‘No shortage of dangers’ and no easy answers for the monarch butterfly

By Justin Cremer

July 11, 2018

North America’s monarch butterfly population is in trouble. That much has been clear for decades but scientists have found that affixing blame to the problem is not so simple. Habitat loss, weather changes and pesticides have all at one time or another been fingered as the primary cause, but the truth is messier and somewhat unsatisfying: there is no easy or single answer. Because of that, just what can be done to stop the monarchs’ decline also remains unclear.

Anurag Agrawal, one of the world’s leading authorities on the subject, wishes he could provide a simple solution to the plight of the monarchs, but he can’t.

“When you look at the 25-year trend, it seems quite dire,” Agrawal, a Cornell University professor of ecology and evolutionary biology and the author of Monarchs and Milkweed, says. “There has been a steep, persistent decline in the number of monarchs that overwinter in Mexico every year. But no one is arguing that the monarch butterflies are threatened or endangered as a species. What we are arguing is that the eastern North American migration, in which hundreds of millions of monarchs travel several thousand kilometers every year, is in serious trouble and may be lost.”

“That’s complicated for people to understand,” he says.

Adding to that complication is the fact that monarchs are also found in large numbers in California, Hawaii, Spain, Australia and New Zealand. While Agrawal specializes in the butterflies that live in the Midwest and Northeastern US and Canada, he says that studies also show a 45-year negative trend for California monarchs.

‘No doubt’ humans are to blame

Agrawal, who recently published an analysis of the monarchs’ migration woes in Science, says “there is no shortage of dangers” facing the monarch population, including extreme weather, disease and the loss of milkweed to industrial agriculture. Though he’s hesitant to quantify those dangers, it’s not hard to see the underlying source: mankind.

“There is no doubt that the decline in monarch butterflies over the last 25 years is caused by humans,” he says.

While a host of human factors are contributing to the stark decline in the number of monarchs overwintering in Mexico each year – some 20 million are thought to be killed by vehicles alone – the conventional wisdom for years has been that widespread herbicide use throughout great swaths of the United States has decimated milkweed, which is the only source of food for monarch caterpillars.

But simply pointing to herbicide use as the butterfly-killing bogeyman is oversimplifying a complex issue, Agrawal warns.

“There is no question that in the core agricultural Midwest there is a lot less milkweed than there used to be. But there are two important questions to ask about that. One is what fraction of the monarchs that go to Mexico come from that core area? It could be as low as ten percent or as high as 60 percent, we really don’t know. The second question is that, while it is clear that there is much less milkweed, is there still enough of it? That is another source of debate and it is very complicated,” he says.

According to a 2017 study from the US Geological Survey, over 860 million milkweed stems were lost in the northern US over the past decade and nearly two billion additional sets of milkweed would be needed for the Eastern migratory population of the monarch butterfly to rebound to a sustainable level.

Anurag Agrawal. Photo: Frank DiMeo

‘Constantly changing narrative’

Although monarchs have been embraced by anti-GMO activists (the Non-GMO Project uses a monarch butterfly in its logo), pesticides are hardly the first culprit to be blamed for the long decline of the species. Agrawal says he’s been digging into media coverage of monarchs over the past several decades and “the narrative is constantly changing.”

After scientists discovered in 1975 that hundreds of millions of butterflies from all across the US east of the Rockies were going to what he calls “12 very tiny mountaintops in central Mexico” each winter, the initial thought was that logging in their winter home was the problem. In the 1980s, after a series of ice storms killed as many as half of the butterfly populations in Mexico, extreme weather was blamed. But the debate was changed irrevocably with the 1999 publication of an initial study by Cornell entomology professor John Losey in the journal Nature that said pollen from genetically-engineered Bt corn increased butterfly mortality.

“The massive amount of attention drawn to this one single-page study was just crazy,” Agrawal says. “It had an incredibly profound effect on the field of agricultural biotechnology. It changed laws in Europe and caused major discussions in the US that really drove the media narrative for more than a decade.”

But Agrawal thinks that the narrative is once again starting to change as scientists and non-scientists alike come to realize the true complexity of the problem. When a species has a life cycle as complicated as that of the monarch butterfly, which has four generations every year and faces a long and treacherous migration, it’s very difficult to put a relative importance to the various dangers it faces.

While Agrawal argues that in the unlikely event that all pesticides were suddenly banned, it would “not necessarily” make monarchs bounce back, he thinks part of the reason the pesticide angle has been so widely embraced is because “it is understandable and it presents a solution: plant milkweed.”

Monarch caterpillar. Photo: Joshua Mayer/Flickr
Monarch caterpillar. Photo: Joshua Mayer/Flickr

‘Icon of nature’

The plight of the monarch butterfly also became somewhat conflated with that of the honeybee, which has also experienced puzzling dips in its population. Unlike bees, however, monarch butterflies are not pollinators and Agrawal acknowledges they are “not particularly special from an ecological perspective”.

“If the monarch disappeared it’s unlikely that there would be considerable impact on the rest of the natural world,” he concedes.

Why then, in a world filled with depressing news, should we care about a species that doesn’t really impact us and whose troubles are multi-faceted and hard to understand?

“The monarch is an icon of nature in part because it is such a spectacular looking insect. In the US in particular, you can go out in your backyard and frequently find them, so unlike the polar bear, many Americans feel a personal connection to monarch butterflies,” Agrawal says.

The monarch’s troubles should also serve as a giant red flag, Agrawal adds.

“Some organisms are sort of like the canary in the coal mine and are indicators of the health of a particular location. One of the things that is so cool about monarch butterflies is that they travel from Canada to central Mexico, drinking nectar all along the way. That makes them a potential indicator species for the health of our entire continent. With their numbers declining like they have been over the past 25 years, that should really set off a light bulb,” he says.

Agrawal, who was animated and talkative throughout our nearly 90-minute chat, was at a temporary loss of words when asked how to save the butterflies.

“We have to take a step back and ask ourselves the harder questions that none of us what to deal with. We want to plant milkweed or give $10 to the Nature Conservancy or have an enviro-friendly garden, and I encourage all of those things. But the truth of the matter is monarchs are health indicators for our continent and they are exhibiting multi-decadal declines that point to very big systemic problems. We shouldn’t fool ourselves.”

Featured image: aka Tman/Flickr