This article from Yale University's environmental news discusses some of the causes, including the disappearance of milkweeds due to use of for herbicides tailored for GMOs:
"A new census found this winter’s population of North American monarch butterflies in Mexico was at the lowest level ever measured. Insect ecologist Orley Taylor talks to Yale Environment 360 about how the planting of genetically modified crops and the resulting use of herbicides has contributed to the monarchs’ decline."
Some of this censusing is done through the Monarch Watch tag and release project, which our class took a part in, catching only and tagging fewer than usual again this year:
At the end of collecting this past week I grabbed some nice dry pods just splitting open and gave them a good shake - POOF!
Take it easy easy down there in Mexico till next year, flutterbys!