Monarch butterflies depend on milkweed. About 30 species of the plant are the only places where North American monarchs lay their eggs, and once those eggs hatch, milkweed serves as the exclusive food source for their iconic stripey caterpillars.

And since "quasi-extinction" now looms over North America's famous migratory monarchs, even after a slight rebound in 2015, it makes sense for our rescue efforts to target such a vital resource — especially since milkweed is also under siege from herbicides. Planting milkweed has thus become a popular way to not only help the butterflies have babies, but to save one of the greatest animal migrations on Earth.

monarch butterfly caterpillarYet while no one doubts that monarchs need milkweed, some scientists who study them have begun to question if planting milkweed is really the best way to solve this specific decline. In fact, a recent study suggests the trouble strikes mainly on the grueling fall migration, after monarch caterpillars have graduated from milkweed to the more diverse, nectar-based diet of adults.

"If the decline is most prevalent at a particular stage of migration, that stage might be more important to study," says Anurag Agrawal, an ecologist at Cornell University and co-author of the new paper, which was published last month in the journal Oikos. "Wouldn't it be a travesty if we spent a lot of effort on the wrong stage?"

That sentiment has grown among some monarch experts, but it's hardly universal. The new study highlights a scientific schism over milkweed's role in the crisis.

"I actually think it could be pretty dangerous, if people conclude that they shouldn't focus on conservation of breeding habitat," says Karen Oberhauser, a prominent monarch expert at the University of Minnesota who has studied the butterflies since 1984. "I'm pretty concerned about the way people interpret this study."

Debates like these may be a healthy part of science, but what are the rest of us supposed to do while scientists sort things out? Can we really stop monarch declines by planting native milkweed, or should we be focusing more on other strategies? To find out, we talked to several experts about what might be hurting the beloved butterflies — and what might make their brief, busy lives a little easier.

monarch butterfly migration map This map depicts monarchs' spring ranges in green, summer in yellow and fall in orange. Click to enlarge. (Image: FWS)

What is quasi-extinction?

First, it's worth a brief reminder of how amazing this migration is. For at least a million years, clouds of fragile insects have endured a yearly relay across North America that spans 2,500 miles and four generations of butterflies, with adults passing the baton to caterpillars who instinctively carry on their parents' mission. Navigating predators, parasites, storms, roads and insecticides, they funnel from huge swaths of the U.S. and southern Canada onto 12 mountains in Mexico.

Millions of monarchs spend every winter on those mountains, drawn by the rare microclimates of oyamel fir forests. They are Generation 4 of one year's migration, and when spring arrives, they restart the cycle by flying north to lay eggs in Northern Mexico and the Southern U.S. Those Generation 1 offspring then quickly mature, mate and continue the journey north, depositing more eggs along the way.

Generation 2 has a similar life, laying eggs across Eastern North America from April to July. Generations 3 and 4 mature more slowly, fueling up on nectar as they start the long return south in late summer and fall. Using sun position, Earth's magnetic field and other variables, they eventually find the same 12 mountains as their great- and great-great grandparents, despite having never been there personally.

As a species, monarchs face no immediate risk of extinction. Yet while they've spread to other continents in modern times, genetics suggest they evolved in North America, which is also the only place where they migrate. And that migratory population is now falling so rapidly that it faces "substantial risk" of quasi-extinction — or crashing too badly to recover — in the next 20 years, according to a 2016 study.

monarch butterfly population graph The total area, in hectares, covered by monarchs wintering in Mexico from 1994 to 2015. (Image: Xerces/USDA)

Got milkweed?

Up to 1 billion monarchs wintered in Mexico as recently as the 1990s, but just a fraction of that show up these days. Only about 35 million monarchs reached Mexico two years ago, and while the 2015 migration was considered good by recent standards, its final estimate was still a relatively meager 140 million.

Asclepias tuberosa milkweedMonarchs have lost about 147 million acres of summer breeding habitat since 1992, according to Monarch Watch, which means fewer places to lay eggs. Native milkweeds like Asclepias tuberosa (pictured) have faded in many areas due to industrialized agriculture, including the cultivation of genetically modified organisms (GMOs) that can tolerate herbicides like glyphosate, aka Roundup. Farmers who use "Roundup-ready" crops can spray glyphosate more liberally, knowing only genetically protected plants will survive.

