Plasticity of worker reproductive strategies in Bombus terrestris: lessons from artificial mixed-species colonies

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We used the experimental paradigm of artificial mixed colonies of two phylogenetically related bumblebee species to analyse the dynamics of the reproductive skew in societies of Bombus terrestris. Artificial mixed-species colonies were set up by introducing callow B. terrestris workers either into a queenright (QR) or a queenless (QL) colony of B. lapidarius. The introduced B. terrestris workers were well integrated into their host B. lapidarius colony and displayed nesting activities that did not differ from those of the resident B. lapidarius workers. However, the introduced B. terrestris workers did show a different reproductive behaviour. While B. lapidarius workers did not develop ovaries in a B. lapidarius QR colony but did so in a B. lapidarius QL group, adopted B. terrestris workers in a B. lapidarius QR colony developed ovaries as if they were under QL conditions. These results indicate that, in mixed-species colonies, B. terrestris workers are irresponsive to the queen's inhibitive action on ovary development. In QL homospecific and heterospecific predominately B. terrestris mixed-worker colonies (1Bl + 5Bt), reproduction was dominated by a single B. terrestris worker, whereas in QR B. lapidarius or QL equally mixed-worker colonies (3Bl + 3Bt), almost all B. terrestris workers developed ovaries. We suggest that in the presence of enough heterospecific workers, B. terrestris workers behave as parasites. This last finding suggests that worker reproduction in B. terrestris is highly plastic and that the experimental paradigm of artificial mixed colonies may provide new insights into the evolution of social parasitism in this taxon.