This was the result of an extensive study conducted by Jack Boyle, a Mellon Postdoctoral Fellow of Environmental Science and Policy at College of William & Mary. Boyle and team's study is published in the Proceedings of the National Academy of Sciences of the USA (PNAS).
The researchers gathered the digital records from museums and herbaria throughout the North America and analyzed the relative abundance of both monarchs and milkweeds for over a century (1900-2016). They found that both monarchs and milkweeds were highly abundant in the early 20th century and the recent declines allegedly attributed to herbicide tolerant crops were actually part of a declining trend observed to begin at around 1950.
The paper concluded that herbicide resistant crops 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.