Monarch butterflies decimated. How climate change is killing them in Michigan

Ask how to reverse declines in monarch butterfly populations, and the answer is often “plant milkweed.”

A new study led by researchers at Michigan State University points toward another major factor driving the migratory pollinators ever closer to the brink of extinction: hotter, drier weather in a changing climate.

The U.S. Fish and Wildlife Service in December identified monarchs as a candidate for Endangered Species Act protections after two decades of precipitous population declines. The eastern population that flutters through Michigan each summer plummeted from about 384 million in 1996 to 14 million in 2013.

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That’s cause for dismay. In addition to their cultural value, monarchs are a key pollinator and a food source for other species. The factors causing their decline threaten many other insect species, sparking fears of a fraying food web.

The study, published July 19 in the journal Nature Ecology & Evolution, relied on annual monarch population surveys and localized data on climate, herbicide use and other factors to conclude that, since 2004, climate has played a nearly seven times more significant role in monarch declines than other factors.

That might surprise some: Conservation plans for the iconic migratory orange and black pollinator have often blamed habitat loss, particularly disappearing milkweed, for dwindling numbers. 

A migratory species, monarchs spend winters in densely-packed colonies in Mexico, before flying northward to the southwest in the spring, where they lay eggs and die. The next generations travel onward to the midwestern and northeastern U.S. and Canada, reproducing along the way. The year’s final offspring turn back south each fall to repeat the life cycle.

Monarchs lay their eggs on milkweed, and the caterpillars eat only milkweed leaves. But development and expanded use of farm herbicides have made the plant more scarce.

Disappearing milkweed played a big role in the first wave of monarch declines during the 1990s and early 2000s. But since then, said Elise Zipkin, the study’s senior author and director of MSU’s Ecology, Evolution and Behavior Program, the herbicides that kill milkweed have become so widely used on corn, wheat and soybean fields, “there’s no more milkweed to lose.”

Bridge Michigan interviewed Zipkin and Erin Zylstra, the study’s lead author and a postdoctoral researcher in MSU’s College of Natural Science, about their research and its implications for monarch recovery efforts. Their responses have been edited for clarity and brevity.

So much of monarch conservation focuses on milkweed loss. What got you interested in the role of climate?

Zipkin: There was a hypothesized loss of milkweed, which coincided with the times of the steepest monarch declines from the mid-90s to the early 2000s.

But there’s been a lot of debate out there about why monarchs are declining. There were other hypotheses about what might be contributing, such as disease or change of habitat, illegal logging or winter storms, or climate change effects. 

We really wanted to figure out, which of those factors is causing the most decline?

You found that climate played a huge role. In what way?

Zylstra: It really seems like it's the breeding season climate — what's happening with spring and summer temperatures and precipitation — that’s causing changes with monarchs in recent years. 

With respect to spring weather in eastern Texas, monarch populations seem to do best when temperatures and precipitation are pretty close to the 15-year average. When temperatures are a lot higher or lower than that, and when precipitation is way higher or lower, monarchs are expected to do worse. 

On the summer breeding grounds, it's more nuanced. There's a positive relationship between summer precipitation and monarch numbers. But with respect to temperatures, it depends where you are. 

In the northern part of their summer range (which includes Michigan), higher temperatures were good for monarchs. But in the southern part — places like Northern Ohio, Northern Illinois and Iowa — when the temperatures were well above normal, it wasn't good for monarchs. 

That’s where we start thinking about climate change: These places are already on the warmer end of monarchs’ summer range, and when temperatures are way above normal, that's a bad thing. 

So what does this mean for monarchs in the long run? 

Zipkin: Erin is working on getting a better understanding of that: Taking all we know from this monarch model, and looking at predictions of how these areas are going to look under our future climate. 

The goal is to figure out, what is likely to be the best places for monarchs?

It's not like we think that they're close to extinction, really. But on the other hand, there are parts of their range that may no longer be good at the times when they've historically been in those areas. 

Zylstra: When we look at climate projections, we’ll have to try to find the sweet spots where the conditions are going to be best for monarchs. The places that are going to keep having exceptionally hot years, those are going to be problematic. So will the places where it’s getting drier.

What additional research is needed to better understand what's happening?

Zylstra: We don't really know what capacity monarchs have to adapt to some of these changes. That's a whole different area.. of science, but it's important to think about. 

Zipkin: We also want to extend this research to other butterflies in the Midwest. How are they doing?

Given this research, what can the average person do to help save monarchs, beyond planting milkweed?

Zylstra: We can think about trying to fight climate change and reduce emissions.

I think planting milkweed is still a good thing to do. But if we’re going to spend money on restoring habitat and planting milkweed, we can think more carefully about making that effort in places where the climate is likely to support monarchs. 

Zipkin: And we can elect officials and support businesses that support policies to reduce emissions.