The effect of soil on the efficacy of a nematode-based biopesticide of slugs

Abstract

Several slug species are serious pests of agriculture and are difficult to control. One popular control method is the nematode Phasmarhabditis hermaphrodita, which has been used in slug control for > 25 years. However, there are reports of it failing to reduce slug numbers and damage in the field for unknown reasons. This may be due to lack of knowledge about how P. hermaphrodita performs when applied to different soils. We therefore assessed the survival, movement and pathogenicity of P. hermaphrodita infective juveniles (IJs) when added to six different soils (compost with and without peat, clay loam, loam, sandy loam and sandy soil). The soils were either frozen or autoclaved before use to eradicate resident nematodes prior to the experiment. P. hermaphrodita survived best in autoclaved compost without peat and in experiments with frozen soils, compost with and without peat was best. Survival of P. hermaphrodita was similar in other soils. Interestingly, in peat-free compost P. hermaphrodita reproduced prolifically, which may affect the long-term success of the nematode in the field as other life stages, apart from the IJ stage, cannot infect slugs. In infection experiments we found P. hermaphrodita added to compost with peat killed slugs faster than nematodes added to a sandy clay loam or sandy soil. In movement experiments, the nematodes remained within 3 cm of the application point in each soil. In summary, soil type severely affects P. hermaphrodita survival, and the ability to kill slugs; therefore it should be assessed by farmers and gardeners before use.