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Are We Losing the Western Monarch Migration?

Experts blame habitat destruction, climate change and insecticide use

A monarch butterfly sits on clover. Photo: Kenneth Dwain Harrelson via Wikimedia Commons

Hundreds of thousands of monarch butterflies used to drift through the treetops at Natural Bridges State Beach in Santa Cruz. This year, official counts estimated that just 550 monarchs visited the park.

Sites around the West Coast experienced a sharp drop this year in an already steeply declining pattern. Experts blame habitat destruction, climate change and insecticide use for the dip in numbers.

“It’s really, really concerning,” says Martha Nitzberg, lead interpreter for Natural Bridges. But Nitzberg says she’s hesitant to give people the numbers, because she wants them to hold onto hope. “I don’t want to turn people off from thinking they can make a difference.”

WHY THEY FLY

Each year, western monarchs migrate south to overwintering sites on the California coast. The butterflies migrate for the same reasons many other animals do—to avoid freezing temperatures, leave behind diseases and find new sources of food.

A few natural cues signal to the butterflies that it’s time to migrate, including the lowering angle of the sun and colder days.

“When the temperature drops, their hormones shift from a mating mode into something we call reproductive diapausing,” Nitzberg says. This more lethargic state allows them to survive the winter.

Along coastal overwintering sites, the lack of native milkweed—the only food source for monarch caterpillars—also helps the shift. “In Santa Cruz, we never had milkweed,” Nitzberg says. “They come, there’s no milkweed, and it helps them shift their hormones to get into this non-reproductive mode.”

Once at the overwintering site, monarchs cluster in trees and reduce their activity in order to save energy. At this stage, volunteers with the Western Monarch Count and scientists around the West Coast estimate the population numbers. 

FALLING PATTERNS

Sarina Jepsen, director of endangered species at the Xerces Society for Invertebrate Conservation, says groups have been tracking major declines for a few years now, but this year’s numbers are particularly concerning. Across the entire state of California, volunteers counted less than 2,000 monarchs.

“Last year and the year prior were alarmingly low as well,” Jepsen says. Surveyors counted about 25,000 monarchs in the winter of 2018, and just under 22,000 the next season. “And prior to that, for as long as I’ve been working in monarchs—which is more than a decade—we’ve had several hundreds of thousands of monarchs counted each winter,” she says.

DISAPPEARING HABITAT

Both Nitzberg and Jepsen cited habitat destruction as a major threat for the butterflies. Development, wildfires and tree-removal all contribute to the loss. At overwintering sites, cutting even a single tree could lead to changes in microclimates that the monarchs need to survive.

“There’s a real need to have people adopt these overwintering sites,” Jepsen says. “When a site gets threatened, to have someone show up to a city council meeting and talk about the importance of that site for monarch butterflies and ensure that monarchs are considered in any changes to the site’s management is a really important role.” 

Monarchs are not currently listed under the Endangered Species Act, even though the U.S. Fish and Wildlife Service determined in December that they warrant protection. Other threatened species took priority, and the current plan will list monarchs as threatened in 2024.

“I understand that the Fish and Wildlife Service lacks sufficient resources to adequately address the extinction crisis that we’re in,” says Jepsen, “but the western populations might not be able to wait that long.”

CLIMATE CHANGE AND PESTICIDES

Protecting migration and overwintering sites addresses one part of the problem, but monarchs face other threats too. Butterflies and their eggs don’t always survive extreme heat waves, and a changing climate means that monarchs tend to leave overwintering sites earlier in the year than they used to—sometimes before their food sources have bloomed.

Pesticides pose another danger. One especially deadly class of insecticide for pollinators are neonicotinoids. Chemically similar to nicotine, neonicotinoids are the most popular type of insecticide in the world.

“Neonicotinoids are particularly concerning for monarchs because they persist in the environment for so long,” Jepsen says. “They can hang around for months or even years, and they leach into other areas.”

The Xerces Society and the University of Nevada, Reno recently collected milkweeds from several environments around the Central Valley and tested them for different types of pesticides. They found that all the plants were contaminated—even those grown in nurseries and seemingly pristine areas.

LIFTING MONARCHS

One way people can help the monarchs, Nitzberg says, is to avoid using these sorts of pesticides in their gardens. Even with the dire numbers, she encourages folks to take action. Planting native wildflowers is another way to contribute.

Although milkweed might seem like a good option, the Xerces Society cautions against planting any variety within five miles of the coast, where western monarchs overwinter. Monarchs with access to milkweed don’t enter their non-reproductive, energy-saving phase and tend not to make it through the winter. Farther inland, native milkweeds provide a healthier option than tropical varieties, which sometimes harbor a parasite known as ophyrocystis elektroscirrha, or OE.

Actions that help monarchs will also likely help other, lesser-known pollinators that face similar threats.

“The monarch is kind of the canary in the coal mine warning us of everything happening for everybody else,” Nitzberg says. “It’s bigger than just a few beautiful butterflies. We have to keep figuring out how to make the world safer for them because then it makes the world safer for us as well.”

2 Comments

2 Comments

  1. Adriana Gores

    February 16, 2021 at 11:08 am

    Although i applaud this article for bringing awareness around the plight of the western monarch, especially with regards to habitat destruction, it should be noted that Xerces’ recommendations and thoughts about planting milkweed in coastal communities is opinion and not based on science. There is research being done on this subject and Xerces’ opinion is considered somewhat controversial. Please consider offering another side to this before locals start ripping out their milkweed!

  2. Eleanore Diane Rose

    February 13, 2021 at 6:43 pm

    I’m the founder of the Santa Cruz Monarch Society. There are perspectives on Monarch Butterflies that differ from Xerces. Let me know if you’d like to chat. Thank you! 😄
    -Eleanore

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