Temperature and chalkbrood development in the alfalfa leafcutting bee, Megachile rotundata1

Ascosphaera aggregata, the causative agent of chalkbrood in the alfalfa leafcutting bee (Megachile rotundata), is a major mortality factor when this bee is used commercially as a pollinator. The effect of temperature on A. aggregata hyphal growth, spore germination, and disease prevalence was tested. The lowest prevalence of chalkbrood occurred at 35 °C, yet this temperature was the optimum for fungal germination and growth on agar, and is stressful to the insect. Sporulation of M. rotundata cadavers that were infected with the fungus was highest at 25 °C. Daily exposures to 40 °C for 6 h did not affect disease incidence, but it negatively impacted spore production. A similar temperature response, that is, where lower temperatures promote the disease, has been seen by others for this bee, and other Ascosphaera spp in other bees, but it is not clear why the greatest likelihood of mycosis does not occur at optimum temperatures for the fungus, or at the temperatures most detrimental to the insect. A few hypotheses have been proposed, but empirical data are lacking. alfalfa leafcutting bee / Megachile rotundata / Ascosphaera aggregata / chalkbrood / temperature stress