but females survive for another few weeks, during which they build new nests. Numerous families of wasps and bees will parasitize Megachile nests, most notably including Gasteruptiidae, Leucospidae, Sapygidae, and various cleptoparasitic Megachilids, including the closely-related genus Coelioxys.
Various species in the genus, especially those in the subgenus Chalicodoma and related groups, do not use cut leaves to form the cells as in other species, but instead use fairly dry plant resin, which they carry in their mandibles. The subgenus Chalicodoma includes the world's largest bee, Megachile pluto, as well as one of the largest Megachilids in the United States, the recently-introduced Asian species, Megachile sculpturalis.
Megachile species have no lobe (arolia) between their claws and thus are unable to climb smooth (glass) walls.
 Leaves showing cuts by a Megachile species |
