MEXICO CITY — Hoping to focus attention on the plight of the monarch butterfly at a North American summit meeting next week, a group of prominent scientists and writers urged the leaders of Mexico, the United States and Canada to commit to restoring the habitat that supports the insect’s extraordinary migration across the continent.

Calling the situation facing the butterfly “grim,” the group issued a letter that outlined a proposal to plant milkweed, the monarch caterpillar’s only food source, along its migratory route in Canada and the United States.

Milkweed has been disappearing from American fields over the past decade as farmers have switched to genetically modified corn and soybeans that are resistant to the herbicide glyphosate that kills other plants. At the same time, subsidies to produce corn for ethanol have increased, expanding the amount of land planted with corn by an estimated 25 percent since 2007.

“We can’t ask farmers to change their habits,” said Homero Aridjis, the Mexican poet who wrote the letter, which was to be released on Friday.