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Kaisa Torri 2007 : Oviposition preference, habitat selection and parasitoids of Melitaea athalia (Lepidoptera: Nymphalidae)

The heath fritillary butterfly (Melitaea athalia) is a common checkerspot butterfly in southern Finland. In this study some aspects of the natural history of Melitaea athalia were investigated in the Åland Islands in southwestern Finland. The main subjects were adult oviposition host plant preference, occurrence of the species in different kinds of habitats, and the complex of parasitoids using M. athalia.

The experiments and observational study were conducted during the spring and summer of 2005. The presence of larval parasitoids of M. athalia were investigated by exposing laboratory-reared postdiapause larvae in two different habitat types in the spring. In order to learn about habitat use of M. athalia, adult butterflies were observed in the field during the
flight season using a set of transect counts. Oviposition host plant preference was studied by conducting an experiment in large outdoor population cage using laboratory-reared butterflies and potted plants.

Three species of parasitoids were found in association with M. athalia in this experiment. Cotesia melitaearum aggregation species emerged from M. athalia larvae, and in addition to these two species of parasitoids emerged from M. athalia pupae. These species were Pteromalus apum and Ichneumon cinxiae. The overall parasitism rate was only 4%, but larvae from all habitat types and host plant species were parasitized.

In the Åland Islands dry and mesic meadows are the main habitat for the species. The number of M. athalia was highest in dry meadows, but the species was also found in mesic meadows, herb-rich road verges and semi- open forests. The habitat use of M. athalia in the Åland Islands differs from that in mainland Finland and the UK.

In the oviposition experiment 455 egg clusters were laid over 14 days. The main oviposition host plants in the Åland Islands turned out to be Veronica chamaedrys, Veronica spicata and Plantago lanceolata. This is different than the mainland Finland and the UK. There was a clear pattern in the effects of microclimate on oviposition. Melitaea athalia frequently laid eggs in areas of sparse vegetation with a large percentage of bare ground.The average egg cluster size was 43, and there were no differences in egg cluster size between oviposition host plant species.