The role of fungicide exposure in Daphnia-Metschnikowia disease dynamics

Background/Question/Methods

Fungicides, an increasingly used pesticide for disease management in corn and soybean fields, commonly enter freshwater habitats as runoff during rainfall and flooding events. Runoff of two particular fungicides, pyraclostrobin and azoxystrobin, is known to increase mortality in Daphnia, a planktonic crustacean found in Midwestern lakes, though its impacts to disease dynamics in Daphnia is largely unstudied. Daphnia dentifera is a known host for the fungal pathogen, Metschnikowia bicuspidata. I hypothesize that, at low doses, pyraclostrobin and azoxystrobin disrupt Metschnikowia-Daphnia disease dynamics by eliminating fungal spores from aquatic environments. From this, I predict that when there is runoff of these fungicides into lake ecosystems at sub-lethal concentrations in Daphnia, there will be a decreased parasite burden of Metschnikowia spores in the water column and thus delayed epidemic emergence with lower infection levels seen in D. dentifera. These predictions are being tested with a combination of field sampling and laboratory experiments. Field sampling is being used to quantify concentrations of these pesticides in four lakes at different times of the year. Laboratory life-tables, with multiple host genotypes known to differ in fitness traits such as susceptibility to the fungal pathogen, are elucidating the balance between these competing stressors: fungus and fungicide.

Results/Conclusions

When three genotypes were exposed to a common home garden fungicide with the active ingredient chlorothalonil at three different doses, 6.58x10-5,6.58x10-3,and 6.58x10-1,all three genotypes saw a statistically significant difference in mortality across doses, but no significant difference in mortality between infected and uninfected individuals across doses for two of the three genotypes. These results invite further experimentation with agriculturally-relevant fungicides used at large scales. Ongoing experiments include laboratory life table analysis and an outdoor mesocosm experiment with simulated runoff using pyraclostrobin and azoxystrobin. These data will provide a foundational and ecologically-relevant understanding of changes to population dynamics and the scope of epidemics in natural populations of D. dentifera facing a Metschnikowia epidemic in the presence of fungicide runoff, highlighting important changes and trade-offs at the individual and population level. As an important component of the lake food web, the health and stability of Daphnia populations serves as an indicator of lake ecosystem health. Understanding the interactions between anthropogenic stressors and Daphnia can help us protect lake ecosystems and the important functions they serve.