Ecological risks of imidacloprid and tebuconazole in tropical freshwater invertebrates: Single and mixture...

Pesticides are extensively used in agriculture worldwide, often reaching freshwater ecosystems and posing risks to non-target aquatic organisms. However, standard ecotoxicological assessments are commonly based on temperate species, potentially underestimating the vulnerability of tropical fauna. This study aimed to evaluate the acute toxicity of the insecticide imidacloprid (IMI) and the fungicide tebuconazole (TEB), individually and in mixture, to seven tropical freshwater invertebrate species: the crustaceans Ceriodaphnia silvestrii, Hyalella meinerti, and Strandesia rondoniensis, the insect Chironomus sancticaroli, and the oligochaetes Allonais inaequalis and Allonais chelata. Assays were conducted under controlled laboratory conditions using five concentrations per compound. Mixture experiments were carried out with the most sensitive species from each biological group (S. rondoniensis, C. sancticaroli, and A. inaequalis). Organism sensitivity was compared with that of temperate organisms using Species sensitivity distributions (SSDs) curves. Risk characterization was performed based on environmental concentrations reported in Brazilian surface waters. IMI was considerably more toxic than TEB to all species, except for C. silvestrii, which exhibited higher tolerance to both compounds. SSD distributions showed that temperate species are more sensitive to both pesticides. Mixture exposure induced synergistic interaction primarily at low concentrations of IMI in C. sancticaroli and S. rondoniensis. Synergism was observed at environmentally relevant concentrations, while antagonism occurred at higher doses of IMI. These findings underscore the ecological risks posed by pesticide mixtures in freshwater ecosystems and emphasize the importance of risk assessment frameworks that consider native biodiversity.