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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.”