Life history traits of the Idas Blue (Plebejus idas (Linnaeus, 1761)) were studied at two sites in the northern Upper Rhine valley in Baden-Württemberg (Germany). The species is monophagous on Broom (Cytisus scoparius (L.) Link) at these localities, as has been the case at numerous surrounding sites of former occurrence. Five species of formicine ants – Formica pratensis Retzius, 1783, F. rufibarbis Fabricius, 1793, F. cunicularia Latreille, 1798, Lasius niger (Linnaeus, 1758) and L. psammophilus Seifert, 1992 – have been found attending caterpillars and one pupa. Oviposition, including size of plants used and height above ground, was also studied. Based on the variety of ant partners and the apparent absence of ant-dependent oviposition, we conclude that the studied population exhibits the traits of facultative myrmecophily. In contrast, other Central European populations of P. idas hitherto studied are obligatorily myrmecophilous, usually showing associations with ants of the Formica cinerea-group. Thus, a fundamental intraspecific difference concerning the degree of myrmecophily has been demonstrated for the first time in a lycaenid species. Patterns of geographic variation of life histories in P. idas are discussed, though available information is scanty. The findings on caterpillar-ant associations in the Upper Rhine valley are highly relevant for conservation issues of the local population of P. idas, which is close to extinction. Shifts in the ant fauna and structure of broom patches are supposedly not very important factors in the species’ regional decline. Nevertheless, protection measures for dry heathland such as soil disturbance may benefit ant density and thus P. idas on a local scale.
