|Graph of returning migration Monarch Butterflies from MonarchWatch.org|
CBS quoted entymologist Lincoln Brower: "The main culprit," he wrote in an email, is now genetically modified "herbicide-resistant corn and soybean crops and herbicides in the USA," which "leads to the wholesale killing of the monarch's principal food plant, common milkweed."
The website MonarchWatch.org has the best in-depth analysis of the triple threat of habitat loss. What to do? Plant milkweed!