Imidacloprid dominates the combined toxicities of neonicotinoid mixtures to stream mayfly nymphs

• Published in: The Science of the total environment 2021-03, Vol.761, p.143263, Article 143263
• Main Authors: Macaulay, Samuel J., Hageman, Kimberly J., Piggott, Jeremy J., Matthaei, Christoph D.
• Format: Article
• Language: English
• Subjects: Animals; Clothianidin; Contaminants; Ephemeroptera; Humans; Insecticides; Insecticides - toxicity; Neonicotinoids - toxicity; Nitro Compounds - toxicity; Pesticides; Rivers; Thiamethoxam; Toxicity; Water Pollutants, Chemical - toxicity
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.143263

Contamination of the environment with toxic chemicals such as pesticides has become a global problem. Understanding the role of chemical contaminants as stressors in ecological systems is therefore an important research need in the 21st century. In surface freshwaters, mixtures of neonicotinoid insecticides are being detected around the world as more monitoring data become available. Combinations of imidacloprid, clothianidin and thiamethoxam are commonly found, but studies testing their combined toxicities to freshwater invertebrates are rare. Taking a multiple-stressor approach, we employed a full-factorial design to investigate the individual and combined chronic toxicities of these three neonicotinoids in a 28-day laboratory experiment using Deleatidium spp. mayfly nymphs. Imidacloprid (1.2 μg/L achieved concentration) reduced mayfly survival (by 50% on Day 28) and mobility (~100%) more than clothianidin (1.1 μg/L, affecting about 25% of individuals across the responses measured) and thiamethoxam (2.9 μg/L, affecting 12%). Imidacloprid interacted with the other two neonicotinoids to cause a greater-than-additive negative effect when combined until 25 days of exposure, after which the strong negative overall effects of imidacloprid prevented these interactions from being observed. Our findings represent a novel contribution to multiple-stressor research by demonstrating the combined effects of chronic exposure to environmentally relevant neonicotinoid concentrations on an ecologically important stream insect taxon. These results emphasise the higher toxicity of imidacloprid to non-target freshwater insects compared to clothianidin and thiamethoxam, implying that stricter regulation to control the use of imidacloprid may need to be prioritised to protect vulnerable aquatic insect populations that provide key links to terrestrial food webs. Finally, our study provides an ecological, multiple-stressor comparison for related ecotoxicological investigations indicating neonicotinoid mixtures can deviate from additive toxicity. [Display omitted] •A multiple-stressor assessment of chronic neonicotinoid mixture toxicities•Environmentally relevant neonicotinoid concentrations strongly affect mayflies.•Imidacloprid exerts stronger chronic toxicity than clothianidin and thiamethoxam•Imidacloprid amplifies effects of clothianidin and thiamethoxam in mixtures.