Data underlying Figs. 1a,b and Fig. 2. Data on monarch abundances from North American Butterfly Association counts from 1977 to 2017. Data on monarch museum specimens extracted from dataset posted to Dryad repository: Boyle JH, Dalgleish HJ, Puzey JR (2019) Data from: Monarch butterfly and milkweed declines substantially predate the use of genetically modified crops. Dryad Digital Repository. https://doi.org/10.5061/dryad.sk37gd2.