An unexpected record of Myrmica schencki EMERY, 1895 as a secondary host ant of the hygrophilous form of a small and isolated population of the Alcon Blue butterfly Phengaris (=Maculinea) alcon (DENIS et SCHIFFERMÜLLER, 1775) (Lepidoptera, Lycaenidae) in NE Poland

Phengaris alcon is a socially parasitic butterfly species showing ecological variation across its distribution range. Host ant specificity was studied on a site (a mineral island surrounded by wetlands) in the Narew National Park in NE Poland, inhabited by a highly isolated and small population (estimated at several dozen individuals). We examined nests of their potential hosts, i.e. Myrmica ants, in patches of habitat where the initial larval food plant i.e. Gentiana pneumonanthe was recorded. The Myrmica species composition was shown to be unusual for such habitats because of the presence of M. schencki together with the dominant M. scabrinodis. In 2013 we found just one prepupa of P. alcon in a nest of M. schencki, which had never yet been recorded as a host of the hygrophilous form of this butterfly, and none of the M. scabrinodis nests were infested. However, in 2014 a very typical pattern for eastern Europe was observed, i.e. larvae were recorded exclusively in M. scabrinodis colonies and the parasitisation rate was 33%. In both seasons we recorded similar numbers of egg shells on gentians, which indicated a similar population size of adult butterflies. The results are discussed in the context of the ecology and conservation of P. alcon as well as the sampling design.