Recognition of a paper wasp social parasite by its host: evidence for a visual signal reducing host aggressiveness

Obligate social parasite insects are specialized in exploiting the parental service of the workers of another social species by invading their colonies. As social insects are usually aggressive towards intruders, social parasites have to circumvent the host’s nestmate recognition system to enter the host colony successfully. Many studies on paper wasps have shown that, after host nest invasion, Polistes social parasites change their chemical profile to match the host’s odour, thus allowing their acceptance into its colony. However, a social parasite’s usurpation strategy may benefit from signals that reduce or eliminate the aggression of the host. We used lure presentation experiments to investigate whether Polistes sulcifer, a social parasite of the paper wasp P. dominulus, is able to reduce the aggressive reaction of its host. We found that the parasite lure elicited lower host aggressiveness than the conspecific lure, suggesting that parasite species have evolved cues able to inhibit host aggressiveness. We investigated separately the effects of chemical and visual patterns on host aggressiveness. The lower host reaction to the parasite lure was not due to chemical cues, but was elicited by the visual facial pattern of the parasite. Experimental manipulation of this visual pattern demonstrated that the black lower part of the clypeus of the parasite is the trait able to reduce host aggression. This pattern can be considered an honest signal since it visually amplifies the mandibular width, giving information about the parasite’s dangerousness.