WASHINGTON, February 3, 2014—The annual migration of the monarch butterfly between Canada and Mexico has reached its lowest levels since 1993, when their numbers were first recorded, according to new data released Wednesday by World Wildlife Fund (WWF)-Telcel Alliance and the Mexican government’s Monarch Butterfly Biosphere Reserve Office.
Every November, millions of monarch butterflies arrive in a small area in the mountains of central Mexico, flying all the way from southern Canada and the northern U.S. They are the final generation in a year-long migration. This year, however, the numbers arriving in thier winter habitat in Mexico were alarmingly low.
Monarch populations are measured by the amount of land they take up. Newly released numbers show a dramatic one-year 43.7-percent decrease in total land occupied by the butterflies in the Monarch Butterfly Biosphere Reserve in central Mexico.
This winter, the monarchs occupied only 0.67 hectares; in 2012, they occupied 1.19 hectares. Both are disturbingly low considering a record high of 21 hectares in 1997.
Thought to be the world’s second-longest insect migration (the longest is that of a species of dragonfly in Africa), it takes monarchs three to four generations to complete one migration cycle. It is currently unknown exactly how monarchs know where to go, but scientists suspect it has to do with a chemical trail left by prior generations along the migration route.
The butterflies arriving in central Mexico in November are the last generation of a cycle that repeats every year. This particular generation of monarchs begins life in southern Canada and northern U.S. emerging in August and traveling south up to 2,800 miles to spend winter clustered together by the millions in the mountains in central Mexico in an amazing natural spectacle.
After spending winter in Mexico, this same generation travels north in the spring, laying their eggs in northern Mexico and southern U.S. in March. A new generation emerges in late April and June, continuing the flight north and laying eggs along the way.
As they fly towards southern Canada, a final generation of monarchs emerges in late August. This generation grows larger and takes in more nectar for its trip south and subsequent winter hibernation. Flying to Mexico over a period of two months, the butterflies arrive by the millions in early November, in time for the Day of the Dead.
There is another smaller migration of monarchs west of the Rocky Mountains. These butterflies winter in the California coast and are seeing similar or worse population declines.
Three major reasons for decline
Researchers blame three major causes for the steep, consistent decline in monarch butterfly migration numbers in the past 20 years in North America: deforestation in their winter habitat in Mexico, severe weather and commercial agriculture along their migratory route.
Despite the Mexican government’s efforts to stop illegal logging in the forests in central Mexico, deforestation of the monarchs’ winter habitat is still a problem. The butterflies depend on the protection provided by the forests as temperatures drop during the winters.
Generally able to survive the low temperatures of the central Mexican winters, monarchs will make it to spring unless they get wet. In 2002, for example, an especially severe storm wiped out around 75 percent of the monarch population in central Mexico, according to a Washington Post interview of Professor Lincoln Brower, a monarch butterfly specialist at Sweet Briar College.
In the last year, monarchs faced several threats from extreme weather conditions involving heavy storms and severe heat waves. Monarchs are extremely sensitive to heat. Monarch butterfly eggs dry out in hot weather and larvae die at temperatures above 95°F, contributing to the significant decreases in population.
However, commercial agriculture may be the most serious long-term threat to the monarch migration. While adult monarchs feed on fruit and the nectar of several types of flowers, their larvae can only survive on milkweed.
As much of the American Midwest is being planted with corn and soy crops that are genetically engineered to withstand powerful herbicides like Roundup, milkweed is being systematically wiped out. Over one million acres of grassland in the Upper Midwest—much of it formerly abundant in milkweed—has been converted to corn and soybean fields in recent years, according to a 2013 study published in the Proceedings of the National Academy of Sciences of the U.S. (PNAS).
Close to 83 percent of all corn and 93 percent of all soybean crops planted in the U.S. in 2013, according to the U.S. Department of Agriculture (USDA), were genetically engineered (GMO) to tolerate specific kinds of herbicides, mainly Monsanto’s glyphosate-based Roundup.
As Roundup resistant crops proliferated in the Midwest, milkweed, which used to commonly grow among all kinds of crops, was reduced by 80 percent, according to a 2012 study co-authored by Karen Oberhauser at the University of Minnesota. Professor Oberhauser also found that the decline in milkweed was accompanied by a similar decline in monarch butterfly egg production.
“We have this smoking gun,” said Professor Oberhauser to Slate. “This is the only thing that we’ve actually been able to correlate with decreasing monarch numbers.”
While monarch butterflies are still abundant in other parts of the world including Australia, Hawaii, New Zealand, Portugal and Spain, the concern is that the natural wonder of the spectacular migration to Mexico will be lost.
“The monarch butterfly as a species is not endangered,” explained Omar Vidal, World Wildlife Fund Mexico director general in an email to National Geographic. “What is endangered is its migratory phenomenon from Canada to Mexico and back.”
The loss of the monarch migration also means an economic loss for several places located on the migration route and winter habitat, especially in Mexico, which depend on monarch-related tourism. The migration is additionally part of Mexican culture in the areas where the butterflies spend the winter.
“They were part of the landscape of the Day of the Dead, when you could see them flitting around the graveyards,” said Gloria Tavera, director of the reserve to the Christian Science Monitor. “This year was the first time in memory that they weren’t there.”
Oberhauser said that she believed the migration can bounce back, given monarch butterflies’ ability to lay several hundred eggs in their lifetime. However, Mexico, the U.S. and Canada must work together to ensure that the butterflies can migrate safely and that their route not be cut off or interrupted.
“[Insects] that have such a high rate of reproduction can recover from short periods of high mortality,” said Professor Oberhauser to Journey North, a teachers’ educational resource. “The key is short periods of high mortality – if this continues, it would affect their future survival.”
There are several organizations that advocate planting milkweed in backyards, giving away millions of milkweed seeds to individuals for planting every year. Others partner with groups like the Girl Scouts and Boy Scouts, schools and even local governments to plant milkweed along highways and in other public spaces. Butterfly releases and milkweed seed favors have become popular at weddings and other events.