Seasonal variation in the sex allocation of a neotropical solitary bee

We carried out a field study on the life history and sex allocation of the ground-nesting solitary bee Diadasina distincta (Hymenoptera: Anthophoridae). This species is multivoltine, undergoing five generations a year between February and September. The numerical sex ratio of this species was female biased overall (approximately 38% males) and showed a strong and consistent seasonal pattern. The numerical sex ratio was extremely female biased (approximately 20% males) from February until May, and then slightly male biased (approximately 60% males) from June until September. Females were 3.26 times the size of males, and so the overall investment ratio was female biased throughout the year. The overall female bias and seasonal variation in sex allocation is unlikely to be explained by models that invoke overlapping generations or competition between brothers for mates (local mate competition). We suggest that a possible explanation for the female bias in the early part of the season is local resource enhancement (LRE): nesting near larger numbers of sisters reduces parasitism. LRE is likely to decrease in importance in the later part of the season, when the biased numerical and investment ratios may be explained by models in which male and female offspring gain different fitness returns from resources invested.