Insecticide tolerance in wood frogs influences pesticide-pathogen interactions
As human populations continue to grow, a fundamental challenge for ecologists is to understand how organisms respond to anthropogenic stressors, such as pesticides. Despite their benefits, pesticides often pose unintended consequences to non-target populations (e.g., mortality). Recent evidence, however, suggests that amphibians can respond to pesticides by developing increased tolerance. Two mechanisms for achieving pesticide tolerance include:  constitutive tolerance (i.e. pesticide tolerance is always expressed)  induced tolerance via phenotypic plasticity (i.e. exposure to sublethal pesticides early in life induces increased tolerance later in life). While pesticide tolerance may allow amphibians facing contaminated habitats to persist, an important next step is to consider whether there are costs of pesticide tolerance and whether the two mechanisms for achieving pesticide tolerance vary in the degree of costs they incur. Exposure to contaminants can compromise the ability for amphibians to initiate effective immune responses to pathogens, thus a potential cost of pesticide tolerance is increased susceptibility to pathogens. To determine whether tadpoles with constitutive vs. induced tolerance vary in their susceptibility to pathogens, we exposed six wood frog populations (three with constitutive tolerance and three with induced tolerance) to a common trematode (Echinostoma trivolvis) in pesticide-free, low pesticide, and high pesticide environments.
We examined the main effects of population type (constitutive vs. induced tolerance), carbaryl concentration (pesticide-free, low, or high), and the interaction between population type*concentration on the proportion of trematodes encysted in tadpoles using generalized linear models. We found no significant main effect of population type or carbaryl concentration on the proportion of trematodes encysted in tadpoles. However, we found a significant interaction between population type and carbaryl concentration on the proportion of trematodes encysted in tadpoles. In pesticide-free environments, the populations that have the ability to induce increased tolerance had significantly fewer trematodes relative to populations with constitutive tolerance. When exposed to low carbaryl, we found no significant differences in number of trematodes across the two types of populations. Finally, when exposed to high carbaryl, populations that have the ability to induce increased tolerance had significantly more trematodes relative to populations with constitutive tolerance. To sum, we found that susceptibility to trematodes is influenced by whether populations express constitutive or induced pesticide tolerance. Pesticides, pathogens, and their interaction are implicated as major contributors to amphibian population declines. Our work suggests that responses to insecticides can play a role in shaping amphibian responses to pathogens.