Risk assessment for the alien species Cacyreus marshalli in an Italian Alpine valley

Originally from Southern Africa, Geranium Bronze (Cacyreus marshalli) is an invasive species in Europe, where its larvae pest Pelargonium spp. Because of the potential threat for native Geranium-consuming lycaenids (Quacchia et al. 2008) to evaluate the risk of naturalisation, we investigate in Gran Paradiso National Park: i) C. marshalli current population size; ii) Pelargonium distribution and abundance; iii) Dispersal abilities of gravid females; iv) Current distribution, through the application of Species Distribution Models (Maxent). We selected 30 study sites, within an isolation (distance from towns) and food plant density gradient. We checked 258 Pelargonium plants and about 53% of them were colonised. The analysis of C. marshalli presence showed that eggs density is higher in areas with lower food plant density. Peripheral areas revealed a lower occurrence of eggs compared to town centre and also the number of residents positively influenced the colonization rate. Females are able to overcome barriers, as trees, but the risk of spreading into natural high altitude areas currently emerges as low. Indeed, about 1% of the protected area has a probability of presence for the species above 0.5 (the peak of presence probability is around 1000 m a.s.l., with a constant decrease at higher altitude). Quacchia, A.; Ferracini, C.; Bonelli, S.; Balletto, E. & Alma, A. (2008). Can the Geranium Bronze, Cacyreus marshalli, Can the Geranium Bronze, Cacyreus marshalli, become a threat for European biodiversity? Biodiversity and Conservation 17: 1429-1437.