Genetically engineered varieties and applied pesticide toxicity in U.S. maize and soybeans: Heterogeneous and evolving impacts

The extensive adoption of genetically engineered (GE) varieties in U.S. agriculture has dramatically changed the patterns of pesticide use. How this process ultimately affects environmental risk remains an open question. Previous studies have typically relied on aggregate trends to infer the impact of GE crop adoption on pesticide use, which fails to address selection bias and unobserved heterogeneity. We overcome this limitation by analyzing applied pesticide toxicity using farm-level fixed effects models, estimated with rich plot-level data on >200,000 seed and pesticide choices by U.S. maize and soybean farmers during the 1998–2016 period. We find that applied toxicity was, on average, lowered by the adoption of GE varieties across four target organism groups: mammals, birds, fish, and honey bees. However, most of the toxicity benefits conferred by GE adoption dissipated over time. For herbicide tolerant varieties, this was due to the increased use of old-line herbicides by GE adopters, a likely consequence of the growing problem of glyphosate weed resistance. Applied honey bee toxicity saw the sharpest increase during the GE era, but most of this increase was driven by the adoption of neonicotinoid seed treatments, rather than GE insect resistant traits.