Tracking the Causes of Sharp Decline of the Monarch Butterfly

A new census found this winter’s population of North American monarch butterflies in Mexico was at the lowest level ever measured. Insect ecologist Orley Taylor talks to Yale Environment 360 about how the planting of genetically modified crops and the resulting use of herbicides has contributed to the monarchs’ decline.

University of Kansas insect ecologist Orley R. “Chip” Taylor has been observing the fragile populations of monarch butterflies for decades, but he says he has never been more concerned about their future.

Monarchs are beloved for their spectacular migration across Canada and the United States to overwintering sites in central Mexico — and back again. But a new census taken at the monarchs’ wintering grounds found their population had declined 59 percent over the previous year and was at the lowest level ever measured.

Orley Chip Taylor

In an interview with Yale Environment 360 contributor Richard Conniff, Taylor — founder and director of Monarch Watch, a conservation and outreach program — talked about the factors that have led to the sharp drop in the monarch population. Among them, Taylor said, is the increased planting of genetically modified corn in the U.S. Midwest, which has led to greater use of herbicides, which in turn kills the milkweed that is a prime food source for the butterflies.

“What we’re seeing here in the United States,” he said, “is a very precipitous decline of monarchs that’s coincident with the adoption of Roundup-ready corn and soybeans.”

Yale Environment 360: The study that happened in December indicates a pretty dramatic decline in the monarch population. Could you describe what that study found?

Orley Taylor: The measure of the overwintering colonies takes place every December down in Mexico, and what that measures is the total area of trees that are occupied by monarchs in up to twelve different overwintering sites. They’ll have a team that will go up into the forest and they will examine all of the trees that have got butterflies on them. The butterflies tend to be grouped, so there might be 47 trees in one area covered by butterflies, and there might be another area which has 427 trees covered with butterflies. They measure the polygons occupied by these fir trees, figure out the area of each one, and add them all together.

It came out to only 1.19 hectares this year, about 2.74 acres, the smallest all-time measure. In 1996, we had an overwintering population that was almost 18 times larger.

e360: What percentage decline did the current study find, over one year?

Taylor: It was a 59 percent decline, but that’s not really important. In 2003-4, the population declined a lot more from one year to the next. So it’s not the total percent decline. It’s the total amount of butterflies that are out there that we’re really concerned about.

e360: In the past they bounced back, and you think the prospects are less likely for that now?

Monarch Butterflies
A cluster of monarch butterflies in Mexico. JIM LOVETT/MONARCH WATCH

Taylor: One of the things that you can say about almost all populations is that when they get really small they get very vulnerable to one perturbation or another. What we’re really worried about here is that there would be some sort of catastrophic event that can send the population spinning downward even more. Then the impetus for conservation of the population could weaken — because if you don’t see them, you don’t have the motivation to do something about it.

e360: A lot of Americans assume the problem is deforestation of the monarch’s winter habitat in Mexico. How has that situation changed?

Taylor: They’ve made a terrific effort to control illegal logging down there [in Mexico], and the last report they had showed that they had completely eliminated illegal logging by the organized mafia-like groups that go in there with guns and cut down a hectare of forest in one night with 15 or 20 trucks and then haul it all off before morning.

e360: That’s stopped?

Taylor: As far as we know, it’s stopped. But there still is a little nibbling at the forest here and there, and that’s very hard to control. You’ve got a lot of people living close to the bone, and each of those mature trees is worth about $300. It’s a fairly big area, it’s remote, and the question is, how do you patrol this area? How do you eliminate the day-to-day things that are going on in remote mountainsides?

e360: Let’s talk about the problem on the American and Canadian end of the migration.

Taylor: What we’re seeing here in the United States is a very precipitous decline of monarchs that’s coincident with the adoption of Roundup-ready corn and soybeans. The first ones were introduced in 1997, soybeans first, then corn. By 2003, 2004, the adoption rate was approaching 50 percent, and then we really began to see a decline in monarchs. And the reason is that the most productive habitat for monarch butterflies in the Midwest, in the Corn Belt, was the corn and soybean fields [where milkweed, which monarchs feed on, grew]. Before Roundup-ready crops, weed control was accomplished by running a tiller through those fields and chopping up the weeds and turning over the soil, but not affecting the crops. The milkweed survives that sort of tillage to some extent. So there were maybe 20, 30, 40 plants per acre out there, enough so that you could see them, you could photograph them.

The use of Roundup ‘has effectively eliminated milkweed from almost all of the habitat monarchs used to use.’

Now you are really hard pressed to find any corn or soybeans that have milkweed in the fields. I haven’t seen any for years now because of the use of Roundup after they planted these crops. They have effectively eliminated milkweed from almost all of the habitat that monarchs used to use.

e360: The amount of herbicide sprayed on these fields has gone up?

Taylor: Oh, yes, it’s gone up. The glyphosate used in agriculture has tripled since 1997, when they first introduced these Roundup-ready crops. The developers of these crops not only provided the seeds that were glyphosate-resistant, but they also provided the glyphosate — the Roundup. And, boy, that was a pretty good system. You could make money on both, right?

e360: Right.

Taylor: For the farmers it looked good too. If I was a farmer and I was holding two jobs to keep my farm and I didn’t want to have my rear end sitting on the tractor too long, I would use that product as well, because the ordinary mechanical tillage took a lot more time and cost a lot more money.

