From Africa to the Alps: risk assessment on an invasion by Cacyreus marshalli (Butler, 1898)

Cacyreus marshalli (Butler, 1898) is the only alien butterfly species in Italy, introduced from Southern Africa via the trade of ornamental Pelargonium plants (family Geraniaceae). In 2008, Quacchia and colleagues demonstrated that if Pelargonium plants are not available, females can lay eggs on Geranium spp., developing fertile offspring. C. marshalli is a thermophilous species, but in recent years some adults have been observed flying far from villages and at high altitudes (up to 2400 m a.s.l.) in the Orco valley (Gran Paradiso National Park, Aosta Valley). Due to the potential threat to native Geranium-consuming lycaenids and to evaluate the risk of naturalisation, we investigated: (i) dispersal abilities of gravid females, outside the National Park to avoid accidental establishment of the invasive species; (ii) pelargonium distribution and abundance; (iii) oviposition behaviour and preimaginal distribution; (iv) citizen care practices with pelargoniums. Pelargoniums were counted in the Orco Valley (5455 plants) and eggs and larvae were counted on 348 pelargoniums chosen on the basis of isolation and altitude. Flight experiments suggested that females were able to overcome barriers and fly at least 550 m looking for host plants. Eggs and larvae were unexpectedly abundant, but models showed that there was a temperature limit which prevented upward expansion of C. marshalli. Citizens were given a questionnaire to complete to investigate their propensity to replace pelargonium with other ornamental plants. We discuss the possibility of eradicating C. marshalli in the protected area in the light of our results.