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MEXICO CITY – The annual migration to Mexico of millions of orange-and-black monarch butterflies is one of the nation’s cherished rituals. But it could come to a virtual halt if the insect’s natural habitat is not urgently salvaged.

That is the conclusion of a long list of scientists, artists and environmentalists who are calling on the leaders of Mexico, the United States and Canada to act swiftly to protect butterfly breeding grounds. President Obama, Mexican President Enrique Peña Nieto and Canadian Prime Minister Stephen Harper are to meet in Mexico next week to discuss economy, trade and other issues.

Though the number of butterflies that travel thousands of miles across North America to Mexico has been declining fairly steadily for years, the migration has never been in more danger than now, the activists say.

Blame has always been put on rampant illegal logging in Mexico that destroys the oyamel fir forests where the insects alight and spend the winter. Increasingly, however, activists say blame must be placed on eradication of the milkweed plants in the U.S. where the butterflies lay their eggs and where monarch caterpillars eat.

“It is ecological genocide,” Homero Aridjis, a prominent Mexican poet and advocate for the butterfly, said in an interview. “By killing the plant, you are killing the monarch butterfly. If they don’t stop the destruction of the milkweed, in a few years the migratory phenomenon could collapse.”

Especially in the U.S. Corn Belt, the planting of genetically modified, herbicide-resistant corn and soybean varieties has grown dramatically. The herbicides that are used, especially glyphosate, destroy all other plant life, including the milkweed — the only plant eaten by monarch caterpillars.

In addition, the big money to be made from ethanol has led to increased planting of corn in previously marginal areas that would have been covered in milkweed. Activists warn that the disappearance of the monarch bodes ill for worldwide ecology, akin to the loss of bees.