Rising insecticide potency outweighs falling application rate to make US farmland increasingly hazardous to insects

Each year, millions of kilograms of insecticides are applied to crops in the US. While insecticide use supports food, fuel, and fiber production, it can also threaten non-target organisms, a concern underscored by mounting evidence of widespread insect decline. Nevertheless, answers to basic questions about the spatiotemporal patterns of insecticide use remain elusive, due in part to the inherent complexity of insecticide use, and exacerbated by the dispersed nature of the relevant data, divided between several government repositories. Here, we integrate these public datasets to generate county-level annual estimates of total ‘insect toxic load’ (honey bee lethal doses) for insecticides applied in the US between 1997-2012, calculated separately for oral and contact toxicity. To explore the underlying drivers of the observed changes, we divide insect toxic load into the components of extent (area treated) and intensity (application rate x potency). We show that while contact-based insect toxic load remained relatively steady over the period of our analysis, oral-based insect toxic load increased roughly 9-fold, with reductions in application rate outweighed by disproportionate increases in potency (toxicity/kg) and increases in extent. This pattern varied markedly by region, with the greatest increases seen in Heartland and Northern Great Plains regions, likely driven by use of neonicotinoid seed treatments in corn and soybean. In this “potency paradox,” US farmland has become more hazardous to insects despite lower volumes of insecticides applied, raising serious concerns about insect conservation and highlighting the importance of integrative approaches to pesticide use monitoring. Significance statement Previous analyses disagree about whether US insecticide use is increasing or decreasing, a question of significant importance given the putative role of insecticides in recent insect declines. We integrated information from multiple national databases to estimate ‘insect toxic load’ (represented as honey bee lethal doses) of the agricultural insecticides applied in each US county from 1997 to 2012, and factors responsible for its change. Across the US, insect toxic load – calculated on the basis of oral toxicity – increased 9-fold. This increase was due to increases in the potency (toxicity/kg) of insecticides applied and in the area treated; the volume of insecticides applied declined. Toxic load increased most dramatically in regions where neonicotinoid seed treatments for field crops are commonly used.