As far as I am concerned, Monarch numbers have dropped for two reasons:
First, and most significantly, the deforestation of the wintering sites in Mexico.
And second, the removal a milkweed from the landscape, which is indeed a byproduct of GMO farmers ability to aggressively spray their crops a couple of time a season. Butterfly larve can only feed on Milkweed, and for that reason females will either find one to lay their eggs on or die trying.
The correct response to the second should be encouraging people to plant more ornamental milkweed in their gardens, and this has been happening over the past few years.