Chalkbrood (Ascosphaera aggregata) resistance in the leafcutting bee (Megachile rotundata). I. Challenge of selected lines

Abstract - Twenty-nine cell series of the leafcutting bee, Megachile rotundata (Fabr), each containing 5 or more healthy larvae, no chalkbrood (Ascosphaera aggregata Skou) and no pollen masses were isolated from a population with over 36% chalkbrood. This selected subpopulation was split and isolated for 4 and 6 generations before being challenged by forcing the females to nest in heavily contaminated media, or by weekly dustings with approximately 175 x 10⁸ A aggregata spores. The incidence of chalkbrood in both challenged lines was markedly lower than that of the wildtype, and comparable to that of the selected lines, suggesting a genetic component for resistance in both lines. The very low incidence of pollen masses (dead eggs or early instar larvae) in the selected lines throughout the 4 yrs of the study indicates that this trait may also be genetically mediated, either linked to, or independent of disease resistance.