 Margus Vilbas margusvilbas@gmail.com 1 Department of Zoology, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia Received: 24 November 2015 / Accepted: 26 September 2016 © Springer International Publishing Switzerland 2016 Host ant use of the Alcon blue butterfy at the northern range margin Margus Vilbas · Toomas Esperk · Tiit Teder J Insect Conserv DOI 10.1007/s10841-016-9921-7 Keywords  ·  · · Host specifcity · Myrmecophily · Endangered population · Distribution margin · Estonia Introduction Associations with ants (myrmecophily) have been docu - mented in numerous arthropod taxa (Hölldobler and Wil - son 1990; McIver and Stonedahl 1993). In butterfies, ant associations are almost exclusively restricted to the rep - sentatives of the family Lycaenidae (Fiedler 1991; Pierce et al. 2002). The degree of larval myrmecophily in this family ranges from loose facultative interactions in which larvae are only occasionally tended by ants to complex obligate associations in which ant attendance is crucial for the butt - fies’ survival (Ballmer and Pratt 1992; Fiedler 1991, 2006; Pierce et al. 2002). Consistently, in the latter species, inter - actions with ants have often been considered a key factor for successful conservation of their populations (Elmes e al. 1998; Als et al. 2004; Thomas et al. 2009). Among the myrmecophilous lycaenids, the Palearc - tic genus Doherty, 1891 (senior synonym of Van Eecke, 1915; Fric et al. 2007) particularly stands out by their intricate interactions with ants. The frst three larval instars of these butterfies develop on fowers and seeds of their host plants, whereas they complete thei development as obligate social parasites in the nests Latreille, 1804 ants (Thomas et al. 1989). Based on how they exploit their hosts, the species of this but - fy genus can be divided into two groups. In particular, the caterpillars of so called cuckoo-feeders (e.g.  ) are mostly fed directly by the worker ants with their regurgita - tions, trophic eggs and dead prey, while predatory species (    and  ) feed In myrmecophilous insects, interactions with ants are often a key factor determining persistence of their populations. Regional variation in host ant use is there - fore 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 butterfy (= ) (Lepidoptera, Lycaenidae). The investigations conducted in peripheral populations in Estonia allowed us to expand the knowledge of its host ant use to the north - ern distribution limit of the species. Our data indicate that in its northernmost populations, the xerophilous ecotype of  is primarily parasitizing a single host ant species,  . The data collected are in line with the emerging evidence suggesting that peripheral and core populations of  use different host ants, and peripheral populations tend to display higher host ant specifcity. We also show that, at its northern range mar- gin,  might be more limited by the availability of its sole larval food plant in the region,  , than the densities of its host ant. Finally, we found a strong negative correlation between  . and  . colony densities, suggesting that interspecifc competition between ants could have a substantial infuence on host ant availability of butterfies, and thus should be taken into account in conservation plans of these species. 1 3