Monarch Butterflies
Hundreds of monarchs fill the sky at their wintering ground in Mexico. JIM LOVETT/MONARCH WATCH

e360: This is not the sort of thing people originally worried about with genetic engineering — it’s not a mutant gene getting loose, it’s not food safety. It’s just a change in conventional farming practice.

Taylor: It’s a collateral damage issue. And one of the things that we’re worried about now is that it looks like there’s going to be a lot of collateral damage from the use of various herbicides and pesticides coming down.

e360: You’re worried about other genetically-engineered crops?

Taylor: Yeah, there’s apparently 15 genetically-engineered crops in the pipeline. One of the concerns is that some of them are stacked. That is to say, they’re genetically engineered to resist not just one herbicide, but two, three, four different classes of herbicide. Some of those herbicides are noteworthy for having a lot of collateral damage already because they are volatile. They tend to be difficult to confine, and so they are likely to be dispersed when they are applied and affect areas outside of the field. And that could have a tremendous impact on the vegetation, on the pollinators and, of course, on the monarch butterflies.

e360: There’s more going on here, though, than genetically-engineered crops?

Taylor: Ethanol is a big issue too. We’ve seen a 25.5 million-acre increase in the amount of corn and soybeans since 2006. And that’s been at the expense of nearly ten million acres of Conservation Reserve Program land, which farmers are paid to set aside for wildlife. The other 15.5 million acres means that farmers had to plant a lot of marginal land — that would be milkweed habitat, pollinator habitat, rangeland, grassland and so on. So there has been a tremendous change in agriculture to accommodate the production of biofuel. The price of corn and the price of soybeans has gone way up. There is also an increase in international markets.

‘We can have an impact if we get the gardeners in this country to help us out by planting milkweed.’

So a combination of things have pushed the corn and soybean acreage up to the highest level since just after the Second World War — 169 million acres of corn and soybeans were planted last year. This is just an unprecedented amount of landscape put into those particular row crops. What farmers are tending to do — and you can’t blame them — is that they are narrowing field margins. They are getting closer and closer to the edge of the road. These strips from the road to the field are often six or eight feet wide, and there’s nothing in there but grass.

e360: And in the past it would have been milkweed?

Taylor: You’re basically creating a desert out there, except for the corn and the soybeans.

e360: It’s ironic that the Conservation Reserve Program (CRP) is suffering because of another government program [for ethanol] that was ostensibly intended to protect the environment.

Taylor: Yes, exactly. We have 27 million acres of CRP land right now, and that’s going to go down to about 24 million acres. Congress has actually dictated that the CRP land should be capped at about 25 million acres. Well, it’s going to go below that because of these other economic forces.

e360: So are there prospects for monarch recovery given all these things weighing against it?

Taylor: Basically for monarch recovery we’re going to have to create a lot of milkweed habitat. And the question is where are we going to do it and how are we going to mobilize people to do it. What people are going to be asked to do is to save wildlife by creating habitats in their gardens.

e360: But gardens are not going to make up for 25.5 million acres of additional corn and soybeans.

Taylor: No. It’s an Alice in Wonderland story of the Red Queen’s race. You have to run as fast as you possibly can to stay in one place, and if you want to get any place, you have to run twice as fast. But I have to believe that we can have an impact if we get the gardeners in this country to help us out by planting milkweed and putting in native plants to stabilize native pollinator communities. So people now have another purpose for creating a garden. The purpose is conservation.

e360: This is the best-known butterfly in America and it’s also the state insect in six or seven states. But it sounds as though it might be on the path to extinction.

Taylor: I wouldn’t say extinction. Monarchs could disappear from the vision of most of us in this country as the migration goes way, way down. But the butterfly will persist. On the other hand, if we start talking about climate change, then we may be looking at an even grimmer scenario.

e360: I saw a recent study projecting a 73-100 percent decline in suitable conditions at those overwintering sites.

Taylor: A study by some Mexican colleagues is projecting that by 2030 the temperature will be so high at those overwintering sites that a lot of the trees will begin to die and the microclimate that butterflies need is going to become vanishingly small. I hate to think that they’re correct. Their projection is based on the temperatures in those mountains increasing by two degrees centigrade in that 17-year period. And if the planet has temperature rises that are that fast, we’re not going to be talking about monarch butterflies. We’re going to be talking about survival of a lot of things.

‘Monarchs could disappear from the vision of most of us in this country as the migration goes way down.’

e360: You teach a course about the world in 2040, when your current students will be in their 50s. What does the fate of the monarch say to you about how that world is going to look?

Taylor: You know, I don’t even put the monarch in that world. I mean you know the population is projected to increase by two billion people by 2040. Well, we can’t do that. We are going to see a lot of changes, a lot of restrictions on how fast populations can grow simply on the basis of our food production, the declining available arable land, the limitations on water. If we don’t get with it and if we don’t start modifying our behavior, then things are going to get really out of whack.

But you know there’s still a chance and there’s still a way that we can deal with things fairly effectively. I mean almost every day there are new things that human beings have come up with which will help us deal with some of these crises ahead. We are going to have to make a lot of adjustments. And if we don’t make these adjustments, life is going to get to be pretty tough.

e360: So in all of this, the decline of the monarch butterflies is a kind of side show.

Taylor: It is. It’s my way of introducing people to the larger issues.