The monarch is probably the best-known butterfly in the world, famed for the beautiful orange and black hues of its wings and its even more remarkable trait of undertaking an annual migratory journey of thousands of kilometres across North America.

Although millions of monarchs exist and the insect is not considered to be in danger of extinction, a new report warns that the butterfly's migration as a natural phenomenon is imperilled because of threats to its habitat. These include the widespread spraying, by farmers and municipal weed-control workers, of herbicides that kill the milkweed on which the monarch's larvae feed. Another problem is the conversion of fallow land into urban sprawl.

The migration of monarchs is “among the most spectacular and unusual of the world's natural events,” says the report, but its decline “is certain unless these threats are addressed.”

The report, issued by the Commission for Environmental Cooperation, the Montreal-based conservation watchdog for the North American free-trade agreement, also says Canada, the U.S. and Mexico each have a responsibility to protect the insect.

“Habitat conservation and restoration are absolutely necessary for monarch survival,” the report says. Mexico and the U.S. have to ensure that a suitable habitat is available in their wintering grounds, while all three countries must ensure that a sufficient breeding and migrating habitat remains available.

“Because monarchs depend upon a wide range of habitats in Canada, the United States and Mexico, conservation of the migratory phenomenon requires trilateral co-operation.”

The principle author of the report was Karen Oberhauser, an ecology professor at the University of Minnesota. In an interview, she said that even homeowners can help the butterflies by growing a few milkweed plants – along with nectar-producing flowers such as mints, daisies and coneflowers – around their homes. Adult butterflies need nectar to power their lengthy migrations.

“If you think of the magnitude of suburban land and think, if all of that land was more appropriate habitat for monarchs … that would be a huge contribution.”

She also noted that pressure to plant more corn for use as ethanol could affect the monarchs; it takes marginal U.S. farmland that has been held fallow in conservation reserves and turns it into herbicide-sprayed cropland.

Monarchs are the only butterfly to have an annual migration, with some of the insects in the eastern population flying more than 2,500 kilometres from Southern Canada to winter grounds in Mexico.

Although monarchs live and breed in Canada during the warmer months of the year, they are viewed as tropical insects because they cannot withstand the rigours of low winter temperatures in mid-latitude areas.

There are two distinct populations of migratory butterflies in North America. One breeds in southern B.C. and adjacent areas of the western U.S. and winters in California, while the second breeds in the eastern U.S. and Eastern Canada and winters in Mexico. It is this eastern population that has the most spectacular migrations.

According to a Canadian population survey cited in the report, the eastern population passing through the Long Point area on the north shore of Lake Erie declined by 3 per cent over a study period lasting from 1995 to 2006.

There are also non-migratory populations in southern Florida, the Caribbean and parts of South America. Florida butterflies are typically wiped out every few years during bouts of freezing weather, and then replenished by in-migration from the eastern population.

Monarchs are also found in countries as diverse as New Zealand, Australia, Spain and Portugal, and in parts of Micronesia. There are even infrequent sightings in England, which raises for Dr. Oberhauser the possibility that the insects are occasionally being blown across the Atlantic Ocean on winds from North America.

Researchers do not know how the butterflies arrive at places outside the Americas, but suspect introductions by humans. Scientists do not know what guides the butterflies on their annual migrations either, nor how they are able to find their Mexican wintering sites, which are found in a 10,000-square-kilometre belt of volcanic mountain ranges and valleys in the central part of the country.