Species of chalkbrood (Ascosphaera) competing within the honey bee (Apis mellifera) host

Competition within the host is an important factor in host–pathogen relationships, yet most studies consider interactions involving only single pathogen species. In this study we investigated the possibility of within-host competition of several pathogens using honey bee larvae Apis mellifera L. as a host. Larvae were in vitro reared and inoculated with three species of Ascosphaera. One pathogen, Ascosphaera apis (Ascomycota: Ascosphaeriaceae) is a causative agent of chalkbrood disease in honey bees. The two other species used are causative agents of chalkbrood in wild bees and rarely found in honey bees. Mortality of the larvae was used as a measure of virulence of each pathogen. We show that A. apis alone is less virulent than A. apis in combination with one of the other species of Ascosphaera. These results show that A. apis virulence can be altered by the presence of other chalkbrood species and that these avirulent pathogens may play a more important role in host life histories.