Plummeting populations of monarch butterflies in Mexico have raised fears about the future of the largest and most remarkable insect migration in the world, but the group that winters in California appears to be holding on after years of decline.

Mexico is the winter destination of hundreds of millions of the striking orange-and-black butterflies, which travel thousands of miles from southern Canada and the United States each year to escape the cold - with some settling along the warmer parts of the California coast.

But this year’s winter count of monarchs in the Sierra Madre of Mexico was the smallest recorded since 1993, when entomologists began keeping records, according to a study by the World Wildlife Fund.

The dramatic decline - from close to 1 billion to an estimated 35 million - has lepidopterists in California in a flutter, but nobody seems to know what effect, if any, the collapse might have on monarchs in the Golden State.

Only 1.65 acres of the pine and fir forests west of Mexico City were covered with monarchs this winter, compared with 2.93 acres last year, according to the report, which was prepared by the Wildlife Fund, Mexico’s Environment Department and the Natural Protected Areas Commission. At their peak in 1995, the butterflies covered 44.5 acres of the forest.

The population is now less than 5 percent of its peak, said Karen Oberhauser, an ecologist who has been studying the monarchs for 30 years.

“It’s really concerning,” said Oberhauser, a professor in the department of fisheries, wildlife and conservation biology at the University of Minnesota. “The numbers have never been this low. Last year was the lowest year ever, and this year is just a little more than half of what we had last year, so that’s two years in a row of record low numbers.”

California rebound

The butterflies in Mexico are the same species as the ones that winter in California, which has recently experienced a rebound in monarch populations.

The California butterflies, which gather for the winter in more than 200 groves along the coast, declined 90 percent between 1997 and 2009. The winter population at Natural Bridges State Beach, near Santa Cruz, dropped from an estimated 120,000 in 1997 to just 1,300 in 2009.

Things have improved in California over the past five years, largely as a result of conservation efforts, according to experts. An estimated 7,800 monarchs were seen this winter at Natural Bridges. That’s compared with 1,500 a year ago, which was a particularly bad year, said Michael Ray, the state beach’s interpretive specialist.

“We had an excellent year,” Ray said. “It was probably the best population we’ve seen in about 15 years.”

The monarch is one of the largest and, many say, most beautiful butterflies in the world. It is found throughout North America, but the species has expanded its range around the globe, including to Hawaii, New Zealand and Australia.

Milkweed feasts

Mexico’s spectacular winter monarch migration is the largest in the world, with tens of thousands of butterflies often covering a single tree. The forest is a kaleidoscope of butterflies, with so many that one can hear the sound of their wings flapping, according to researchers.

The insects begin their trek south in Canada and the Pacific Northwest, where the caterpillars live on milkweed until they are ready to fly. The trip south involves a unique genetic directional imprint that scientists still do not understand.

The migrating butterflies divide at the Rocky Mountains into eastern and western migratory populations. The western group spends November through March in California and the eastern population winters in Mexico, more than 2,500 miles away from where they started.

The study blames the decline of the eastern population on urban sprawl and a lack of milkweed- and nectar-bearing flowers along their migratory route through the Midwest. The problem, Oberhauser said, is that corn and soybean crops have been genetically modified to be resistant to herbicides, particularly Roundup, which is a Monsanto trademark. As Midwest farmers increased herbicide use, milkweed died off.

The report also blamed a dramatic reduction in butterfly habitat in Mexico as a result of illegal logging and extreme weather in Canada, the U.S. and Mexico.

Concern for state’s numbers

The extermination of milkweed is not as big of a problem in the Pacific Northwest, which may be why the monarch population in California hasn’t been hit as hard. Still, experts say, the butterfly population in California is by no means safe.

The California butterflies are being threatened by urban sprawl, pesticides and loss of habitat.

“Here in the western portion of their range the numbers have been going down” over time, said Chris Nagano, a senior scientist for the endangered species program for the U.S. Fish and Wildlife Service. “I wouldn’t say the monarch as a species could be in trouble, but the phenomenon where they winter in California and Mexico, that’s a concern.”

It is not clear how much the western and eastern populations mix - some monarchs tagged in Arizona have been found in both Mexico and California - but their journey back and forth from their wintering grounds is phenomenal.

In March, the males die and the females start heading back north, but each butterfly lasts only a couple hundred miles, where they lay eggs on milkweed and die. The trip back to Canada is essentially a relay race involving four generations of adult butterflies, which each feed on flowers along the way before breeding and dying. The monarch that returns to Mexico or California the next year is consequently five generations removed from the last ancestor that wintered there. Scientists have not figured out how the insects know where to go.

“It’s really amazing,” Oberhauser said. “Certainly there are other organisms that migrate and other insects that migrate, but the migration of monarchs to the same spot year after year is a unique phenomenon, and the density of their population is unprecedented.”

The World Wildlife Fund is urging Canada, the United States and Mexico to take action to protect monarch migration at the North American Leaders’ Summit on Feb. 19 in Toluca, Mexico.

“What’s happening to monarchs is probably happening to lots of species,” Oberhauser said. “This is a species, unlike most other insects, that we can count and look at what we’ve done to it. So this really should serve as a wake-up call.”