Monsanto Years
You never know what the future holds in the shallow soil of Monsanto, Monsanto
The moon is full and the seeds are sown while the farmer toils for Monsanto, Monsanto
When these seeds rise they're ready for the pesticide
And Roundup comes and brings the poison tide of Monsanto, Monsanto

lyrics by Neil Young

Monsanto really are the bad guys here. They gave us Agent Orange. While GMO proponents will say “GMO does not mean Monsanto” it owns most of the GMO seeds, the monoculture mega crops and Roundup, the herbicide it invented. Monsanto controls 80 percent of the GM corn market, and 93 percent of the GM soy market. That's a pretty big bite.
Monsanto dominates global market in GM seeds

In flies the beautiful Monarch Butterfly Danaus plexippus

Range and Migration Map of the Monarch Butterfly

Enter the villain: Monsanto
Monsanto = Roundup + glyphosate + GMO crops + mega farming + loss of milkweed = decimation of Monarch butterflies

Tracking the Causes of Sharp Decline of the Monarch Butterfly
The use of Roundup has effectively eliminated milkweed from almost all of the habitat monarchs used to use.
Taylor: What we’re seeing here in the United States is a very precipitous decline of monarchs that’s coincident with the adoption of Roundup-ready corn and soybeans. The first ones were introduced in 1997, soybeans first, then corn. By 2003, 2004, the adoption rate was approaching 50 percent, and then we really began to see a decline in monarchs. And the reason is that the most productive habitat for monarch butterflies in the Midwest, in the Corn Belt, was the corn and soybean fields [where milkweed, which monarchs feed on, grew]. Before Roundup-ready crops, weed control was accomplished by running a tiller through those fields and chopping up the weeds and turning over the soil, but not affecting the crops. The milkweed survives that sort of tillage to some extent. So there were maybe 20, 30, 40 plants per acre out there, enough so that you could see them, you could photograph them.
[…]

Taylor: Oh, yes, it’s gone up. The glyphosate used in agriculture has tripled since 1997, when they first introduced these Roundup-ready crops. The developers of these crops not only provided the seeds that were glyphosate-resistant, but they also provided the glyphosate — the Roundup. And, boy, that was a pretty good system. You could make money on both, right?

Risks they face
Monarch butterflies are currently facing three major risks: illegal logging, lack of milkweed plants and climate change. WWF’s 2013-14 report from Mexico showed that the number of monarch butterflies wintering there was at its lowest in 20 years. This finding was determined by measuring the amount of forest they occupy; in 2013, the number of butterfly acres decreased to 1.65 acres compared to 27.5 acres in 2003.

Monarchs need mountain forests in Mexico for their winter habitat, however nearby human communities also rely on them and create pressure on forests through agriculture and tourism activities.
In Canada and the U.S., monarchs need places to reproduce and feed. Herbicide use is also decreasing the availability of their primary food source, the milkweed plant.

Climate change threatens to disrupt the monarch’s annual migration pattern by affecting weather conditions both in wintering and summer breeding grounds.

What can we do?
Plant milkweed, with caution, be careful to plant only the right species of milkweed. There are over 70 species of milkweed so look for the one that is native to your area in the guide below. The one to avoid is the tropical Asclepias curassavica.
Plan to save monarch butterflies backfires

Planting the Wrong Kind of Milkweed May Harm Monarch Butterflies

Here's a planting guide:
Milkweeds: A Conservation Practitioner’s Guide