Chemical and behavioural interactions between ants and Microdon hoverflies

Microdon mutabilis is a myrmecophilous hoverfly that is a host-specific social parasite of Formica lemani ants. Although F. lemani is a geographically widespread ant. M. mutabilis is only found in a few populations and is a ‘nationally notable’ species in the British red data book. Previous experiments suggested that M. mutabilis was rare in comparison to its host because M. mutabilis eggs survived only in F. lemani colonies close to their maternal colony (extreme host specificity). It was hypothesised that eggs were covered in chemicals that mimicked the recognition cues of certain host colonies. Analysis of M. mutabilis eggs showed that they are covered in chemicals, deposited by the female at oviposition. These chemicals do not mimic the cuticular hydrocarbon recognition system of F. lemani workers or brood, but are a mixture of terpenoids, including nephthenol, 12-oxogeranylgeraniol (a novel natural product) and around 13 minor compounds, F. lemani workers treated M. mutabilis eggs less aggressively than foreign ant eggs, but not differently from chemically inert objects. Terpenoids have been found in the exocrine glands of other ant species, so the eggs may be mimicking other F. lemani semiochemicals. Preliminary tests also suggest that the chemicals have fungistatic or fungicidal properties. Profiles of the egg chemicals were compared between M. mutabilis and M. myrmicae, a cryptic sister species that parasites Myrmica sp. ants. The profiles were qualitatively distinct, due to an additional compound on M. myrmicae eggs and differences in the relative abundance of the minor compounds. M. myrmicae from sites across Europe had different proportions of nephthenol and 12-oxogeranylgerniol, but there is no difference between M. myrmicae using different Myrmica ants as hosts. There were no chemicals found on M. mutabilis larvae. M. mutabilis and M. myrmicae should be treated separately in future conservation efforts.