Pesticides pose a significant threat to amphibians, particularly during their vulnerable embryonic and larval stages, as pesticides can easily reach waterways through runoff and some systemic pesticides may persist in soil for long periods of time. Neonicotinoids are a common class of systemic pesticides used in the U.S. and often have sublethal effects on amphibian immune systems, as well as growth, development, and behavior. Imidacloprid, a neonicotinoid pesticide, is extensively applied throughout the eastern U.S. to control the spread of invasive hemlock wooly adelgids (Adelges tsugae), without fully understanding the impacts on aquatic and terrestrial stages of pool-breeding amphibians. Furthermore, no attempts have been made to quantify the response of larval amphibians to an interaction of imidacloprid exposure and hydroperiod length. As hydroperiod varies widely across vernal pools and wetlands and can have direct negative effects on amphibian development and terrestrial fitness, we sought to understand the combined effects of these stressors on larval development and survival and juvenile locomotor performance and behavior of the wood frog (Rana sylvatica). We manipulated hydroperiod length (nondrying vs drying) in 1000 liter cattletank mesocosms with and without Imidacloprid exposure (at a concentration of 10 ppb) for a total of four aquatic treatments. During the larval stage, we assessed the effects of aquatic treatments on larval growth and development and survival to metamorphosis. Upon emergence, we assessed locomotor ability through endurance trials before and after 12-hour exposure to a low concentration of the pesticide. Finally, we performed behavior trials of juvenile frogs from each treatment to assess potential pesticide avoidance behavior.
Results/Conclusions
While we did not find any significant differences in morphology (mass/SVL) of metamorphs from the four treatments, larval survival to metamorphosis did vary significantly, with the pesticide x hydroperiod treatment having the lowest survival rates. Juveniles from pesticide treatments performed significantly better during initial endurance trials, however, post-exposure individuals experienced greater declines in performance compared to non-pesticide treatment individuals. We found no significant avoidance behavior of juveniles to pesticide exposure, indicating a lack of pesticide assessment ability. The biological and physiological insights from this work can be used to better understand the impacts multiple aquatic stressors have on amphibian larval development and help to better inform management decisions of the application and use of such systemic pesticides.