Intense monitoring of alfalfa leafcutting bees and their nests to better understand reproductive loss

Commercial populations of Megachile rotundata, the alfalfa leafcutting bee, are difficult to sustain in the United States for annual pollination of alfalfa for seed production. Most seed growers are thus compelled to purchase bees from Canada, where populations thrive and can increase. An intense observational and monitoring study was performed to determine the cause of reproductive loss in M. rotundata. The most frequent mortality is seen as failed bee cells called “pollen balls” where mass provisions remain because no larvae developed on them. Other causes of mortality or population loss are pathogens, parasites, and the emergence of a summer generation of bees. Frequent assessment of nest progression of individually-marked female bees revealed when cells failed, when cells were created that contained immatures that succumbed to a disease or parasites, or when cells were created that produced a summer-emerging bee. Cell destinies could be correlated with variables such as temperature and relative humidity, the age of the nesting bee, the sequence of the cell in relation to other cells or nests belonging to the same bee, and the availability of floral resources.