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

In social insects, reproduction is clearly biased in favour of the queen. However, worker sterility is conditional and social-context-dependent. Under normal colony conditions the mechanism and the flexibility level of worker reproductive strategy are often masked to the researcher. We employed a system of mixed colonies of two bumblebee species, Bombus terrestris workers hosted by B. lapidarius colonies which revealed some traits of this reproductive plasticity.
The introduced B. terrestris workers were well integrated into their host B. lapidarius colony in all aspects except for reproduction. Unlike the resident B. lapidarius workers they were not inhibited by the queen and laid eggs from which B. terrestris males emerged. Moreover, workers B. terrestris in mixed species groups exhibited a completely different reproductive strategy from equivalent workers reared in queenless homospecific colonies. Instead of exhibiting a reproductive skew in favour of one or two workers, all of them developed ovaries and laid eggs, behaving as parasites in the presence of heterospecific workers. This finding suggests that the experimental paradigm of artificial mixed colonies may also provide new insights into the evolution of social parasitism in social insects.

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