Chronic effects of an environmentally-relevant, short-term neonicotinoid insecticide pulse on four aquatic invertebrates

Highlights: • Four aquatic invertebrates were exposed to a pulse of imidacloprid or thiamethoxam. • Two insect species were immobilized after exposure to 9 μg L⁻¹ imidacloprid. • Exposure to thiamethoxam had no adverse effects. • Immobilized organisms recovered after placement into clean water. • No long-term effects observed relative to negative control. Neonicotinoid insecticides used in agriculture can enter freshwater environments in pulses; that is, a short-term period of a higher concentration, followed by a period of a comparatively lower concentration. Non-target aquatic arthropods are exposed to these fluctuating concentrations of neonicotinoids. The present study investigated the potential latent effects of a single environmentally-relevant 24-h pulse of imidacloprid and thiamethoxam, in separate experiments, on the early life-stages of four aquatic arthropods (Hyalella azteca, Chironomus dilutus, Hexagenia spp., and Neocloeon triangulifer). At least three nominal pulse concentrations were tested for each neonicotinoid-species combination: 2.5, 5, and 10 μg L⁻¹, which were based on environmental monitoring in Ontario, Canada. After exposure to the pulse, organisms were assessed for survival and immobilization. Surviving organisms were then moved into clean water for a chronic post-treatment period, where endpoints including survival, growth, reproduction or emergence, depending on the species, were evaluated. Immediately after the 24-h pulse, immobilization was seen in C. dilutus and N. triangulifer in the highest imidacloprid concentrations tested (8.8 and 8.9 μg L⁻¹, respectively). After transfer to clean water, immobilized organisms recovered, and no latent toxicity was seen for any of the evaluated endpoints. H. azteca and Hexagenia spp. showed no effects immediately after the imidacloprid pulse, or after the chronic post-treatment period. No effects were seen in any species after the thiamethoxam pulse, or the post-treatment period. The present study shows that toxic effects due to short-term pulse exposures of ~9 μg L⁻¹ imidacloprid can occur in sensitive insect species. However, organisms can recover when the stressor ceases, with no long-term effects on test organisms.