Host ant use of the Alcon blue butterfly at the northern range margin

In myrmecophilous insects, interactions with ants are often a key factor determining persistence of their populations. Regional variation in host ant use is therefore an essential aspect to consider to provide adequate conservation practices for such species. In this study, we examined this important facet of species’ ecology in an endangered myrmecophilous butterfly Phengaris (=Maculinea) alcon (Lepidoptera, Lycaenidae). The investigations conducted in peripheral populations in Estonia allowed us to expand the knowledge of its host ant use to the northern distribution limit of the species. Our data indicate that in its northernmost populations, the xerophilous ecotype of Phengaris alcon is primarily parasitizing a single host ant species, Myrmica schencki. The data collected are in line with the emerging evidence suggesting that peripheral and core populations of P. alcon use different host ants, and peripheral populations tend to display higher host ant specificity. We also show that, at its northern range margin, P. alcon might be more limited by the availability of its sole larval food plant in the region, Gentiana cruciata, than the densities of its host ant. Finally, we found a strong negative correlation between Myrmica spp. and Lasius spp. colony densities, suggesting that interspecific competition between ants could have a substantial influence on host ant availability of Phengaris butterflies, and thus should be taken into account in conservation plans of these species.