Assessing the Effects of Chronic Neonicotinoid Insecticide Exposure on Aquatic Insects using Multiple Experimental Approaches

Abstract

Neonicotinoid insecticides are among the most widely used plant protection products in industrialized agriculture, and are hypothesized to be contributing to losses in non-target insect biodiversity. The lack of toxicological data evaluating the effects of neonicotinoids restricts the ability of regulators to adequately protect sensitive insects particularly those with aquatic larval stages. Many Prairie wetlands are in regions of intensive agriculture and are directly at risk of neonicotinoid contamination and require special considerations to best protect these ecologically important areas. Under laboratory conditions, I compared the effects of imidacloprid, clothianidin, thiamethoxam on the model aquatic insect, Chironomus dilutus. Reduced emergence success, advanced emergence timing, and male-biased sex ratios were observed across all compounds tested. Imidacloprid and clothianidin reduced emergence success at similar concentrations whereas thiamethoxam required a concentration an order of magnitude greater to observe similar toxicity. Normalizing the clothianidin and thiamethoxam toxic responses to imidacloprid, I calculated acute (lethality) 14-day toxic equivalency factors (TEFs) for imidacloprid, clothianidin and thiamethoxam as 1.00, 1.05 and 0.14, respectively, and chronic (emergence inhibition) 40-day TEFs as 1.00, 1.62 and 0.11, respectively. To expand upon these single-species laboratory assessments, in situ limnocorrals were used to determine the chronic effects of the three neonicotinoids to emerging aquatic insect communities in a Prairie wetland. Imidacloprid and clothianidin treatments had similar community responses and non-biting midge (Diptera: Chironomidae) and damselfly (Odonata: Zygoptera) emerged significantly earlier than the controls (18 to 25 days earlier). An additional limnocorral study with clothianidin was inconclusive. While laboratory and limnocorral studies were useful to isolate neonicotinoid effects, multiple anthropogenic stressors were hypothesized to cumulatively influence insect emergence from natural Prairie wetlands surrounded by neonicotinoid-treated canola fields. Multivariate analysis showed neonicotinoid concentration, turbidity, vegetation disturbance and continuity of grasses were significant factors modifying the abundance and composition of emerging insects. Total insect abundance was negatively affected by neonicotinoids (β ± S.E.=-0.61 ± 0.14, P<0.001) but positively affected by vegetation disturbance (β ± S.E.=0.34 ± 0.11, P<0.001). Collectively, these data suggest more rigorous water quality guidelines and agricultural management strategies are needed to protect aquatic insects and the higher trophic organisms that rely on this important food source.