While I have ready plenty of articles that attribute the lesser population of Monarchs' in Michoacan to deforestation in Mexico, I thought that the following article brings some interesting information to the table:
http://www.kansascity.com/2011/07/26/3038300/disappearing-monarchs-have-an.html"The monarchs’ numbers have been cut in half in recent years, some researchers say, and they put much of the blame on hardier farm crops here in the Midwest.
Monarchs lay their eggs on milkweed plants. But thanks to genetically modified corn and soybeans that withstand herbicides, farmers can now wipe out milkweed from their land without damaging their crops.
That means monarchs can no longer find milkweed on 100 million acres of farmland.
Combine that loss with deforestation in Mexico, where monarchs winter, and you have a vanishing species...................................................................