Responses of the earthworm Lumbricus terrestris (L.) to iron phosphate and metaldehyde slug pellet formulations

Earthworms are susceptible to many pesticides, including some slug pellet formulations, but very little is known about the effects of iron phosphate formulations on non-target fauna. In artificial burrows, the study provides the first evidence of deleterious impacts on the survival and behaviour of the earthworm Lumbricus terrestris of pelleted iron phosphate. This occurred at eight times recommended field application rates, but it is noteworthy that this type of trial requires higher numbers of pellets to be applied as animals usually contact only very few pellets. Effects were stronger than another widely applied pelleted formulation (metaldehyde) which did not differ significantly from control groups, even when this treatment was applied at very high rates (35 times recommended field application rates). Again few pellets were contacted by the earthworms, but more than in iron phosphate assays. Exposure to iron phosphate increased earthworm mortality, and surviving individuals gained less mass than those exposed to metaldehyde (significantly less than the control group; metaldehyde did not differ significantly from the other two treatments). Significant reductions in the rates at which pellets and leaves were removed from the soil surface were observed after exposure to iron phosphate, indicating reduced surface foraging activity. Findings are discussed in relation to the chemical formulations and other pellet characteristics and highlight the requirement for dose-dependent earthworm assays with iron phosphate formulations.