Megachile rotundata
Megachile rotundata (Alfalfa leafcutter bee)
Taxonomy
Megachile rotundata, commonly known as the "alfalfa leafcutting bee" is in the subgenus Eutricharaea within the genus Megachile. The genus Megachile is known by the common name “leafcutter bees” because many members cut out circular or oval pieces of leaves and use them to line their nests. There are some exeptions; some Megachile use tree resin and others use chewed vegetation and mud in addition to leaf pieces. Megachile rotundata was introduced to the United States from Europe for use in alfalfa pollination. It was a fairly common leafcutter bee in Bee Atlas blocks in southern Minnesota, and most nests we found were in USDA hardiness zone 4b.
Description
Megachile are primarily characterized by having large chewing mandibles and scopae (pollen-collecting hairs) on the underside of females' abdomens. Megachile rotundata, like many Megachile, is a mostly dark bee with light hair bands across the abdomens of both females and males. Males have unmodified forelegs and look similar to females with extra long antenna and no scopae. Bees range from 7-9 mm long. Megachile rotundata is a fairly small robust bee, half as long as a honey bee worker. They look similar to other Megachile species, but if you can get a good look at a pollen-less scopa from behind and underneath, you can see that below the longer light-colored scopal hairs, there is a second band of short, dense hairs along the edge of each abdominal segment, between the bands of long scopal hairs. Megachile rotundata are introduced to North America from Europe and are managed as an important pollinator of alfalfa.
Nest Structure
Megachile rotundata use leaf pieces to build their nests and nest plugs. You can frequently see overlapping leaf pieces in a plug and leaf pieces will often have visible veins. They also sometimes use petal pieces to build their nests.
Hole Sizes
M. rotundata most frequently nested in lower column 3 (3/16") holes in Bee Atlas blocks, but also sometimes used larger diameter tunnels. (see graph in pictures)
Voltinism
Reports from one Bee Atlas volunteer with an alternative nest trap suggest 2 generations per year are possible.
Megachile rotundata nest, showing petals used to line the cells. The mud/sand mix partitions were made by another insect, probably a wasp, that emerged before the bee started building its nest. (Photo courtesy of Thea Evans)
Wild strawberry (Fragaria vesca) leaf with holes cut by Megachile rotundata (Photo courtesy of Thea Evans)
Activity Period
M. rotundata is most active in the later summer. Most M. rotundata nests in the Bee Atlas project were completed in August. (see graph in pictures)
References
No information at this time.