COS 105-2 - Fertilizer increases the rate of evolved pesticide tolerance in freshwater copepod populations

Abstract: In agricultural settings, fertilizers and pesticides are commonly applied in tandem with the goal of maximizing crop yield. Fertilizers act as an enriching agent, providing valuable nutrients to crops. Pesticides are applied to control pests that threaten crop species. We hypothesize fertilizer use provides nutrients to pest species increasing their reproductive output, thus accelerating the evolution of tolerance to pesticides. To test this hypothesis, we conducted a 14-month freshwater mesocosm experiment crossing two levels of pesticide (control and chlorpyrifos insecticide) and two levels of nitrate- and phosphate-based fertilizer (low N+P vs. high N+P), focusing on the model herbivore-producer system of freshwater copepods and algal resources. To quantify the rate of chlorpyrifos tolerance development in copepods, we repeatedly sampled copepods from mesocosms and conducted time-to-immobilization toxicity tests. As hypothesized, we found that copepods exposed to high N+P developed chlorpyrifos tolerance at a significantly faster rate than copepods exposed to low N+P. We are in the midst of processing genomic samples to identify the genomic changes associated with insecticide tolerance. Additionally, we are conducting genetic metabarcoding analyses to determine whether copepod community composition differs significantly among the pesticide and fertilizer treatments. This will allow us to discern whether the increased rate of tolerance development in fertilizer treatments is a result of 1) changing copepod community composition (i.e. a reduction in the abundance of less-tolerant species) or 2) increased rate of evolved tolerance within copepod populations. Regardless of the outcome of these genomic analyses, our findings support the hypothesis that fertilizers can reduce the effective lifespan of pesticides by accelerating tolerance development.