Toxicity and Hazard Assessment of Neonicotinoid Insecticides to Aquatic Invertebrates

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Raby, Melanie
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University of Guelph

Neonicotinoid insecticides are a group of water-soluble pesticides commonly used in agriculture. Their transportation to nearby waterways presents the potential for toxicity to non-target aquatic invertebrates—especially aquatic insects. The lack of toxicity data for developing water quality guidelines has been highlighted as a key knowledge gap by jurisdictions including Canada. The objective of this thesis, therefore, was to generate toxicity data for six neonicotinoids (acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, thiamethoxam) for a suite of aquatic invertebrates under different exposure scenarios: acute (2- to 4-day), pulse (24-hour, then chronic recovery), and chronic (7- to 56-day). These data were used to evaluate the potential hazard of neonicotinoids to aquatic invertebrates in a southern Ontario context. As neonicotinoids are commonly applied with other pesticides, toxicity of imidacloprid was also examined in a binary mixture with the fungicide, tebuconazole. Acute toxicity data generated for ≥ 20 aquatic invertebrates showed the laboratory-cultured insects Chironomus dilutus and Neocloeon triangulifer were generally most sensitive to neonicotinoids, with LC50s < 12 µg L-1. Hazard assessment showed a moderate acute hazard to aquatic invertebrates for imidacloprid; for other neonicotinoids, no or low hazard was concluded. Single-pulse exposures showed C. dilutus and N. triangulifer were immobilized after 24-hour exposure to 9 µg L-1 imidacloprid but recovered when transferred to clean water with no long-term effects on emergence. Chronic toxicity data for C. dilutus and N. triangulifer showed effects on survival, growth, and emergence at concentrations < 2 μg L-1 for most neonicotinoids. Ceriodaphnia dubia was several orders of magnitude less sensitive compared to insects. Chronic hazard assessment concluded a moderate hazard to aquatic invertebrates for clothianidin, and a high hazard for imidacloprid. Binary mixtures of imidacloprid and tebuconazole showed no conclusive synergistic effects; only effects on Hyalella azteca deviated from additivity (independent action model) and were dose-ratio antagonistic. Tebuconazole was less potent than imidacloprid; when combined with the lack of synergistic effects, it was concluded that tebuconazole does not pose a greater acute hazard to aquatic invertebrates than imidacloprid alone. Data presented in this thesis will contribute to developing water quality guidelines and risk assessments in Canada and abroad.

pesticides, aquatic invertebrates, aquatic toxicology