Veronica longifolia L. as an important initial larval food plant of Scarce Fritillary Euphydryas maturna (LINNAEUS, 1758) (Lepidoptera, Nymphalidae): the ecological uniqueness of populations from the Natura 2000 area “Dolina Biebrzy” (Biebrza Valley) in NE Poland

The Euro-Siberian Scarce Fritillary Euphydryas maturna is considered a vulnerable species in the European Union and is listed in Annexes II and IV of the Habitats Directive. The butterfly shows a complex pattern of larval plant use throughout its range. In central Europe females oviposit on some trees and bushes, especially Fraxinus spp., on which hatched larvae spend their pre-diapause phase of development feeding gregariously in conspicuous webs. However, some herbal plants are also reported in the north and east. During a study performed in the Natura 2000 area “Dolina Biebrzy” (Biebrza Valley) in NE Poland we recorded populations showing a unique mixture of ecological characteristics. Both Fraxinus excelsior and Veronica longifolia were used as larval food plants before hibernation, and some local populations seemed to be completely dependent on the latter plant. Moreover, in the spring, at one site, we observed larvae feeding on Salix rosmarinifolia – the first host record for this plant species. The importance of our findings for conservation, as well as for the monitoring of the butterfly, is discussed.