Shifts in cuticular hydrocarbon profiles of bees and wasps primarily reflect temperature variations rather than the amount of sealed ground surfaces in an urban landscape

The primary function of insect cuticular hydrocarbons (CHC) is to protect the bearer against desiccation, with long-chain saturated (n-alkanes) compounds being the substance class with the strongest effect. The Urban Heat Island (UHI) effect is the process by which cities become hotter than their surrounding semi-natural areas. This can potentially subject insects to heat and desiccation stress and likely induce shifts in their CHC profile. This hypothesis was tested in three different aculeate Hymenoptera: the bees Halictus scabiosae and Osmia cornuta and the wasp Polistes dominula, sampled in different urban settings in the metropolitan city of Milan (Italy). We expected that warmer and more urbanized areas favour insects with a higher abundance of n-alkanes and a lower abundance of alkenes and/or methyl-branched alkanes. In addition, we expect CHC profiles with longer medium chain length of hydrocarbons and particularly of n-alkanes in these areas. We found that temperature, more than the amount of sealed ground surfaces (a proxy of urbanization), explained the shifts in the CHC profiles of all the three species. The strongest shifts were found in O. cornuta and P. dominula compared with H. scabiosae. P. dominula shifted to longer mean hydrocarbon chain length and longer mean n-alkanes chain length with increasing monthly site temperature. In addition, P. dominula showed a CHC profile poorer in n-alkanes and richer in methyl-branched alkanes only under hotter conditions in the most urbanized sites. This suggests complex interactions between temperature and the degree of urbanization. The species-specific responses to temperature can be explained by the inherent characteristics of the CHC profiles, as the two species with the lowest relative abundance of n-alkanes (O. cornuta and P. dominula) showed the strongest shifts in their CHC profiles. These results show that variations in CHC profiles within a species can be detected even at small spatial scales, such as along urbanization gradients.