Effects ofPhasmarhabditis hermaphrodita on non-target molluscs

Two field experiments were done in which plots of oilseed rape, immediately adjacent to the field margin, were left untreated or treated with the slug-parasitic rhabditid nematode Phasmarhabditis hermaphrodita. Nematodes were applied at a rate of 3 × 10 9 ha -1 . Slug populations within plots were low and nematode application had little detectable effect on slug numbers, biomass or slug damage to the crop. There was a large snail community in the field margin in both experiments, but P hermaphrodita had no detectable effect on the abundance of any of the nine snail species recorded. The susceptibility of seven common hedgerow species (Monacha cantiana, Cepaea hortensis, Cepaea nemoralis, Pomatias elegans, Oxychilus helveticus, Clausilia bidentata and Discus rotundatus) to P hermaphrodita was tested in the laboratory. Snails were kept confined for three weeks on soil without nematodes, or with P hermaphrodita applied at the recommended application rate or five times this rate. Soil treatment with the recommended rate of nematodes caused significant mortality only for the snail M cantiana and the susceptible slug Deroceras reticulatum. Soil treatment with five times the recommended rate caused significant mortality of the slug D reticulatum and the snails M cantiana and C hortensis, but not any other species. Reasons why application of P. hermaphrodita to arable crops in Britain is unlikely to pose a substantial threat to non-target molluscs are discussed.