The monarch butterflies are vanishing. Over the past 20 years, fewer and fewer butterflies have been making the long journey down to Mexico to survive the winter.
Scientists have offered a few possible reasons for the decline, from deforestation in Mexico to bouts of unusually severe winter weather. But a new paper this month in the Journal of Animal Ecology argues that the biggest culprit is likely the decline of milkweed plants in the United States — the main food for monarch caterpillars before they turn into butterflies.
The study doesn't identify a precise cause for the decline of milkweed, but the authors suggest that the growth of herbicide-intensive agriculture in places like the Midwest may be killing off native plants. (It's worth noting that not everyone is convinced herbicides are solely responsible here.) Either way, the paper also points to one way to help revive monarch populations: namely, more milkweed.
Monarch butterflies are vanishing
Here's how monarch butterfly migrations are supposed to work: Every spring, millions of monarchs fly up from Mexico through Texas, laying eggs on milkweed plants as they go. That spring generation will, in turn, produce two successive summer generations that gradually make their way up to Canada.
Then, in the fall, that third generation of monarchs will fly thousands of miles back down to the Oyamel fir forests in central Mexico to survive the winter.
Over the past 20 years, however, scientists have noticed that fewer and fewer monarchs are actually making it down to Mexico each winter. They can tell this by measuring the areas of the Oyamel forest with trees that contain butterflies (the butterflies only go to 12 locations in Mexico, so this sort of counting is doable).
As the graph below shows, the area in Mexico occupied by monarch colonies keeps shrinking. Back in the winter of 2003, the monarchs occupied some 11 hectares (or 27.5 acres). This past winter, the monarchs occupied just 0.67 hectares (or 1.65 acres).
That translates to an estimated 33 million butterflies left:
(By the way, not all monarchs go to Mexico — as the map above shows, there's also a smaller population of butterflies west of the Rocky Mountains that spend the winter along the California coast. As best anyone can tell, those butterflies are also in decline, with just 200,000 observed last winter.)
The decline of milkweed may be a major factor
So what's going on with the monarchs?
Earlier this year, I asked Lincoln Brower, a biologist at Sweet Briar College who has studied monarch migrations for decades. He suggested three main factors for the monarchs' decline: The winter habitat in Mexico has come under pressure from deforestation. Harsh weather can disrupt the monarchs' migration. And milkweed plants in the United States are disappearing from grasslands. Given that monarchs lay their eggs on milkweed and the plant is the main source of food for caterpillars, that's a big deal.
Now a new study in the Journal of Animal Ecology suggests that the decline of milkweed is likely the most important culprit here. The study, led by a team of researchers from the University of Guelph, modeled variations in monarch populations. They found that habitat destruction in Mexico was no longer driving the decline — possibly because Mexico has put new conservation measures in place to protect those forests.
But butterfly populations are very sensitive to loss of milkweed plants in the United States, the study found. And the study found that milkweed plants had declined 21 percent between 1995 and 2013. These losses were concentrated in areas where monarchs breed — and 70 percent of the milkweed loss was located in agricultural areas. (The rest of the decline was on conservation lands or in public areas such as the medians of highways.)
And the outlook wasn't great — the authors forecast that the monarch population would decline another 14 percent if milkweed loss continues apace.
Are GMOs or herbicides to blame?
The University of Guelph study didn't investigate exactly why milkweed was declining. But the authors pointed to the rise of soy and corn crops that have been genetically modified to be resistant to herbicides — which, in turn, can lead to increased herbicide use. The worry is that more spraying is killing off crucial native flora like milkweed.
The timing here is at least suggestive: Other studies have found that milkweed plants in regions like the Midwest seem to have declined at around the time that genetically modified crops became more common. Proving a direct connection, however, would require more careful study.
Meanwhile, not everyone's convinced that herbicides are the only reason for the decline of native plants near agricultural fields. Another recent study by scientists at the US Department of Agriculture and Penn State found that herbicide-tolerant native plants around farmland in Pennsylvania were declining at the same rate as less-tolerant plants. That study suggests that other factors may be at work here.
The Penn State researchers pointed out that farmers have made a lot of changes in recent decades apart from rising herbicide use — they've simplified their crop rotations, segregated crops and livestock, and employed new mechanical farming methods. What's more, woodlots, hedgerows, pastures, and wetlands have all been cleared to make way for bigger fields. So there may be more going on than just GMOs and herbicides.
(For more on this question, see this post by the University of Wyoming's Andrew Kniss, who notes that herbicides have likely played some role in the decline of milkweed, but is much more cautious about insisting that it's the only factor at play here.)
In any case, there's broad agreement that milkweed plants are declining sharply. And that appears to be a major reason monarch butterflies are vanishing. Figuring out exactly why those plants are declining — and how to protect them — is the crucial next step.
Is there any way to bring back the monarchs?
Some groups, like Monarch Watch, have encouraged people to plant more milkweed in their gardens and on other public and private lands.
Along similar lines, Brower suggested in our interview that state highway departments could be more careful about mowing and spraying their highway medians (which often contain plenty of milkweed), particularly during the butterfly breeding season.
Still, the University of Guelph paper suggested that only about 26 percent of milkweed loss since 1995 had occurred in public areas or parks. By contrast, 70 percent of the loss had occurred in "agricultural-intensive landscapes" — particularly in the south and central United States. So figuring out how to preserve undeveloped grasslands and bringing back milkweed plants seems to be a more crucial task here.
Further reading: Here's a more in-depth interview with Lincoln Brower about the decline of the monarch butterflies.