Water activity of the bee fungal pathogen Ascosphaera apis in relation to colony conditions

We report factors that are limiting for Ascosphaera apis growth: water activity (aw) and temperature. Features of A. apis were a high water activity, 0.85–0.90 aw at 25/30 °C and 0.90–0.95 aw at 35 °C. A minimum water activity of 0.95 aw was required to produce spores. None of these characteristics changed from cycling or descending/ascending between low and high water activity media. Growth rates varied between isolates. This new study provides the biological data for monitoring and early prediction of disease outbreaks. Beekeepers should note that A. apis has a high water activity for spore production. Healthy bees that keep the internal brood area hot and dry can critically limit incidence of infection and transmission via spores.