HISTORICAL OVERVIEW of CANADIAN and USA MANAGEMENT

It is believed that Megachile rotundata was accidentally introduced from the Mediterranean into the continental USA as cocoons from tunnels in ship wooden crating material in the 1940s. The species became feral in the mid western states and was first managed as a commercial pollinator by American alfalfa seed producers. They fashioned leafcutting bee nests by drilling holes in wooden blocks - this was the beginning of the closed cell system of propagation that dominated American leafcutting bee management for decades.

Hobbs introduced leafcutters from the USA to Canada in the 1960s and popularized the open cell system of management whereby fall harvested nests were taken apart and the harvested cells were refrigerated over the winter months. Harvested cells were then placed in incubation trays and released into fields shortly after the beginning of the spring alfalfa bloom. The essential difference between the American and Canadian management systems was that the American system did not harvest cells from fall harvested nests but rather released bees from hive incubated nests placed directly in alfalfa fields.

The USA closed cell system was susceptible to the development of disease. The main limiting disease was chalkbrood  (Ascosphaera aggregata ) which was rapidly spread from spores adhering to the surface exoskeleton of bees. Thus during spring emergence from unharvested 'closed' hives, bees chewed an exit to the surface of the hive. In the process, bees chewed through dead chalkbrood diseased cadavers and contaminated their body surface with chalkbrood spores (Vandenburg et al. 1980). Spores were thereby carried throughout the major seed producing areas of the USA. This process was exacerbated by the the tendency of commercial leafcutter prepupae to break diapause and emerge in the field prior to the autumn removal of hives - field emerged bees are referred to as second generation progeny. Second generation progeny became a reservoir for the feral population which, in turn, became a reservoir for contamination with Ascosphaera aggregata. Thus second generation spread of chalkbrood from wild and commercial leafcutting populations became an established feature throughout American alfalfa seed producing areas. American seed producers thus became dependent on the importation of Canadian leafcutter bees. Since commercial Canadian leafcutter bees exhibit a high degree of second generation under USA management conditions, the chalkbrood disease cycle continues to limit USA bee propagation. Repeated attempts to introduce Megachile rotundata into Argentina have been thwarted by a similar scenario.

Recognition of these features has led to a closure of the Canadian border to USA produced leafcutting cocoons and an estimated yearly sale of 300,000,000 to 500,000,000 prepupae to American producers.

Vandenburg, J.D., Fichter, B.L. and W. Stephen. 1980. Spore load of Ascosphaera species on emerging adults of the alfalfa leafcutting bee, Megachile rotundata. Applied and Environmental Microbiology. 39:650-655.