Monarch butterfly (Danaus plexippus) decline over the past 25 years has received considerable public and scientific attention, in large part because its decline, and that of its milkweed (Asclepias spp.) host plant, have been linked to genetically modified (GM) crops and associated herbicide use. Here, we use museum and herbaria specimens to extend our knowledge of the dynamics of both monarchs and milkweeds in the United States to more than a century, from 1900 to 2016. We show that both monarchs and milkweeds increased during the early 20th century and that recent declines are actually part of a much longer-term decline in both monarchs and milkweed beginning around 1950. Herbicide-resistant crops, therefore, are clearly not the only culprit and, likely, not even the primary culprit: Not only did monarch and milkweed declines begin decades before GM crops were introduced, but other variables, particularly a decline in the number of farms, predict common milkweed trends more strongly over the period studied here.

We are pleased to see Wepprich (1) and Ries et al. (2) engaging with the museum records data presented in our original study (3). One of the strengths of digitized specimen data is that its portability allows precisely this kind of reanalysis.The main point of our original study was that genetically modified (GM) crops were unlikely to be an important factor in monarch declines because milkweed and monarchs began their declines decades before the introduction of GM crops (3). Both Wepprich (1) and Ries et al. (2) propose alternative methodologies for using museum data to calculate monarch butterfly abundance over time, and both suggest that the currently available data are insufficient to estimate the true monarch abundance trends over the past century. If Wepprich (1) and Ries et al.(2) are correct that monarch abundance over the 20th century is still unknown (we discuss the specifics of their proposals below), this does not change our primary conclusion. Farming of GM crops has been proposed to impact monarch populations by the effect of GM-associated herbicides on the monarch's milkweed host plants (e.g., refs. 4-7). However, the decades-long declines we show across milkweed species, both on and off agricultural fields, do not support this hypothesis, and these declines in milkweeds are not questioned by either Wepprich (1) or Ries et al. (2). Milkweed trends are likely more robust than monarch trends for 2 reasons: First, there are about 30 times more milkweed specimen records than there are monarch records. Second, the question of what taxonomic group comprises an appropriate comparison group appears to be less fraught in plants, probably because botanists show less taxon specificity in collection methods than do entomologists. As long as milkweed declines begin decades before GM crops, this is strong evidence against the milkweed-limitation hypothesis driven by GM crops, regardless of when monarch declines began.Turning to the specifics of their approaches, both Wepprich (1) and Ries et al.(2) argue that it is inappropriate to use the number of Lepidoptera specimens to estimate monarch collection effort. They note that Lepidoptera includes both butterflies and moths, 2 sets of organisms with very different life histories. We find this argument plausible, and the effect shown by Wepprich (1) is an important consideration for future studies using these kinds of data. In theory, many comparison groups are possible: smaller ones (e.g., a sister species, or members of the same genus) include species that are most similar to the focal species, sharing similar trait-and taxon-specific collection biases; but using a small number of species will produce more variable estimates of collection effort. Using larger comparison groups (class, phylum, etc.) will reduce this variance but will necessarily include less similar organisms. The proper comparison group is thus a tradeoff, and it is not possible to determine a "correct" or "appropriate" comparison group a priori. When possible, a good comparison group may be de...