Imidacloprid, a neonicotinoid pesticide, is used to prevent the spread of the hemlock woolly adelgid, currently affecting Eastern Hemlock trees across North America. When the pesticide is sprayed directly onto soil around infested trees (soil drenching), it can run off into aquatic systems, with potential negative effects on biota. Simultaneously, climate change may lead to faster pool drying, which acts as an additional stressor for sensitive species such as amphibians. We evaluated the sublethal effects of imidacloprid (10 ppb), and interaction with shorter hydroperiods on the larval behavior, growth, and survival of a model organism, the wood frog (Rana sylvatica). We performed 3 behavioral experiments evaluating swimming speed, time spent swimming, and distance the larvae swam. We found that larvae raised in 10 ppb imidacloprid or shorter hydroperiod did not differ in their swimming time, distance, and speed from nonexposed larvae. Naïve larvae exposed for 20 min to 10-to 500-ppb concentrations also showed similar performance to nonexposed larvae. However, when we applied a stimulus halfway through each experiment, we found that larvae exposed to 10 ppb imidacloprid (short and long term) swam shorter distances and spent less time swimming, suggesting that imidacloprid exposure may slow reaction time, potentially increasing the risk of predation. To minimize impacts on pool-breeding amphibians, imidacloprid application to combat the invasive hemlock woolly adelgid should use trunk injection and avoid soil drenching.

Neonicotinoid pesticides are widely used to combat agricultural and forest pests, yet the extent of their biological effects and synergies with other stressors is not well understood, particularly for species with complex life cycles such as amphibians. While there is extensive research on the impacts of agricultural chemicals on amphibians (particularly larval stage), research on the effects of the new class of widely used neonicotinoid pesticides across life stages is lacking. In this study, we used aquatic mesocosm manipulations and terrestrial locomotor and behavior trials to evaluate demographic, behavioral, and fitness carryover effects of sublethal neonicotinoid pesticide (imidacloprid) exposure and hydroperiod length on the wood frog (Rana sylvatica), a model organism. We found that the interaction of imidacloprid exposure and shorter hydroperiod led to decreased larval survival to metamorphosis (0.54 AE 0.14, compared to control 0.75 AE 0.04). When exposed to their thermal optimum (18°C), individuals exposed to imidacloprid as larvae had higher terrestrial locomotor performance (284.08 AE 28.62 body lengths traveled) but also experienced the largest decreases in performance (147.88 AE 19.27 fewer body lengths traveled) after terrestrial imidacloprid exposure. In a 48-h substrate choice experiment, postmetamorphic frogs did not show behavioral avoidance of imidacloprid-treated substrates. Finally, we observed a skewed juvenile sex ratio from imidacloprid treatments (~10% fewer males compared to control), and we were not able to assign 15.7% of individuals from imidacloprid treatments to either sex due to ambiguous reproductive organ morphology. Our empirical assessment of carryover effects of chemical exposure and pond drying provides insights into the physiological capacity of taxa with complex lifecycles to respond to contaminants experienced at multiple life stages and informs best practices for neonicotinoid pesticide use in forest settings and conservation strategies for pond-breeding amphibians.