Looking for the ants: selection of oviposition sites by two myrmecophilous butterfly species

Obligate myrmecophilous butterfly species, such as Maculinea teleius and M. nausithous that hibernate as caterpillar in nests of the ant species Myrmica scabrinodis and M. rubra respectively, have narrowly defined habitat requirements. One would expect that these butterflies are able to select for sites that meet all their requirements. Both butterfly species occur in habitats where their initial larval resource, the host plant Sanguisorba officinalis, is abundant, while the ant nests are relatively less abundant. With ants in the proximity of a host plant, caterpillars have a chance of being found by their host ant species, while the host plant could become a sink as caterpillars will die when there are no ants close to the host plant. We tested whether females oviposit on host plants in the close proximity of host ants (ant-mediated oviposition) or whether there is random oviposition, using 587 1-m2 plots in and around a nature reserve in The Netherlands where both species were reintroduced. We found that females of both species indeed more often deposit eggs on host plants nearby ants than on host plants without ants. Visits of adult butterflies to plots with the host plant but without the host ants only occurred in years with high butterfly densities. We argue that understanding site selection for oviposition is important for these highly endangered butterfly species with a specialised life cycle, especially after disturbances or reintroduction