The influence of a neonicotinoid insecticide on aquatic organisms and their interactions
Neonicotinoids are a newer generation of agricultural insecticides that are now the most widely used across the globe. Despite their efficiency at controlling pests, there are growing concerns over the hazards neonicotinoids pose to natural communities. In particular, neonicotinoids pose a significant threat to aquatic systems because of their high water solubility and low level of binding to soil. However, there is limited knowledge of the environmental fate and impacts of neonicotinoids in these systems. Using controlled laboratory toxicity experiments combined with a semi-natural mesocosm experiment, we addressed the effects of the neonicotinoid clothianidin on pond communities. We began by conducting a series of LC50 experiments on aquatic invertebrates to develop baseline information on the relative toxicity of clothianidin. Then, we conducted a 2x3 mesocosm experiment consisting of 3 clothianidin concentrations (0, 10, and 500 ppb) crossed with the presence or absence of invertebrate predators. Our predator-free communities contained vertebrate (tadpoles) and invertebrate herbivores (snails and zooplankton). The predator communities contained four species of invertebrate predators. Assuming that clothianidin is toxic to invertebrate predators, we predicted increased clothianidin-associated predator mortality, which would indirectly benefit the survival of herbivores.
Our LC50 experiments revealed dramatic differences (several orders of magnitude) in clothianidin toxicity to aquatic invertebrates. While the herbivore D. pulex displayed high tolerance to clothianidin (LC50 = 46.04 ppm), the predatory species (Anax junius, Belostoma flumineum, Orconectes propinquus, Acilius mediatus, and Lestes sp.) displayed high sensitivity (LC50 values from 0.014 to 1.87 ppm). Given that predatory invertebrates were highly sensitive to clothianidin, we used a mesocosm experiment to explore the community level effects of pesticide exposure. In the absence of invertebrate predators, clothianidin had no effect on the mortality (average = 11%) of herbivores in the community. When invertebrate predators were present, prey mortality increased to 60% in the pesticide-free treatment resulting in a 50% reduction in prey biomass. However, the predator effect on prey mortality was reduced to 30% when clothianidin (500 ppb) was present in the tanks. This effect was mainly driven by a 50% increase in the mortality of invertebrate predators in the tanks as predicted by the LC50 experiments. While our results suggest that the effects of clothianidin are density-mediated, additional research is necessary to determine whether predator behavior (i.e. trait-mediated effects) is altered at lower concentrations.