The drought certainly fits, but the linkage to herbicide tolerant crops makes no sense. First of all, those have been around for 16 years now and in large numbers for at least 12. Before there where herbicide tolerant crops there were selective herbicides used so weed control was already good. Herbicides have been in use since the 1960s. Before that weeds were controlled with mechanical means. Milkweed still thrives outside of fields and at field margins - but not in severe drought years. I'm actually amazed the drop was as small as 59% considering conditions over so much of the US last summer

Rachel Carson, please come back.

I suggest that we build corridors for butterflies, bees, and migrating birds of non-GMO plants...plant insect-friendly plants in our backyards and on boulevards and help out our little critter friends.

The monarchs have been kidnapped and imprisoned at the Minnesota Zoo Butterfly Gulag. If you ever want to see them alive again, deliver five hundred pounds of Elephant food to the zoo, or we will feed the butterflies to the black billed groesbeaks.

Plant native milkweed between here and Mexico along every single highway and roadway where it's possible to grow plants.

I believe the culprit is windmills. They're taking out both bald eagles and monarchs.

Again...nobody talking about the impact of human population growth in the USA.

At least they're not trying to blame it on global warming. (That scheme worked re: the reduction of songbirds -- until we learned that the real reason was feral cats.) I agree that the huge Midwestern drought was the most likely reason. Next year the numbers will be up and the alarmists will have to find some other crisis to headline.

Loss of habitat too. Family farms are being sold and tilled under making way to plant more crops. Good for farmers and hungry Americans but bad for the animals that lived there. There are fewer places for the little critters to stop and rest along the way.

The monarch population in Mexico has been dropping for over a decade, so yes it is related to the use of genetically modified corn (and it does affect plants on the fringes of fields). It is also related to the increased use of marginal land for crop production - land that wouldn't be tilled if it weren't for federal insurance subsidies that ensure the farmer can't lose--so why not? Those areas no longer contain milkweed that have long supported monarchs. Another big factor is corn production for ethanol. There's no good reason to use corn for ethanol - it's inefficient, only works because it's subsidized, hard on cars, gets lower mileage. Those subsidies (crop insurance on marginal land and ethanol)should go away NOW.