You are the owner of this article.

Monarch butterfly numbers decline again in Mexican wintering grounds

  • ()
  • 3 min to read

The number of monarch butterflies counted at the insects’ wintering grounds in the mountains of Mexico in late December is down again from the previous year, in spite of higher numbers of the East Coast population migrating through Cape May this fall.

“We could lose the monarch butterfly if we don’t take immediate action to rein in pesticide use and curb global climate change,” said Tierra Curry, a senior scientist at the Center for Biological Diversity and co-author of a 2014 petition to protect monarchs under the Endangered Species Act.

The U.S. Fish and Wildlife Service is still studying the issue and has agreed to make a final decision by June 2019.

World Wildlife Fund Mexico and Comisión Nacional de Áreas Naturales Protegidas registered nine colonies of monarch butterflies that covered 2.48 hectares of forest in Mexico, the organizations recently announced.

Monarch butterfly numbers

It is a 14.77 percent decrease from last year, when the monarchs covered 2.91 hectares, the groups said.

But it is dramatically low compared with 1996-97, when the butterflies covered more than 18 hectares.

One hectare in the metric system equals about 2.5 acres.

Overall, monarchs have declined by more than 80 percent over the past two decades.

This year’s decrease is partly due to tropical storms and hurricanes in mid-September, when the migration began, said Jorge Rickards, director general of the World Wide Fund for Nature Mexico. High temperatures in the United States also caused a late migration that resulted in fewer reaching Mexico, he said.

The Monarch Monitoring Project at New Jersey Audubon’s Cape May Bird Observatory reported much higher numbers of the insect migrated through Cape May Point in the fall of 2017 than 2016. But the numbers were still only about half of those from 2012.

The East Coast population is a small subset of the overall monarch population, said project Director Mark Garland, who recently returned from leading a group to visit three of the overwintering sites in Mexico.

“It was still totally overwhelming when you are there — an astounding sight,” he said of seeing millions of monarchs clustered on trees.

He said he heard anecdotally that Midwestern numbers were low this year.

“The long-term trend on the Atlantic Coast is a lot better than across the country. It’s too bad ours is a small percentage of continental monarchs,” Garland said. “It’s reinforcing the most widely held theory that what is limiting the population of monarchs is widespread herbicide use in the (Midwestern) farm belt.”

Garland said crops that have been genetically modified to resist herbicides such as glyphosate, marketed as RoundUp, have allowed farmers to use much more of the chemical, killing off the native milkweed plants on which monarchs lay their eggs.

Monarch butterfly migration graphic

The monarch caterpillar eats only milkweed, so when that native plant declines, the monarch population follows.

The East Coast and Central U.S. populations of monarchs migrate to the mountains of Mexico in the fall and stay there in a dormant state until spring. Then they mate and start the migration back north, laying eggs along the way.

Scientists estimate the population in Mexico by measuring the area of trees covered by the butterflies as they cluster together to survive the winter.

That population has been dangerously low since 2008, the center said. In the mid-1990s, the population was estimated at nearly 1 billion butterflies, but this year’s population is down to about 93 million.

A 2016 study by the U.S. Geological Survey estimated the monarch population needs to get back to at least 225 million butterflies to be safe from eventual extinction.

Of the nine colonies counted this year, five were within the Monarch Butterfly Biosphere Reserve and four outside it, census takers said.

Monarchs west of the Rocky Mountains overwinter on the central coast of California, where counts have found their numbers continued to drop as well.

A Thanksgiving 2017 count in California found only about 200,000 butterflies, down from 1.2 million two decades ago.

Never miss breaking news as it happens! Sign up now to receive alerts delivered to your inbox.

Contact: 609-272-7219 Twitter @MichelleBPost

In my first job after college got paid to read the New York Times and summarize articles for an early online data base. First reporting job was with The Daily Record in Parsippany. I have also worked in nonprofits, and have been with The Press since 1990.

Recommended for you

Welcome to the discussion.

Keep it Clean. Please avoid obscene, vulgar, lewd, racist or sexually-oriented language.
Don't Threaten. Threats of harming another person will not be tolerated.
Be Truthful. Don't knowingly lie about anyone or anything.
Be Nice. No racism, sexism or any sort of -ism that is degrading to another person.
Be Proactive. Use the 'Report' link on each comment to let us know of abusive posts.
Share with Us. We'd love to hear eyewitness accounts, the history behind an article.