Assessing the quality of different ant species as partners of a myrmecophilous butterfly.

We assessed the quality of different ant species as partners of the facultatively myrmecophilous lycaenid butterfly Glaucopsyche lygdamus. We compared disappearance and parasitism rates of G. lygdamus larvae in the field, and development of non-feeding prepupae in the laboratory, when individuals were untended or tended by one of four ant species. Formica podzolica was the only ant species to provide a clear benefit to G. lygdamus, in the form of reduced larval parasitism relative to untended larvae. F. 'neogagates' (F. neogagates + F. lasioides) and Tapinoma sessile were essentially neutral partners, providing no significant cost or benefit for any of the parameters measured. Relative to untended individuals, association with F. obscuripes significantly increased larval disappearance and significantly decreased pupal mass. Thus, F. obscuripes may act as a parasite of the general association between G. lygdamus and ants under certain conditions. Ant species also differed in their persistence as tenders of G. lygdamus larvae once an interaction was established. Over the lifetime of a larva, F. podzolica and F. obscuripes usually remained as the attendant ant species on plants over consecutive census dates, while F. 'neogagates' and T. sessile were frequently replaced, most commonly by F. obscuripes. It remains to be determined if disappearance and developmental outcomes reported here reflect true fitness costs (i.e. reduced survivorship and lower reproductive success) for G. lygdamus. The potential and limitations for specialization in association between G. lygdamus and high quality ant partners are discussed.