Exposure to the herbicide 2,4-D exacerbates infection outcome of a global amphibian pathogen

Exposure to multiple environmental stressors threatens amphibian survival globally. While infections and pesticides are capable of driving declines, interactions may exacerbate their impact. Here, we study experimental exposure of painted frogs (Discoglossus pictus) to a commonly used herbicide (2,4-D) and/or to a presumedly highly (JEL423) or low (BdBE3) virulent strain of the global panzootic lineage of Batrachochytrium dendrobatidis (BdGPL), a pathogenic fungus involved in amphibian declines and extinctions. Inoculation of both tadpoles and metamorphs with JEL423 impaired growth and reduced body condition of metamorphs, while inoculation with BdBE3 resulted in higher level infections that only reduced metamorph growth when inoculated at the metamorph stage. A higher pathogen burden at a lower cost for the anuran host corroborates the hypothesis of low virulent endemic BdGPL strains (BdBE3) versus highly virulent exotic strains (JEL423). Exposure of tadpoles to 2,4-D shortly before metamorphosis reduced froglet size at metamorphosis in a dose-dependent manner. Co-exposure to both 2,4-D during the tadpole stage and the BdGPL strains resulted in significant loss of body condition in the low virulent BdBE3 inoculated frogs, bringing its negative impact on par with that observed after inoculation with the highly virulent JEL423 strain. We demonstrate how the use of herbicides may exacerbate the impact of endemic infections on early life stage amphibians.