Monarch butterfly declines reported in Boyle et al. (2019) are biased by unexamined changes in museum collections over time

Museum records provide an underutilized source of information for documenting long-term changes in phenology, species interactions, and trait evolution. However, non-systematic collection data must be treated carefully if they are to approximate abundance, as trends may be confounded with spatial or temporal changes in sampling effort. Boyle et al . (2019b) argue that the relative abundance of Eastern North American Monarch butterflies ( Danaus plexippus ) has been in a long-term decline since the mid-20th century, following a similar decline in milkweed ( Asclepias spp.) herbarium records. I demonstrate that this reported abundance trend is biased by the choice to standardize Monarch records as a proportion of all Lepidoptera collected. The sampling of Lepidoptera has changed systematically over time to favor moths, causing the apparent trend in Monarch records. With the data standardized more appropriately, I show that the trend in Monarch records shows no mid-century decline and increases over recent decades. As the trend in Monarch museum specimens contradicts the recent trend in Monarch abundance documented from systematic population monitoring, I argue that these records are unreliable for abundance estimates. The conclusion in Boyle et al . (2019b) that Monarch declines started in the mid-20th century is unwarranted both because the trend is biased by sampling changes in museum records and because the trend in Monarch records, when corrected, does not correspond with real-world population abundance.