The role of parasite release in invasion of the USA by European slugs

Several species of European slugs are invasive in the USA, threatening native species and damaging agricultural and horticultural crops. One possible explanation for the success of these invaders is parasite release. To test this hypothesis we collected European slugs in part of their native range (United Kingdom) and in the USA and compared prevalence, distribution and species richness of their nematode parasites. All slugs were dissected and examined for the presence of nematodes. In the UK, nematodes were present at 93% of study sites and 16.4% of all slugs examined were associated with nematodes whereas in the USA the respective figures were 34% of sites and 5.4% of slugs. Nematode species richness was greater in the UK with 12 species being found, seven of which were thought to be truly parasitic as opposed to being phoretic or necromenic. Nine species of nematode were found in the USA, four of which were truly parasitic. Four of the ten European slug species examined in the USA, were entirely free of truly parasitic nematodes whereas all were infected by nematodes in some sites in the UK. There was a significant difference in the prevalence of truly parasitic nematodes in five of these species when comparing their home versus invasive range. A significant difference in parasite prevalence was observed when comparing native and introduced slug species in the USA, however, this was not significant in the UK. Our data support a role for parasite release during the invasion of the USA by European slugs.