The annual migration of Monarch butterflies from the U.S. and Canada is "in danger of disappearing" after the number of the insects currently in Mexico dropped to its lowest level since 1993, according to the Associated Press.

In a recent report released by the World Wildlife Fund, the Natural Protected Areas Commission and Mexico's Environment Department have blamed the decline in Monarch butterflies due to the "displacement" of milkweed, which is genetically modified crops and urban sprawl in the U.S., along with the reduction of the Monarch's habitat in Mexico.

The announcement comes right before the 20th anniversary of the North American Free Trade Agreement, which is an agreement made by Canada, Mexico, and the U.S. to "protect migratory species" like Monarch butterflies, according to AP.

"Twenty years after the signing of NAFTA, the Monarch migration, the symbol of the three countries' cooperation, is at serious risk of disappearing," said Omar Vidal, Omar Vidal, the World Wildlife Fund director in Mexico, according to AP.

After steady declines the last three years, the butterflies only cover approximately 1.65 acres in the pine and fir forests located west of Mexico City. The butterflies covered 2.93 acres in the Mexican forests during 2013, and 44.5 acres at their peak in 1995, according to AP.

The butterflies are counted by the area they cover since they usually bundle together by the thousands.

Lincoln Brower, a leading entomologist at Sweet Briar College in Virginia, said that "the migration is definitely proving to be an endangered biological phenomenon."

"The main culprit is now GMO herbicide-resistant corn and soybean crops and herbicides in the USA," which "leads to the wholesale killing of the monarch's principal food plant, common milkweed," Brower wrote in an email, according to AP.

Weather conditions have also played a role in the decline of Monarch butterflies, like unusually heavy rain or droughts in all three countries.

Experts currently don't know what would happen to the species if they no longer migrated, but they can survive in warm climates year-round. They can't however survive long in locations that get cold like Canada and most parts of the U.S.

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