Monitoring the effects of pesticide pellets to address farmers' concerns on soil fauna, specifically earthworms

Slugs are a major pest in arable agriculture that are typically controlled by using pesticide-impregnated pellets (molluscicides). We received multiple requests from farmers in 2018 for research into the impact of slug pellets on earthworms because they had been told that ferric-based pellets were ‘worm-friendly’. We measured soil fauna feeding activities within Lumbricus terrestris middens on a replicated field plot experiment comparing the recommended product rates of a metaldehyde (4%) based or ferric phosphate EDDS (3%) based slug pellets. There was a significant decrease in feeding activity in the presence of either type of slug pellet, particularly in the top 25 mm of the soil indicating movement away from the pesticide. This could be explained by the significantly lower abundance (numbers and biomass) of earthworms in the topsoil. Four months after ferric phosphate-based slug pellet application, there was no significant difference between the control and treated plots in earthworm abundance, indicating the effects were temporary. In the laboratory, individual Lumbricus terrestris earthworms consumed up to 4 slug pellets over 14 days and no significant differences in biomass were measured. A burrowing fitness assay indicated that earthworms exposed to ferric phosphate-based slug pellets burrowed to a greater depth. This suggests an avoidance behaviour response, which could also explain the field results. A consultation with farmers (n = 68) rated this information service as useful and indicated widespread interest in minimizing the effects of pesticides on non-target soil fauna.