The effect of queen-worker conflict on caste determination in the bumblebee Bombus terrestris

Endocrine analyses were used to investigate the well-known association betw een queen production and the onset of worker reproduction (termed the compe tition phase, CPh) in Bombus terrestris. Larvae that reached the age of 5 d ays before the CPh had a worker-like profile: low juvenile hormone (JH) bio synthesis rates and low JH hemolymph titers. In contrast, larvae that reach ed the age of 5 days during the CPh had a queen-like profile: high JH biosy nthesis rates and high hemolymph JH levels. Larval fate could be manipulate d by transplanting egg cells into host colonies with different social struc tures. There was a steep rise in JH production in larvae transplanted into colonies near or during the CPh. This indicates that during colony developm ent, larvae switch from the "worker developmental pathway" to the "queen de velopmental pathway," and that the switch is socially regulated. In small r earing groups, larvae reared with queens before the CPh developed into work ers, whereas those reared with queens after the CPh developed into queens. Variation in worker type (naive or experienced) did not affect caste determ ination. Therefore, we hypothesize that queens produce a pheromone that dir ectly inhibits queen differentiation by larvae. We also present two alterna tive scenarios that explain the timing of gyne production in B. terrestris, one based on ecological constraints and the other based on queen-worker co mpetition.