Chalkbrood co-infections of leafcutting bees (Megachile rotundata): Understanding the arms race between obligate, facultative, and non-pathogenic fungal species

Fungi in the genus Ascosphaera are the causative agents of chalkbrood, the most economically devastating fungal disease affecting both social and solitary bees. There are 26 known species of Ascosphaera, and although not all of them are pathogens of bees, all are associated with bees or bee nesting materials. A previous study of diseased bees in the northwest United States found that for chalkbrood infected alfalfa leafcutting bees (Megachile rotundata), more than one pathogenic Ascosphaera were found in the bee. We studied the competitive dynamics between some pathogenic Ascosphaera species, as well as competition with an associated saprophytic fungus thorough growth studies with various temperatures and culture media. Ascosphaera aggregata, an obligate pathogen had the best growth success on specialized media and at a narrow temperature range. Ascosphaera proliperda a facultative pathogen, grew well at all temperatures and media formulations. Chrysoporium farinicola, a closely related and ecologically associated saprophytic fungal species to the Ascosphaera had growth that was the most limited of the three fungi studied and had the strictest media requirement, only growing on media with low available water. The ability of A. proliperda to succeed at a wider range of temperatures and medias than either A. aggregata or C. farinicola indicates that pathogenesis via a facultative pathogen is controlled by a factor other than its ability to be temporally and spatially present in environments.