Although generally capable of producing males, workers in most hymenopteran societies (bees, ants and wasps) perform little or no reproduction in the presence of the queen. We investigated why workers do not reproduce in the European hornet Vespa crabro (L.). Previous genetic and behavioural work on this species had shown that, although queen mating frequency is low (effective paternity, 1.1) causing workers to be more related to workers' sons than to the queen's sons, workers do not lay eggs and the males reared are all the queen's sons. This suggested that workers are under queen pheromonal control. Here we show that this is not the case. Using egg introduction experiments, we show that worker policing behaviour occurs. We introduced queen-laid and worker-laid eggs into four discriminator colonies in five trials. In colonies with a queen, workers removed significantly more worker-laid than queen-laid eggs (1/79 worker-laid eggs but 46/72 queen-laid eggs remained after 16 h, p < 0.001). In colonies without a queen, workers removed significantly more queen-laid than worker laid eggs (30/44 worker-laid eggs but 13/41 queen-laid eggs remained after 16 h, p≤ 0.001). The presence of worker policing in queenright hornet colonies provides a proximate explanation for the absence of worker reproduction. Workers are not under queen control but instead are collectively enforcing their own sterility. Worker policing at low paternity may have been selected for because it enhances colony productivity by eliminating costly conflicts over reproduction.
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