Every November, millions of monarch butterflies arrive at the Oyamel fir forests in central Mexico, where they migrate to survive through the winter. Or at least that's what's supposed to happen.

In recent years, however, scientists have noticed a disturbing trend: The number of butterflies migrating to Mexico has been dropping sharply. In 2012, just 60 million monarchs arrived at their overwintering habitats, a record low. And fresh data from WWF Mexico suggests that the current winter's numbers are even lower — down to just 33 million butterflies across 0.67 hectares (1.65 acres).

That's the lowest area on record:

So why are the monarchs vanishing? Last month, I interviewed Lincoln Brower, a professor of biology at Sweet Briar College who has studied the monarch migrations for decades. In a 2012 paper, he cited three big reasons the populations are dwindling: Deforestation in Mexico, recent bouts of severe weather, and the growth of herbicide-based agriculture destroying crucial milkweed flora in the Midwest.

So is it too late to stop them from vanishing altogether? Or are there still things we can do to reverse the decline? An updated transcript of our conversation follows:

Brad Plumer: What do the numbers tell us about the decline of monarch butterflies so far?

Lincoln Brower: They've been going down for the last 20 years that we have data. This data comes from our visiting the places in Mexico where the butterflies are known to winter and measuring the total area of occupied trees:



The total annual area occupied by overwintering monarch butterflies from 1994 through 2013 has declined significantly, with the all-time smallest area reported during the 2012–13 overwintering season. (Source: Brower et. al 2012 updated with data from the author for the past two years)

The colonies are spread across twelve mountain ranges in Mexico, and in the biggest year on record they covered 21 hectares. Two years ago, that was down to 2.89 hectares. Last year, it was down to about 1.19 hectares. That's a highly significant trend.

It's possible that even fewer monarch butterflies are reaching Mexico this year than last year — and last year was the lowest it's ever been since the butterfly colonies were first discovered in 1975. [Update: The new data is in and, according to WWF, the butterflies occupy just 0.67 hectares this winter.]

BP: So how many butterflies is that in all?

LB: There are roughly 50 million butterflies per hectare, so [that means there are, as a very rough estimate, around 33 million butterflies this year.]

BP: Why has there been such a sharp decline in monarchs?

LB: The three big reasons: Severe weather was working against the butterflies for the last two years. Another is the progressive deterioration of the overwintering habitat in Mexico due to illegal deforestation. But the third and probably the most egregious problem is the result of industrialized agriculture in the Midwest.

BP: Let's take these one at a time. What's happening with illegal deforestation in Mexico?

LB: The first time I ever went down to the overwintering sites was in 1977, and it was just absolutely incredible to walk through this forest and see this wall of monarch butterflies. That year, the colony we saw was about 3 hectares, and the total area [of all colonies across Mexico] may have been as high as 15 to 20 hectares altogether, although we don't know for sure.

Since then, there's been heavy deforestation from illegal logging. The World Wildlife Fund has published quite a bit of documentation of that. In 2000, a new decree was announced by the president of Mexico, but even after that, there was still illegal logging, with several hundred hectares of forest clear-cut. The Mexican government in the last five years has made a strong effort to stop illegal logging, but there’s still small-scale logging going on, with a few trees taken here or there. This progressively deteriorates the forest.

The forest serves as a blanket and umbrella for the butterflies. Even though the area is in the tropics, because it’s such high elevation, 11,000 feet up, it gets cold during January and February. The temperatures can drop to 10°C below freezing. Butterflies can tolerate that provided they're not wet — but if they get wet, they lose their resistance.

In 2002, [the overwintering habitats had] the worst storm on record. We counted the number of dead butterflies in the colonies and estimated that about 75 percent of the population was killed, by a combination of rain, snow, and freezing. If that were to happen again this year, it could be catastrophic.

BP: You mentioned severe weather as another factor in the last two years. What happened?

LB: Two years ago, Texas had that massive drought. All the monarchs that migrate down to Mexico have to go through Texas during the fall. And as they migrate, they feed on nectar from composites, wildflowers, goldenrod, various species of sunflower. That nectar has sugar in it which the adult butterflies convert into lipids, which fuels their winter survival for five months. There's insufficient nectar at the overwintering site in Mexico to support them, so building it up beforehand is important.

That was a really serious drought. There was a very strong possibility that not enough butterflies made it to Mexico with enough energy to survive the winter.

Then, this year, the numbers were already down, only about 60 million. So the migration comes back at the end of February and the middle of March and they move northward. These are the 5-month-old survivors that leave Mexico, migrate up through Texas, and lay eggs on milkweed. And the new generation that's produced in about three weeks moves all the way up to Canada. But when that new generation tried to migrate this spring, they were held back by a cold front that was in place for a long time. And that prevented a really good spring re-migration up into the northern breeding range.

Normally over the course of the summer, that first spring generation will produce two more summer generations. But data indicates that the butterflies are missing a whole generation this year. And reports coming all over the Eastern range, from the Rockies all the way to Maine and Virginia, there are very few butterflies breeding anywhere. So the numbers getting back to Mexico are really low. And that's what we're waiting to hear, just how low are they?

