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Monarch butterflies gather on a tree at the El Rosario Butterfly Sanctuary near Angangueo, Mexico.KIRSTEN LUCE/The Associated Press

The latest count of monarch butterflies overwintering in Mexico suggests the insect's North American migratory population has hit a new low and may be at risk of disappearing altogether.

The dire forecast, announced Wednesday, is based on the the amount of Mexican woodland occupied by the orange-and-black butterfly during its annual hibernation. In December, that space shrank to a mere two-thirds of a hectare, an area not much larger than a football field and 44 per cent smaller than that inhabited by monarchs during the previous winter.

The result is the lowest since conservationists began tracking monarch winter populations and it has sparked a renewed call for pro-active measures.

"We're gambling with this," said Omar Vidal, director of the World Wildlife Fund Mexico, which gathered the monarch data. "If we don't take immediate action, we are very close to losing this migratory phenomenon."

While monarchs as a species are widespread around the globe, it's the astonishing migration of the eastern North American population – the most highly evolved in the insect world – that experts fear is at risk.

Each spring, waves of butterflies move northward from a region of high-altitude forest in Mexico's Sierra Madre and fan out across the U.S. and Canada from the Rockies to the eastern seaboard. Since the butterflies normally live no longer than six weeks, it typically takes three generations for them to reach the northern limits of their range. At that point, regulated by as-yet unknown genetic signals, a fourth "super generation" emerges with the ability to survive up to eight months, during which individual butterflies endure a 4,000-kilometre flight back to Mexico.

The areas where they congregate to spend the winter are so densely packed with butterflies that conservationists do not attempt to count their numbers but instead use GPS to make an accurate estimate of the total area covered by the insects. In past years, that area has topped more than 18 hectares in as many as 17 discrete clusters. This winter, Mr. Vidal said, only seven clusters were found.

The overwintering phenomenon was first documented in 1976 by University of Toronto zoologist Fred Urquhart, whose startling find was popularized in the Imax film Flight of the Butterflies. Conservationists have been measuring the butterflies' total overwintering area since 1993, and have observed a steady decline in its size over the past 10 years. This has mirrored a lower number of monarchs returning north each year, with the insects seeming to have all but disappeared from Canadian meadows and gardens last summer.

"It was a very quick and shallow migration last fall," said Don Davis, who chairs the Monarch Butterfly Fund, a conservation charity, and who has participated in butterfly counts in Ontario since 1967 when he began working with the late Dr. Urquhart.

Experts cite multiple reasons for the downward trend. Efforts by the Mexican government to curtail illegal logging in the monarch region is having a positive effect, conservation groups say, but that has been outweighed by the negative effects of intensified agriculture and the use of herbicides in the U.S. Midwest. This has eliminated areas where milkweed, a plant that is crucial to the monarch life cycle, can grow.

Land-use changes in Canada could also be threatening monarch survival. Phil Schappert, a Halifax-based insect biologist and author, noted that the Canadian government's reworking of the Navigable Waters Protection Act in 2012 "removes all protection of the lands bordering waterways" including monarch breeding areas.

Mr. Vidal said that the low winter number was a wake-up call that should put butterflies on the agenda when the leaders of all three countries meet in Mexico next month. "I think it's time now for [them] to sit and agree on an immediate plan of action to conserve the monarch," he said.