Optimal timing of the production of sexuals in bumblebee colonies

Colonies of annual eusocial insects often produce only workers until they switch to the production of sexuals (queens and males). This switch from the ergonomic phase to the reproductive phase has been modelled by Macevicz & Oster (1976). Their model predicts that the production of sexuals should take place approximately one generation (one brood developmental period) before resources are depleted in order to maximise the number of sexuals. Bumblebees, however, produce sexuals early (relative to colony lifetime) when resources are still abundant and thus the model of Macevicz & Oster does not hold for these insects.

Here we show that Macevicz & Oster's prediction hinges critically on their assumption that the egg laying rate of the queen is a function of the foraging effort of the workers. This assumption does not hold for bumblebees, where queens lay eggs at a constant rate. Taking this feature of bumblebees into account the model predicts early production of sexuals, since it does not pay to produce more workers and collect more resources than needed to raise sexuals.