Megachile rotundata, the alfalfa leafcutting bee, is a European bee that has been introduced to various regions around the world. As a solitary bee species, it does not build colonies or store honey, but is a very efficient pollinator of alfalfa, carrots, other vegetables, and some fruits. Because of this, farmers often use M. rotundata as a pollination aid by distributing M. rotundata prepupae around their crops. Each female constructs and provisions her own nest, which is built in old trees or log tunnels. Being a leafcutter bee, these nests are lined with cut leaves. These bees feed on pollen and nectar and display sexual dimorphism. This species has been known to bite and sting, but it poses no overall danger unless it is threatened or harmed, and its sting has been described as half as painful as a honey bee's.
Taxonomy and phylogeny
Megachile rotundata is a member of the subfamily Megachilinae, which includes more than 4,000 bee species; this currently makes this family the second-largest among all bee families. This subfamily is one of four other subfamilies of Megachilidae, the other three being the Fideliinae, Pararhophitinae, and Lithurginae. Phylogenetic studies suggest that this subfamily is monophyletic. More specifically, it belongs to the genus Megachile, which contains 52 subgenera and 1,478 species. The genus Megachile consists of bees that cut leaf pieces to line their nests.
Description and identification
Megachile rotundata is a European leaf-cutting bee placed in the subgenus Eutricharia, the "small leaf-cutting bees"; they are in length. They are partially bivoltine, meaning that under the right conditions they can produce two generations per year. These bees present a sexual dimorphism, in which the males are smaller than the females and differently marked. Megachile rotundata bees are a dark grey color. Females have white hairs all over their bodies, including on their scopae. In contrast, males have white and yellow spots on their abdomens.
Distribution and habitat
Megachile rotundata is currently found on all continents except Antarctica. In North America, the species was deliberately imported to assist in the pollination of food crops, but has now become feral and widespread. In New Zealand and Australia, M. rotundata was also introduced to assist in the pollination of alfalfa (known locally as lucerne), in 1971 in New Zealand, and 1987 in Australia.
Nest construction
Females construct tubular nests in a variety of sites, including rotting wood, flower stems, reeds, and soda straws. In the wild, females also create nests in small holes in the ground or in available cracks/crevices in trees or buildings. The nests are composed of a string of individual cells, as many as the space will allow. When managed for pollination, the females are induced to nest in paper cylinders similar to drinking straws or drilled blocks of wood.
Each cell is made from circular disks cut from plant leaves using the bee's mandibles, hence the name "leafcutter". Females use about 15 leaves per cell, called a concave bottom, overlapping the leaves to produce the thimble-shaped cell. While the bees do not store honey, females do collect pollen and nectar which they store in the cells of their nests. Specifically, females first regurgitate the nectar they have provisioned into the cell and then transfer the pollen that is attached to their scopa on top of the nectar. Each cell contains one pollen and nectar ball, and one egg with each cell containing a 2:1 nectar-to-pollen ratio. The completion of one cell in the nest requires between 15 and 20 provisioning trips. After the female lays her eggs, she seals the cell with circular leaf pieces.