Milkweed has been considered a pest for some time, as its name implies, so being targeted on farms is nothing new. But the rise of Roundup-ready GMOs has let farmers kill it more thoroughly by ramping up herbicide use, even after crops have emerged in spring. Herbicide-tolerant (HT) soybeans debuted in 1996, for example, and by 2014 they totaled 94 percent of U.S. soybean acreage, according to the USDA. Adoption of both HT corn and cotton in the U.S. is now about 90 percent.

Removing small patches of milkweed can make it harder for female monarchs to reach their egg-laying potential, a 2010 study found, since they must spend more time searching for a suitable place. And as Oberhauser noted in a 2013 study, steeper declines among monarchs in the U.S. Midwest seem to indicate their woes are related to milkweed loss, since HT crops are more common in the Midwest than in regions with stabler monarch populations, such as the U.S. Northeast and southern Canada. Findings like these have led to the widespread popularity of milkweed replenishment, from local efforts by schools and garden centers to federal incentives for farmers.

monarch butterflies on goldenrod Goldenrod is one of many nectar sources visited by monarchs as they move south in autumn. (Photo: Rachel Laubhan/FWS)

Southern discomfort

Although milkweed is undeniably important, the new study suggests it's not the main factor in monarchs' low winter numbers. This isn't the first research to propose that, but thanks to a wealth of data from annual butterfly counts, it may be the most compelling so far. It's a "game-changer in monarch conservation," according to a blog post by University of Georgia ecologist Andrew Davis, a monarch researcher who has raised similar questions but wasn't involved in the new Oikos paper.

"This study is one more that shows monarchs may not be declining in breeding season after all. They may be declining on the way to Mexico," Davis tells MNN. "This is sort of controversial. Studies like this polarize the monarch community a bit."

For the new study, researchers wanted to figure out which part of the annual migration is most dangerous for monarchs — and thus where we should focus our efforts to help. They analyzed 22 years of citizen-science data from four monitoring programs across North America, studying populations at various migratory stages.

They saw a sharp yearly decline in Generation 1, which they blame on "progressively smaller numbers of spring migrants from the overwintering grounds." But monarch numbers then grew regionally during summers, they add, with no sign of statistically significant declines until they arrived in Mexico. That suggests the breakdown occurs somewhere along the fall migration route, the researchers write.

So if the biggest threats facing monarchs emerge on the way to Mexico, what are they? The study's authors aren't sure, but they identify three possibilities: habitat fragmentation, bad weather and too little nectar available in autumn.

monarch butterflies on radar This radar image shows a cloud of monarchs flying south near St. Louis during the 2014 fall migration. (Photo: NWS)

Humans have sliced up monarchs' ancient migration routes in a variety of ways, but highways are among the deadliest. One study from 2001, for example, estimated that cars and trucks killed 500,000 monarchs during a single week in central Illinois. "I extrapolated that number across the entire flyway, and I came up with 25 million dying just from crossing roads," Davis says. "To put that into context, two years ago we believe the entire overwintering population was about 50 million."

Severe weather in the Southern U.S. is also a likely culprit, Agrawal explains, including storms that make flying difficult and droughts that limit water and nectar.

"We cannot overestimate the importance of the Texas drought," Agrawal says, referring to the historic dry spell from 2010 to 2013. "It was the most severe drought in Texas in 50 to 100 years. Spring rains typically promote lush milkweed, and then in the fall, the goldenrod and other flowers the monarchs rely on during their southern migration. Climate is super important in predicting monarch numbers."

The idea that nectar shortages are reducing monarch populations is still speculative, but Agrawal says droughts or flooding could both disrupt plants' production of nectar, which, along with water, is crucial to adult monarchs throughout their migration — and especially on their marathon flight back to Mexico.

While some GMO advocates have cheered this study as a vindication of glyphosate and GMOs, the authors aren't drawing broad conclusions. This study is about butterflies, not GMOs, and even if milkweed loss isn't a major factor in recent monarch declines, this research hardly absolves herbicides of ecological harm. In fact, Agrawal points out, the same agricultural practices killing milkweed up north could also be limiting nectar — and therefore adult monarchs — farther south.

"Frankly, herbicides and industrial agriculture could also be a factor for those nectar sources," he says. "If there are fewer flowering plants, that could be an issue."

tractor spraying pesticides Migrating monarchs face many forms of habitat loss, from herbicides to highways. (Photo: Sean Gallup/Getty Images)

Why the controversy?

A few critics have faulted the study's reliance on citizen-science data, Agrawal says, but that's not why Oberhauser is dubious. "I'm a firm believer in citizen science," she says. "I've done a lot of work in studying the importance of citizen science, so I have strong confidence in the data and the people collecting the data. I just have reservations with the way those data were interpreted and analyzed."

