Comparisons of Chalkbrood susceptibility between a French-derived univoltine strain and a Canadian strain of the alfalfa leafcutter bee, Megachile rotundata
Mark S Goetiel; Grant M Duke; Gerald H Rank
The susceptibility of the alfalfa leafcutter bee, Megachile rotundata, to Ascosphaera aggregata, the causative organism of chalkbrood, was investigated during 1990-1993. Under laboratory conditions, the eggs of a French-derived univoltine strain and a Canadian, partially bivoltine, strain of M. rotundata were exposed to different doses of A. aggregata spores (from 20 to 220 000 spores/egg) on tissue culture plates. Percentage mortality of larvae due to chalkbrood and total mortality were determined. The two strains were equally susceptible to chalkbrood when reared on an artificial medium; when reared on natural provisions, the French strain was either equally or more susceptible. Comparison of these laboratory bioassay results to published field results suggests that voltinism plays an important role in resistance of field populations to chalkbrood.