REPRODUCTIVE DECISIONS IN BUMBLE-BEE COLONIES - THE INFLUENCE OF WORKER MORTALITY IN BOMBUS-TERRESTRIS (HYMENOPTERA, APIDAE)

1. The timing of reproductive decisions represents a major life-histor y decision that all organisms must make. In social Hymenoptera this de cision takes the form of when to curtail worker production (equivalent to growth) in favour of male and queen production (reproduction). 2. In bumble-bees (Hymenoptera: Apidae), queens produce one to several br oods of pure workers (females) before switching to sexual production. Queens are determined environmentally, via food levels provided to dip loid larvae, whereas, as in all Hymenoptera, females suppress fertiliz ation of eggs to produce males. After the decision to begin reproducti on is made, mixtures of workers, males and sometimes queens are produc ed, but the proportion of fertilized eggs decreases over time until on ly haploid eggs are laid. 3. Here we assess the two hypotheses that ha ve been proposed to explain the onset of haploid egg (male) production in bumble-bee colonies. We generate a list of clear predictions deriv ed from the two hypotheses and test these with data from laboratory an d field-reared nests of the bumble-bee Bombus terrestris. 4. We show t hat the two main hypotheses available, about worker density and about oviposition record by the queen, are insufficient to explain the obser ved patterns in field and laboratory bumble-bee nests. We suggest the possible alternative that this reproductive decision may be made by wo rkers through selective culling of eggs or larvae rather than by the q ueen.