Mixing it up: The role insecticide mixtures in aquatic wetlands

Background/Question/Methods The global use of insecticides is critical to disease control and efficient crop production, but they may also pose unintended consequences to non-target organisms. Aquatic systems are exposed to a variety of individual pesticides that vary in breakdown rates and toxicities. More often, however, aquatic systems are exposed to complex mixtures of pesticides and at relatively low concentrations (<1 ppm). Given the diversity of amphibians and their life histories, it is imperative to consider the effect of mixtures on multiple species of amphibians. We used aquatic mesocosms to assess the impact of four common acetylcholine-esterase inhibiting insecticides (chlorpyrifos, diazinon, endosulfan, malathion) applied individually at nominal concentrations of 10 and 40 ppb and as a mixture using both an additive and substitutive experimental design. This design allowed us to address the direct and indirect impact of insecticides as they are applied individually and as a mixture. Over a 5-month period, we quantified the abundances of 16 taxa (zooplankton, phytoplankton, periphyton, and larval amphibians) and abiotic conditions (pH, temperature, light attenuation, dissolved oxygen). Results/Conclusions The direct and indirect effects of separate and combined insecticides were species- and insecticide-specific. When applied individually, endosulfan was lethal to larval toads, wood frogs, and leopard frogs with 90-100% mortality, malathion was lethal to gray tree frogs with 52% mortality, but all four insecticides were similarly lethal to peepers at 29-55% mortality. The mechanisms behind these effects also varied. Endosulfan caused amphibian mortality via direct toxic effects whereas malathion caused amphibian mortality via an indirect effect through the food web. Diazinon and chlorpyrifos had no direct nor indirect significant effects on amphibians but they did dramatically alter the communities by suppressing cladocerans and initiating an indirect chain of events that affected the phytoplankton and periphyton. Amphibian mortality associated with mixtures was also species-specific. Mixtures killed 87-90% toads and leopards and 60-65% of peepers and wood frogs. Interestingly, gray tree frogs, which were added four weeks after the other anuran species to simulate their natural breeding phenology, only experienced 2% mortality. Further, gray tree frogs emerged significantly larger in these mixture treatments compared to the control. Thus, insecticides applied individually and as mixtures have unique impacts on aquatic communities that must be considered for more effective conservation efforts.