Sexual harassment by males reduces female fecundity in the alfalfa leafcutting bee, Megachile rotundata

Under sexual conflict, males evolve traits to increase their mating and reproductive success that impose costs on females. Females evolve counteradaptations to resist males and reduce those costs. Sexual harassment is a form of sexual conflict in which males make repeated, costly attempts to mate. Costs to female foraging or predation risk have been measured in several species, but quantitative measurements of direct fitness costs are rare. In the alfalfa leafcutting bee, Megachile rotundata (Fabricius; Hymenoptera: Megachilidae), males harass females, and females resist all mating attempts. We placed bees in large, outdoor cages with various male-biased sex ratios. Harassment rate, nest progression, offspring production, temperature, and food availability were measured daily for 7 days. Harassment rates were highest at intermediate sex ratios. Harassment reduced the number of foraging trips and increased the duration of foraging trips made by females. Females produced offspring at a slower rate when subjected to higher rates of harassment. This shows a direct link from sex ratio to harassment to female fitness under natural conditions. We also discuss an alternative explanation that female resistance is a mechanism for mate choice for high-quality males, which would require that indirect benefits accrue through either daughters or grandsons, because all sons in haplodiploid species arise from unfertilized eggs.