Disease transmission in solitary bees—the case of chalkbrood in Megachile rotundata

Chalkbrood is a disease of bee larvae caused by fungi in the genus Ascosphaera. Entomopathogenic Ascosphaera infect through the larval gut after the spores are consumed. But how do larvae of solitary bees get exposed to the spores such that they might eat them? The larvae are all but immobile, occur deep inside the nests of solitary bees and are not exposed to sporulating cadavers from the previous year. Adult female Megachile rotundata, the alfalfa leafcutting bee, become contaminated with spores from dead, infected siblings when they emerge from the nest. This bee is used for pollination of alfalfa for seed crops, and farmers typically remove cocoons from the nesting boards before winter storage to reduce this contamination source. We show that high levels of adult contamination occur even when this is done. We document chalkbrood levels in the field during the course of the summer, including density of spores on nesting females, density in the pollen and nectar of food provisions, and actual disease prevalence. We reduced disease incidence by treating the extracted cocoons with a fungicide, but only when combined with the use of disinfected or new nesting boards. The use of disease-free nesting boards did not reduce disease prevalence when used as the only control measure. Thus, transmission of the spores appears to occur both in re-used nesting boards and via the mother bee, but the combination of these sources may be necessary before the larvae contract a sufficient dose for infection.