Technical Abstract: Alfalfa leafcutting bees, Megachile rotundata Fabricius, are excellent pollinators of alfalfa for seed production. Alfalfa leafcutting bee nests consist of a series of cells in a cavity. The female bee lays a single egg in each cell on a pollen-nectar provision. In alfalfa seed production systems, nests are constructed in manufactured bee boards placed in field domiciles. For this study, nests containing bee progeny were collected from commercial boards in late summer from several northwestern U.S. states and from Manitoba, Canada. X-radiography was used to identify “pollen balls” (i.e., cells in which the pollen-nectar provision remains but the egg or larva, if present, is not detectable on an x-radiograph). Most U.S. samples had higher proportions of pollen ball cells than Manitoba samples. Pollen ball cells were dissected to determine the moisture condition of the mass provision and true contents of the cell. Most pollen ball cells from Manitoba samples were overgrown with fungal hyphae. The frequency of fungus-filled cells in individual samples was positively correlated with cool, wet weather. In the U.S., most pollen ball cells had moist provisions, but many of them lacked young brood. Correlation analysis revealed that pollen ball cells occurred in greater proportions in fields with more hot days (above 27°C). Broodless pollen ball cells occurred in greater proportions under cool conditions, but pollen ball cells with dead eggs or small larvae occurred in greater proportions under hot conditions.