Since around 30 percent of the monarch habitat has been destroyed by development, conversion to row crops, and herbicide resistant crop systems, other things that may have been minor threats to the monarchs become more important.
Milkweeds that grow along the rows of corn or soybean crops would seem to be good sources of remaining habitat. But are they really? If the systemic insecticides added to seed coatings remain in the soil, do they become a part of the milkweed plant? Do they discourage female monarchs from laying eggs? Do the females lay eggs there anyway, but then do the insecticides kill the caterpillars? I don’t know the answer–maybe someone needs to research the question.
Another possibly minor thing–if you have traveled Iowa lately, you have seen the huge wind farms that have grown up.
So the windmills are known to kill a few birds here and there. I find it unlikely that they are directly killing monarchs. But what they have been shown to do is locally modify the air temperatures around them. Specifically, they even out the air temperature downwind from them.
When butterflies migrate, do they ride on little thermals resulting from uneven air temperatures? And if so, what happens to those thermals when the air becomes more uniform in temperature?