Megachile rotundata

The alfalfa leafcutting bee, Megachile rotundata F. (Megachildae) is a Eurasian solitary bee species that was inadvertently introduced to North America sometime before the 1940s. By the mid 1950s, M. rotundata had become established in the farming regions of western United States. With the discovery of M. rotundata’s pollination impact on alfalfa seed production, early efforts to increase its populations near alfalfa fields were undertaken a few years later. Currently, M. rotundata is the most intensely managed solitary bee species in the world and is surpassed only by the honey bee for its economic impact.

Females are gregarious cavity nesters constructing nests composed of leaf pieces in a linear series of cells in naturally occurring cavities or in artificial nesting boards. In most North American latitudes, M. rotundata emerge in late June and early July. Females provision each cell with nectar and pollen, lay a single egg and seal the cell before starting the construction of the next cell. The larvae will develop through five larval instars, spin a cocoon and enter a prepupal diapause and overwinter. A portion of the larvae laid in early spring will avert diapause and produce a second generation of bees. The second generation is problematic to farmers. Depending on the length of the growing season the larvae of the summer generation of females may enter diapause and overwinter. Therefore, in some years the second generation will result in an increased number of bees entering diapause and available for the next growing season. But if the growing season is too short, the larvae will not have sufficient time to complete development and will not be able to enter diapause. Besides influencing the total number of bees entering diapause each year, the second generation has been implicated as a major factor in the spread of chalk brood, the primary disease of M. rotundata. The development of a M. rotundata genome database is an important advancement for understanding basic physiology and disease management of this important pollinator.