Insecticides, more than herbicides, land use, and climate, are associated with declines in butterfly species richness and abundance in the American Midwest

Abstract

Mounting evidence points to a global collapse in insect populations, with some of the best documented evidence among butterfly species. While habitat loss, climate change, and agricultural pesticides have all been implicated, researchers have pointed to glyphosate herbicides and neonicotinoid insecticides specifically as potential causes of insect declines. But the focus on individual pesticides or individual insect species or groups fails to grapple with the complex dynamics of how agricultural practices vary and correlate through time and space. By examining a comprehensive set of insecticide and herbicide classes from thousands of farm records over the years 1998-2014 and linking them with systematic butterfly abundance surveys across 60 Midwestern U.S. counties, this research disentangles correlated pesticide effects across butterfly species. For the median county in our sample, we find an 8% decline in total butterfly abundance due to changing pest control systems, with declines pronounced among forb-hosted specialist species including monarchs, for which populations declined 33.5% relative to predictions under historic pest control technologies. Neonicotinoid-treated seeds bear primary responsibility, while there appears to be no consistent negative effect from crop seed that incorporate Bacillus thuringiensis (Bt) proteins nor from glyphosate herbicide. Better public data are needed on pesticides applied to crop seed in order inform policy about environmental safety.

Advancing bee conservation in the US: gaps and opportunities in data collection and reporting

“…For example, occupancy models, which can be powerful for detecting trends while accounting for some of these issues, have only recently begun to be developed for bee species (Graves et al, 2020;Otto et al, 2021, Otto et al, 2023Boone et al, 2023a;Boone et al, 2023b), and the data needed to calculate these models are not often collected in routine field surveys. Recent research from other groups of insects (especially butterflies) highlights declines in broadly distributed, formerly abundant species (e.g., Wepprich et al, 2019;Forister et al, 2021;Van Deynze et al, 2022;Forister et al, 2023). If similar declines are occurring in the wild bee fauna, as available assessments suggest, they are potentially going unnoticed.…”

Advancing bee conservation in the US: gaps and opportunities in data collection and reporting

“…Recent research from other groups of insects (especially butterflies) highlights declines in broadly distributed, formerly abundant species (e.g. Wepprich et al, 2019;Forister et al, 2021Forister et al, , 2023Van Deynze et al, 2022). If similar declines are occurring in the wild bee fauna, as available assessments suggest, they are potentially going unnoticed.…”