Anthropogenic activities can have significant ecological and evolutionary consequences on populations in natural systems. In the Midwest, neonicotinoid insecticides are widespread across the agricultural landscape and frequently detected in stream systems. The effects on heptageniidae mayflies is a major concern because they are highly sensitive to neonicotinoids and have some of the lowest reported EC50 values of any organism. Here we conducted EC50 tests on populations of heptageniids across a gradient of agricultural landcover to assess patterns of tolerance to the neonicotinoids clothianidin and thiamethoxam. Additionally, we collected water samples to assess temporal patterns of neonicotinoid presence in stream habitats.
Results/Conclusions
We found variation in neonicotinoid tolerance with EC50 values from 4.9 µg/L to 32 µg/L and 19.8 µg/L to 86.5 µg/L for clothianidin and thiamethoxam, respectively. Stream water samples indicated peak insecticide concentrations of 0.43 µg/L for clothianidin and 0.07 µg/L for thiamethoxam during rain events. However, agricultural landcover was not associated with neonicotinoid tolerance. Moreover, water samples demonstrated that the amount of agricultural landcover was not a strong predictor of neonicotinoids presence in the stream. Our data suggest that populations of heptageniidae mayflies can vary substantially in neonicotinoid tolerance. As mayflies are widely used in rapid bioassessment protocols, an important next step is determining whether these patterns are consistent with evolved tolerance.