Conclusions
A better understanding of the effects of multiple stressors, particularly the complex interactions between pesticide exposure and resistance for host-pathogen outcomes, is critical for effective pest and vector control strategies and the protection of important insect pollinators . Here, we identified antagonistic interactions between Bt and OP that mitigated the effects of pesticides on fitness-related host traits. Our results suggest that both pesticides and Bt individually influence important demographic parameters like development time and mortality rate, but that co-exposed hosts may actually perform better than those exposed to pesticides alone. We also observed dampened expression of important candidate genes upon dual exposure to OP andBtt and in Pyr-resistant individuals exposed to Bttcompared to the main effect of Btt exposure. Are these within-host interaction effects sufficient to alter the dynamics of disease in insect populations? Mathematical models would be useful to address this question, as the answer will likely depend on feedbacks from ecological variables like intraspecific competition, relative transmission among single- and co-exposed hosts, and the relative costs of pesticides and infection to host reproductive rates . Moreover, our data suggest that the rise of pesticide resistance within populations could reduce the magnitude of the interaction effects, likely altering population outcomes. Overall, our results shine light on the physiological and evolutionary processes that shape interactions among two important ecological stressors and should prompt further study of their interactions at multiple levels of biological organization on ecological and evolutionary time scales.
Acknowledgements: We would like to thank three anonymous reviewers for comments that helped improve previous versions of the manuscript. We would like to thank William Galardi for assistance in preliminary dose response experiments and Siqin Liu for assistance with population maintenance. We are also grateful to Justin Critchlow and Justin Buchannan for guidance with in vitro bacterial growth curves. Last, we would like to thank Juan Luis Jurat-Fuentes for comments on the manuscript. This work is funded by USDA NIFA 2019-67012-29659 provided to S.S.L.B. N.K.E.S. and A.T.T. were supported in part by Alfred P. Sloan Foundation Fellowship FG-2020-12949 to A.T.T.