Effects of four common pesticides on the emerging chytrid fungal pathogen Batrachochytrium dendrobatidis

Background/Question/Methods

Amphibian populations are experiencing declines across the globe due to an emerging infectious disease caused by the chytrid fugal pathogen, Batrachochytrium dendrobatidis (Bd). Several stressors have been implicated in these declines such as habitat loss and global climate change, yet Bd has been singled out as a primary factor. Chemical stressors (insecticides, herbicides, fungicides) can have lethal, sub-lethal, direct, or indirect effects on amphibians. There may be interactive effects of chemical stressors on Bd susceptibility. Yet there is little known about how chemical contaminants impact Bd directly. In this study, we examine four commonly used pesticides on Bd growth in culture. We examined the insecticides carbaryl and malathion, the herbicide glyphosate, and the fungicide propiconazole in a standard toxicity test using commercial formulations of each pesticide. Bd was grown in 96 well microplates, and zoospore production was estimated over two weeks time using absorbance values via spectrophotometry. Six ten-fold dilutions of each pesticide were used to estimate the inhibitory concentration of each. Estimates are nominal values as verified concentrations were not yet available. The median inhibitory concentration (IC₅₀) for each contaminant was calculated in R version 3.0.1 using the package drc. 

Results/Conclusions

Bd was most sensitive to the insecticide carbaryl with an IC₅₀ of 148 μg/L.  For the fungicide propiconazole the IC₅₀ was 1.4763mg/L and for the insecticide malathion it was 15.809mg/L.  Bd was least sensitive to glyphosate (29.406mg/L). Overall, Bd was highly tolerant of all pesticide exposures except carbaryl as median inhibitory concentrations were in the mg/L range while environmental exposures are more typically in the μg/L range. These results suggest that exposure to these common pesticides are unlikely to significantly inhibit Bd growth in polluted waterways except under highly contaminated situations. Yet, acute toxicity tests do not capture the entire dynamic between host and pathogen. Further work testing the effects of pesticide exposure on infection rates of Bd in several species of amphibians and using other commonly used pesticides, particularly fungicides, needs to be conducted to better understand host pathogen dynamics in polluted systems.