Evaluating high-throughput assays for pesticide ecological risk assessment

Evaluating pesticide ecological risk is essential for regulation, but traditional vertebrate testing is resource-intensive and ethically challenging. New approach methodologies, such as high-throughput assays (HTAs), offer cost-effective, mechanistically explicit alternatives that reduce animal use. The US EPA's ToxCast program houses HTA data for chemical screening, but its use in ecological risk assessment (ERA) remains underutilized. We applied ToxCast data directly to ERA metrics, comparing assay-derived exposure-activity ratios to in vivo risk quotients (RQs) from regulatory assessments. Uniquely, our study focuses on pesticides and is risk-focused rather than hazard-focused, leveraging data drawn directly from the same standardized regulatory risk assessments that inform decision-making. While ToxCast assays generally underestimated risks compared to in vivo RQs-particularly for chronic endpoints-certain assays, such as cytochrome P450 assays, demonstrated strong alignment for herbicides and fungicides. In contrast, assay performance was weaker for neurotoxic insecticides and herbicides targeting photosynthesis, reflecting gaps in HTA coverage for these modes of action. Our findings underscore the potential of HTAs as complementary tools for ERA, particularly for screening and prioritization, though further assay development is needed for chronic and mode-of-action-specific risks. Integrating HTA data into risk metrics lays the groundwork for promoting more accurate, efficient, and ethical approaches to pesticide evaluation.