Sex ratio variation in the bumblebee Bombus terrestris

Patterns of sex allocation in bumblebees have been enigmatic and difficult to interpret in either a Fisherian context or in a kin-selection perspective. We gathered data on several hundred laboratory-reared colonies of the bumblebee Bombus terrestris and analyzed sex allocation as a function of diapause duration and a series of variables describing colony development. Our analyses addressed both sex allocation patterns across different cohorts of laboratory colonies reared at different times and sex allocation patterns across individual colonies within these cohorts. We used path analysis to test a hypothetical model linking a sequence of colony development variables to the crucial reproductive parameters at the end of the colony life cycle. We show that (1) population-wide patterns of sex allocation show equal investment in the sexes and are thus consistent with queen control, but not with worker control. (2) A significant part of the colony-level and cohort-specific variation in sex allocation is related to the hibernation conditions of founding queens: Queens with longer than average winter diapause produce larger cohorts of first and second brood workers, switch to haploid eggs early, and produce colonies that raise mostly males and few new queens and vice versa. (3) Colony-level sex allocation is significantly related to the time span between the switch point (date of first haploid egg laid by the queen) and the competition point (date of first haploid egg laid by one of the workers): the longer this period, the more male biased the sex ratio. (4) The breeding constraints of an annual life cycle, the short reproductive season, and the presumably high premium on early produced males imply that bumblebee workers have no realistic options to capitalize on their relatedness asymmetry toward the different kinds of reproductive brood by biasing the sex ratio.