Pesticide application rates and their toxicological impacts: why do they vary so widely across the US?

Pesticide usage in the U.S. has more than doubled since 1960, raising concerns on its human and ecological health implications. The literature indicates that pesticide application rates for the same crop vary widely across geographies, while the magnitude of variation and its underlying drivers are poorly understood. Here, we present a new dataset on farm-level pesticide application for maize in the U.S. Using the dataset, we derived four human and ecological health impact metrics, (1) environmental impact quotient, (2) acute hazard quotient, (3) chronic hazard quotient, and (4) freshwater ecotoxicity, and analyzed their relationships with various climatic and biophysical factors including precipitation, growing degree days (GDD), soil conductivity, and irrigation practices. Our results show that the potential human and ecological health impact of pesticide use per unit maize harvested vary by 5-7 orders of magnitude across the 891 maize-producing counties in the U.S. All four best-fitted models are statistically significant, explaining 21% to 28% of the variations in the impact intensities across counties. Among the climatic and biophysical factors examined, GDD was the most significant variable for all four metrics. This suggests that climate change may adversely affect human and ecological health impact intensities of pesticide use for maize, which may increase 22%-471% by 2100 under the 2 degrees C warming scenario. Besides, electrical conductivity and the percentage of cropland irrigated were significant for multiple impacts. The large remaining variability unexplained by our analysis suggests that behavioral and management factors, which were not captured in our model, play a crucial role in pesticide use, calling for the interventions targeting them.