Her main qualm is about the sites where the data were collected, which she describes as inadequate for estimating overall monarch populations in breeding season.

"The studies they used were conducted year after year in the same locations," she says. "Just by the nature of the data collection, these are good sites for monarchs. People chose them because they're good habitats. There are a lot of things we can learn from the numbers, and I've been involved in studies that have used data from those projects. But as a means for monitoring the whole population, it's not appropriate to use data from a few spots that have not been changing."

Back when the citizen-science monitoring projects began, she explains, monarchs still had a lot of other habitat that wasn't being monitored. "But those habitats are now gone. So the habitat that's available to monarchs has shrunk." And just because the number of monarchs hasn't dropped in remaining habitats, she adds, that doesn't necessarily mean the overall population size is unchanged before autumn.

Agrawal counters that all monitoring programs predicted monarch numbers at other sites, even when different people collected the data. "That's just not going to happen unless the data are valid," he argues. Despite this dispute, however, both researchers are quick to downplay the discord. "I have respect for the authors of that study," Oberhauser says. "I just think they didn't think carefully enough about how they needed to use those adult count data." Agrawal adds that "I'm a huge fan of Karen's. She's one of the most important monarch scientists out there."

monarch butterfly egg on milkweed Monarchs lay one egg per milkweed plant so each caterpillar has enough food after hatching. (Photo: Joanna Gilkeson/FWS)

What does all this mean?

While scientists figure out what's wrong with monarchs, surely there's no harm in planting milkweed, right? Well, it depends on the species and place, since some non-native milkweeds can actually make things worse. (To find out which species are native near you, check out this article by MNN's Tom Oder, or this Milkweed Finder from the Xerces Society.) Plus, as Davis points out, even planting native milkweeds may be futile if we don't help monarchs in later stages of migration, too.

"I look at it this way: If they're really having that many problems during the southern migration, then producing more monarchs during the breeding season will just be sending more monarchs to their deaths," Davis says. "I'm not sure that simply sending more along the way is going to fix the problem."

Davis qualifies, however, that "it can't hurt to plant native milkweed," a sentiment echoed by Agrawal. "I don't think planting milkweed is a bad thing," he says. "They're good-looking, they're going to attract other insects. Should we be planting milkweed? Sure. But is it going to solve the problem? Almost certainly not."

Experts tend to agree that monarchs need lots of help. The butterflies need better protection of milkweed in their breeding range, better protection of native flowering plants in the Southern U.S., and better protection of oyamel forests in Mexico. (They'd probably appreciate less habitat fragmentation and insecticide use, too.) The dispute is mainly about where, and how, our help is needed most urgently.

monarch butterfly and bees Plants that feed monarchs often benefit other insects, too, like these bees in Wisconsin. (Photo: Richard Hurd/Flickr)

"One of their conclusions is that we need to look at all parts of the migratory cycle, and certainly that southern part is very important for monarchs," Oberhauser says. "It's crucially important that they have good habitats to migrate through, so I'm not arguing that the south isn't important. But just because they don't see a correlation between these few monitoring sites up here and in Mexico, that doesn't mean that what happens up here [in the northern breeding range] isn't important."

That's true, says Dara Satterfield, a Ph.D. candidate at the University of Georgia who studies monarch ecology. But after years of focusing on milkweed, the new study suggests it's time to publicize the importance of many other native plants, too. Beyond planting milkweeds, Satterfield recommends we also revive the kinds of biodiverse habitats where migratory monarchs have thrived for millennia.

"This paper reminds us that monarchs need protection across their entire migratory range — from Manitoba to Mississippi to Michoacan," she says. "Planting milkweed is still critical. Milkweeds are still where life begins for monarchs. It's also important for us to remember that, for their whole lives, monarchs depend on plants — different plants. As caterpillars, they need milkweed. As migrating and breeding adults, they need frostweeds, thistles, sunflowers, mistflowers, many types of flowers. As overwintering butterflies, they need high-altitude fir trees in Mexico.

"Monarchs have lost millions of these plants over the past few decades," Satterfield continues. "These recent data remind us that we need to not only protect and provide milkweed, but to plant nectar sources, to preserve forests in Mexico, and to keep studying monarchs across their range."

And, Oberhauser adds, we all need to keep doing whatever we can to preserve these incredible creatures — from big decisions about land stewardship to picking out plants for our backyards. "The amazing thing about monarchs is that individual people can make a difference," she says. "Monarchs can use so many different types of habitat, so people really can make all kinds of individual differences."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.