Effects of a Thiamethoxam-Based Insecticide on the Life History of Chironomus xanthus

Thiamethoxam (TMX) is one of the most widely used neonicotinoid insecticides for pest control in agricultural crops. However, information on its acute and chronic toxicity for freshwater non-target insects is still limited. We aimed to evaluate the lethal and sub-lethal effects of TMX-based insecticide on Chironomus xanthus using laboratory ecotoxicological assays. Besides survival, 28-day partial life-cycle tests assessed the effects of chronic exposures on larval growth and emergence of midges. The estimated 48-h LC50 was 32g a. i./L of TMX. Chronic tests revealed that chironomids exposed to sub-lethal concentrations of TMX showed reduced larval growth (LOEC=0.4 and 1.6g a.i/L of TMX for head capsule width and growth rate, respectively) and emergencerate (LOEC of 1.6g a.i/L of TMX). This study provides important ecotoxicological data concerning effects of TMX-based insecticides on tropical ecosystems and reveals that environmentally relevant concentrations of TMX can adversely affect chironomids populations. Our study also validates C. xanthus as a sensitive species to monitor the ecological integrity of tropical aquatic systems near intensive agricultural areas and to help understand the effects caused by TMX on freshwater insects.