Alfalfa Leafcutter Bee

Megachile rotundata — the alfalfa leafcutter bee — is one of the most agriculturally valuable solitary bees in the world and the dominant managed pollinator of alfalfa seed production in North America (a $5+ billion industry). The species was introduced to North America from Eurasia in the 1930s and has since become the cornerstone of large-scale alfalfa seed pollination because alfalfa flowers have a 'tripping' mechanism (a spring-loaded staminal column) that strikes pollinators in the head — honey bees learn to avoid alfalfa, but leafcutter bees tolerate the trip and continue working. The species' best-known behavior is leaf cutting: females use their mandibles to slice perfectly round disc-shaped pieces from leaves of rose, lilac, redbud, ash, and other deciduous trees. The cuts are so geometrically precise (an essentially perfect circle, ~6 mm diameter) that early botanists blamed leaf-mining beetles. Females stack the discs into cigar-shaped brood cell capsules inside pre-existing cavities (hollow stems, beetle holes, or commercial nest blocks): one circular disc lines the bottom, multiple sub-discs form the side walls, the female fills the cell with pollen and nectar, lays an egg, and seals it with another disc. A single tube may contain 5-15 stacked cells. The leaf-cutting causes essentially no damage to the source plants. Leafcutter bees are solitary and gentle — males are stingless, females sting only when handled.