Toxicity of imidacloprid to soil organisms: biological responses in plants, earthworms, and millipedes

Imidacloprid is the most widely used neonicotinoid insecticide worldwide, but it poses serious risks to non-target soil organisms. This study evaluated the toxicity of the commercial formulation Warrant^® (700 mg a.i. kg^-1) using three bioindicators. <i>Lactuca sativa</i> germination, root, and shoot growth were inhibited at concentrations between 0.1 and 2 mg L^-1. All earthworms (<i>Eisenia andrei</i>) died after exposure to filter paper contaminated with 0.1-2 mg L^-1 of imidacloprid, and soil concentrations of 1-2 mg kg^-1 induced morphological alterations that led to mortality. Sublethal concentrations (0.2-0.8 mg a.i. kg^-1) caused avoidance behavior and histopathological damage to the body wall (0.1-0.8 mg a.i. kg^-1) and intestine (0.4-0.8 mg a.i. kg^-1). Although imidacloprid was not lethal to millipedes (<i>Rhinocricus padbergi</i>), it caused histopathological alterations in the midgut at 0.5-2 mg a.i. kg^-1. The extent of sublethal effects increased with imidacloprid concentration. This study emphasizes the importance of evaluating sublethal concentrations and natural soil assays to assess environmental toxicity. It also shows that soil physicochemical properties may influence imidacloprid bioavailability and, consequently, the magnitude of biological responses, underscoring the need for caution in imidacloprid applications to protect soil ecosystems.