It was not that long ago that a routine spectacle of autumn was to see monarch butterflies fill the air. There would be hundreds filling the sky as you drove along in late September. On occasion you would be fortunate enough to see a “monarch tree" where hundreds of butterflies would roost overnight in the same tree as they passed through the area on their migration-route from Canada and Minnesota to central Mexico. Check out the migration of monarchs through Iowa as documented in a blog by Robert D. Woodward.
Unfortunately, the days of plentiful migrations may be over, as monarch butterfly populations have fallen on hard times and the numbers are down. Dramatically. The decline in the number of monarchs has occurred for a variety of reasons:
- Loss of milkweeds
- Intensive farming
- Urban development
- Deforestation in Mexico
- Global climate change and unfavorable weather for monarch reproduction and growth
According to Pleasants & Oberhauser (Insect Conservation & Diversity, 2012), milkweeds, the food source of monarch caterpillars, are disappearing in both agricultural fields and non-agricultural habitats in the Midwest. By some estimates, half of the overwintering monarchs that go to Mexico in the fall come from the Midwest. Without their food source, they struggle to thrive. Between 1999 and 2010 "there was a 31% decline for non-agricultural milkweeds and an 81% decline for agricultural milkweeds with a 58% overall decline for total milkweeds." One cause of milkweed decline is the use of glyphosate herbicide on genetically modified, glyphosate-tolerant corn and soybeans.
An estimated 25.5 million more acres of corn and soybeans are now planted compared to just a few years ago. At the same time, Conservation Reserve Program land, grassland and pasture land (places milkweeds are likely to grow) have decreased. If the current trend continues, the amount of non-cropped farmland will continue to decrease, putting further pressure on monarchs.
Wet springs and summer droughts in the have decreased monarch survival and reproduction. And warmer temperatures on Mexican forest hillsides threaten the forest trees where the monarch spend the winter.
As a result of these multiple factors, last winter saw the fewest monarchs in Mexico since record keeping began. The population last winter was one-twentieth what it was just 16 years ago. With fewer monarchs overwintering in Mexico, there are fewer to return to the upper Midwest. With less food resource available to monarch caterpillars in the summer, fewer monarchs migrate south the following year. It's a vicious, downward spiral that may eliminate monarchs as we have known them.
At the moment, individual actions to increase the number of milkweeds are the best hope, but replacing the loss of milkweeds from 174.4 million acres of corn and soybeans is a tall order. As Chip Taylor (Monarch Watch) has said, "To assure a future for monarchs, conservation and restoration of milkweeds needs to become a national priority."