Two Pellioditis biocontrol nematode species infect Ariolimax columbianus , the Pacific banana slug, and increase mortality in laboratory infectivity trials

Gastropods are a highly diverse and often overlooked taxonomic group of significant ecological and economic importance. Some terrestrial gastropods are critical pests of commercial agriculture and home gardens worldwide. Malacopathogenic nematodes offer an effective biological control method of managing pest slugs and snails as a ‘natural enemy’. Pellioditis (syn. Phasmarhabditis ) hermaphrodita and Pellioditis (syn. Phasmarhabditis ) californica are two species of biocontrol nematodes that have been commercialized, sold as Nemaslug® and Nemaslug® 2.0 respectively on three continents. Although there is interest in bringing Nemaslug® products to the US, they are currently not permitted due to limited knowledge on their North American distribution and effects on non-target and native species. In this study, we investigated the impact of P. hermaphrodita and P. californica on Ariolimax columbianus across two slug-host life stages, in laboratory infectivity assays. The objectives were to 1. determine whether P. hermaphrodita and P. californica nematodes impact survival of A. columbianus , and 2. evaluate whether there are differential effects on survival in juvenile and adult life stages of A. columbianus , in laboratory infectivity trials. We found that P. hermaphrodita caused significant mortality in A. columbianus with 100% mortality observed in both juvenile and adult slug hosts. The P. californica treatment had significant effects on the juvenile A. columbianus group only, with 80% mortality. By contrast, only 16% of unexposed control juveniles and 4% of control adult slugs died during the experiment. These results indicate that P. hermaphrodita and P. californica are lethal to the native, non-target Pacific banana slug ( A. columbianus ) under laboratory conditions, with mortality differing between juvenile and adult host life stages. Given the ecological importance of A. columbianus , these findings raise concerns for potential non-target effects of P. californica and P. hermaphrodita on terrestrial gastropod communities and emphasize the need for testing biocontrol agents against multiple life stages.