Facultative social parasites mark host nests with branched hydrocarbons

The chemical integration strategies of facultative social parasites of social insects have not received the scientific attention they deserve, even though there is considerable research being done on the strategies of obligate social parasites. We simulated intraspecific nest usurpations in the social paper wasp, Polistes biglumis, by dividing each nest into two parts and putting one half in the care of the original foundress and the other half in the care of a usurper. After 8 days, we removed and killed foundresses and usurpers, and later tested the responses of naïve, sister-offspring to them. In each half-colony, the offspring were more tolerant to the female that was last on the nest, regardless of whether she was the foundress or a usurper. This suggested that usurpers had the chemical means to be tolerated by the host offspring. Comparisons between the epicuticular hydrocarbon profiles of foundresses and usurpers showed that usurpers were neither chemically insignificant nor transparent, nor were they mimetic, as obligate parasites often are. Instead, usurpers had chemical profiles richer in methyl-branched hydrocarbons than those of the foundresses. Analyses of the hydrocarbon profiles of nest paper revealed that usurpers supplemented host nests with their own hydrocarbons, a sort of nest marking. As a result, the chemical profiles of the host nests became qualitatively more similar to those of the usurpers. These chemical strategies illustrate that branched hydrocarbons play a role as semiochemicals and that facultative parasites may not all be on the main pathway to obligate parasitism.