Predation by nymphs of Picromerus bidens (Heteroptera Pentatomidae Asopinae) on caterpillars of Euphydryas aurinia provincialis (Lepidoptera Nymphalidae) in Italy

Euphydryas aurinia (Rottemburg 1775) is one of the most seriously threatened European butterflies, as it suffered a severe decline in most countries and it is today listed in the European Community Habitats and Species Directive (92/43/EEC). Although its ecology and biology have been increasingly studied during the 20th Century, the factors affecting its mortality have not been described in detail, and the role of natural enemies on its population dynamics is largely unknown. In this paper, we present new data on predation events of P. bidens (Linnaeus, 1758) on caterpillars of Euphydryas aurinia spp. provincialis (Boisduval, 1828) in Central Italy, and discuss the stages during which predation could have a stronger impact. Our results show that II to IV instar nymphs of P. bidens attack the pre-diapause 3rd instar caterpillars when they live in a communal silken nest, while adults were found on butterfly larval webs, but never preying on caterpillars. The bugs visited a larval group more than once a day and at different times, and showed preference for cooler habitats (i.e., they were not observed on webs of warmer habitats). All together, our observations point out to the potential significant impact that gregarious nymphal instars can have on the population dynamics of E. aurinia in cooler habitats, where predator and prey coexist.