COMPREHENSIVE CHARACTERIZATION OF THE ACUTE AND CHRONIC TOXICITY OF THE NEONICOTINOID INSECTICIDE THIAMETHOXAM TO A SUITE OF AQUATIC PRIMARY PRODUCERS, INVERTEBRATES, AND FISH.

Abstract

Thiamethoxam is a neonicotinoid insecticide used widely in agriculture to control a broad spectrum of chewing and sucking insect pests. Recent detection of thiamethoxam in surface waters has raised interest in characterizing the potential impacts of this insecticide to aquatic organisms. Here we report the results of toxicity testing (acute and chronic) conducted under good laboratory practices (GLP) for over 30 freshwater species (i.e., insects, molluscs, crustaceans, algae, macrophytes, fish) and four marine species (an alga, a mollusc, a crustacean, and a fish). As would be anticipated for a neonicotinoid, aquatic primary producers and fish were the least sensitive organisms tested, with acute median effect concentrations (LC50 / EC50) observed to be ≥ 80 mg/L in all cases, which far exceeds surface water exposure concentrations. Tested molluscs, worms, and rotifers were similarly insensitive (EC50 ≥ 100 mg/L), except for Lumbriculus sp., with an EC50 of 7.7 mg/L. In general, insects were the most sensitive group in the study, with most acute EC50 values < 1 mg/L. However, the crustaceans Asellus aquaticus and Ostracoda exhibited a sensitivity similar to insects (acute EC50 < 1 mg/L), while the midge larvae Chaoborus sp. were relatively insensitive compared to other insects (EC50 > 5.5 mg/L). The most sensitive chronic response was for Chironomus riparius with a 30-d NOEC (emergence) of 0.01 mg/L. Observed toxicity to the tested marine organisms was comparable to that of freshwater species. We used the reported data to construct species sensitivity distributions for thiamethoxam, calculate HC5s for acute data (freshwater invertebrates), and compared these to measured concentrations from relevant North American surface waters. Overall, based on acute toxicity endpoints, the potential acute risk to freshwater organisms was found to be minimal (i.e., likelihood of exceeding HC5s < 1%)This article is protected by copyright. All rights reserved.