Competition induces adaptive shifts in caste ratios of a polyembryonic wasp

An important transition in insect life-history evolution was the shift from a solitary existence to living in groups comprising specialized castes. Caste-forming species produce some individuals that reproduce and others with worker functions that have few or no offspring1. Morphologically specialized castes are well known in eusocial species like ants and termites1, but castes have also evolved in less-studied groups like thrips, aphids and polyembryonic wasps2,3,4,5. Because selection acts at both the individual and the colony level, ratios of investment in different castes are predicted to vary with environmental factors like competition and resources6,7,8. However, experimental evidence for adaptive shifts in caste ratios is limited9 owing to the experimental difficulty of manipulating factors thought to influence caste ratios10,11,12,13, and because some species produce behaviourally flexible castes that switch tasks in response to colony needs14,15. Unlike other caste-forming species, the broods of polyembryonic wasps develop clonally, so that increased production of one caste probably results in decreased production of the other16. Here we show that the polyembryonic wasp Copidosoma floridanum alters caste ratios in response to interspecific competition. Our results reveal a distinct trade-off by C. floridanum between reproduction and defence, and show experimentally that caste ratios shift in an adaptive manner.