Pesticides are ubiquitous in freshwater habitats and may alter host-pathogen interactions, ultimately influencing the distribution of pathogens across a landscape. Pesticide exposure of amphibians, which occurs mainly in the aquatic stage, could lead to differential effects on infection prevalence by a chytrid fungal pathogen (Batrachochytrium dendrobatidis [Bd]) across host life stages. Previous research suggests amphibians exposed to pesticides as larvae can face a decreased risk to Bd in the aquatic stage due to direct negative effects on the fungus, and an increased risk to Bd in the terrestrial stage caused by increased susceptibility to the pathogen. We predict that in the aquatic stage, infection prevalence will be negatively associated with pesticide use, and in the terrestrial stage, infection prevalence will be positively associated with pesticide use. To test these predictions, we use species distribution models and multimodel inference approaches to assess the influence of 1) total pesticide use, 2) pesticide use by type (herbicide, insecticide, fungicide), and 3) the most commonly used pesticide compounds on Bd infection prevalence in amphibian populations across life stages throughout the contiguous United States, controlling for the influence of environmental and biotic factors.
These models reveal that infection prevalence is negatively associated with total pesticide use in the aquatic stage but positively associated in the terrestrial life stage, which is mainly driven by the effect of herbicides. Based on our findings, we suggest that early-life exposure of amphibians to pesticides in the aquatic stage can reduce the likelihood of infection in larval hosts but can cause latent negative effects on host susceptibility in the terrestrial stage.