BP: Is climate change expected to affect the butterflies in any way?

LB: We don’t really know how much climate change is causing problems. There are several models that have been published that indicate that with global warming, the fir forests in Mexico that provide a habitat for the butterflies will decline, it will become too warm to support that forest. My feeling is that’s mainly a problem in the future, and probably not a problem yet — though it very well could be.

BP: You also mentioned Midwestern agriculture as a third factor in the decline. What's happening there?

LB: The most catastrophic thing from the point of view of the monarch butterfly has been the expansion of crops that are planted on an unbelievably wide scale throughout the Midwest and have been genetically manipulated to be resistant to the powerful herbicide Roundup.

These crops are planted in the grassland ecosystems of the United States, where the monarchs do most of their breeding. And normally in that area there are milkweed growing all over the place on the agricultural fields and the edges of fields and the sides of roads. There are 108 species of milkweed in the United States — the whole monarch migration evolved in relation to evolution of this milkweed flora.

Anyway, where they use these herbicides, it kills all emergent seedlings and all the emergent perennial plants. A paper last year by John Pleasants of the University of Iowa and Karen Oberhauser of the University of Minnesota estimated that 60 percent of milkweed has been eliminated from the grassland ecosystem. We're not just talking about one species, we're talking about the entire native flora being eliminated.

The other thing herbicides do is kill sources of nectar. This is important: When monarchs come back [to the United States] they lay their eggs on milkweed, the caterpillars hatch out in four or five days and develop over a period of two or three weeks, then form the chrysalis, then a week later it hatches into an adult. These adults initially have about 20 milligrams of fat in their body that's carried over from larval development. But the butterflies that migrate back [to Mexico] have about 125 mg of fat. All that additional fat is gotten from drinking nectar from wildflowers. And this agriculture is killing off the wildflowers.

BP: Why should we care about the decline of monarch butterflies? Because they're an iconic species? Or are they important to broader ecosystems?

LB: If the monarch goes extinct, it's no different ecologically from the loss of the passenger pigeon. It’s just like going into a museum and pulling a rare painting off the wall and destroying it. So there's the aesthetic aspect.

There's also the fact that monarch butterfly is used in teaching biology to kids on a scale that's unequaled by any other insect in the world. So it's an incredibly strong educational tool. And the biology of the migration is absolutely unique.

So you get back to the philosophical aspect of it: How many natural phenomena are we going to kill off? I think the monarch is the canary in the coal mine telling us that things are beginning to go really wrong, when you can take a widespread migration of this sort and completely dismantle it as a result of human activity.

BP: What's the best-case scenario for the monarch butterfly?

LB: A lot of people are pretty confident that it will recover, but it will be slow. Monarchs can lay 300 or 400 eggs, and their reproductive capacity is rather extraordinary. Like cockroaches, they've managed to survive eons, through thick and thin, through asteroids and everything. Part of the reason insects are capable of surviving like that is that their reproductive capacity is so high, unlike mammals.

So if the conditions are right, the monarch has the chance of coming back fast. If we have really good weather and healthy milkweeds and no freezing in Mexico and no drought in Texas... they may be able to come back. But I’m beginning to have doubts.

BP: Why do you have doubts they'll recover? What's the worst-case scenario?

LB: We have a hypothesis about this. Every year, the butterflies fly from all over eastern North America to a tiny 30 by 60 square mile part of Mexico 3,000 miles away that they have never been to before. They don't go anywhere else that we know of, and we've been looking all over.

So the question is how do they find the same trees year after year? Our hypothesis — for which there is weak evidence, no specific chemical evidence yet — is that they may be marking the trees chemically, so that they can orient themselves when they come back the following year.

My concern is that if that hypothesis is true, then the signal could get so weak because so few butterflies are getting back that next year the butterflies might not even be able to find the area. The whole migration could collapse. This is a real concern, but it's going to be controversial, because we don't have hard evidence for it. We're working on it.

BP: Are there things people could do to halt the decline in monarch populations? You mentioned deforestation in Mexico above. What about here in the United States?

LB: Are you familiar with the group Monarch Watch? One of their programs is to get people to plant milkweeds in their gardens. I think that's a positive response, although it's not going to be adequate to restore monarch populations, compared with the scale of natural native milkweeds growing in a million square miles of land, it's not the answer.

You can also look at highways. Drive along Interstate 81 through south Virginia, there's tons of milkweeds growing in the median, and much of that is being eliminated over such a huge area by highway departments. One of the problems is that they mow the sides of the road, and if they hit that area during the peak of monarch breeding, that can kill everything.

So one of the things we've suggested, there are millions of miles of roadside in the eastern United States, and if we could get highway departments everywhere to manage roadsides for wildlife rather than just cutting them down when it's convenient or spraying them with herbicides, that could be a boon to insects in general. That's the one mitigation I can think of.

The other is to try to get the people using herbicides in agriculture to have a system of not spraying 100 percent of their fields. Don't spray along 100 feet along the edge of everyone's field, so that there's some native habitat left. So there are some things that can be done.

Interview has been lightly edited for length and clarity.

Related: It's not just monarchs. Bees are dying too, and the U.S. and Europe have different theories on why.