Monarch butterfly numbers have taken a nosedive. There were reports about this all summer, and a recent report over at the Texas Butterfly Ranch blog.
The caterpillar host plants of monarchs are milkweeds of various species. With the herbicide resistant crops, the milkweeds that were once common in the rows between crops are gone. I suspect we are mainly talking about common milkweed, Asclepias syriaca, and possibly blunt-leaved milkweed, A. amplexicaulis. Whorled milkweed, A. verticillata was probably also present in some numbers, but I have my doubts about it as a host plant for monarchs.
Common milkweed is still present in the ditches around Iowa, and is easy to find. But the total number of plants has to be way down. How much area is in the thirty foot line around the perimeter of the field, compared to the area of the field?
Monarch habitat may be easier to maintain than habitat for more stationary butterflies. If you do something to the habitat in the winter–mow too closely, burn, or something similar, you might eliminate other species altogether. Monarchs repopulate each year from populations that overwinter in the south, and might survive habitat modifications that wipe out others.