Monarch butterflies and their principal host plant, the milkweed, have been
declining in abundance even before GM crops were planted. 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.