The migratory population of the monarch butterfly has reached an “ominous” low, researchers in Mexico announced Wednesday.

Scientists are attributing the decline of this essential pollinating population to the ongoing drought and the “explosive” increase in the use of genetically modified crops in the American corn belt.

“Because farmers have planted over 120 million acres of crops resistant to the milkweed-killing herbicide glyphosate, the monarchs’ essential food supply has been all but destroyed.”

Released by the Mexican government along with the World Wildlife Fund (WWF), a recent survey found a 59% decrease in the area occupied by monarch colonies wintering in the forests of central Mexico, the Los Angeles Times reports.

Because of the difficulty in counting butterflies, scientists rely on measurements of the area that butterflies occupy to estimate their numbers; 1 hectare may contain as many as 50 million butterflies.

“The report of the dwindling Monarch butterfly winter residence in Mexico is ominous,” said leading entomologist Lincoln Brower.

According to Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, the decline was “hastened” by North America’s ongoing drought and record-breaking heat.(Photo by farflungphotos / Flickr)

However, the more “alarming” source of decline, according to Taylor and Omar Vidal, head of WWF in Mexico, is the “explosive increase” in the use of herbicide-tolerant genetically modified soybean and corn crops across America’s farmland which, the New York Times reports, has “enabled farmers to wipe out the milkweed,” on which monarch larvae feed almost exclusively.

“The American Midwest’s corn belt is a critical feeding ground for monarchs, which once found a ready source of milkweed growing between the rows of millions of acres of soybean and corn,” they continue. However, as farmers have planted over 120 million acres of crops resistant to the milkweed-killing herbicide glyphosate, the monarchs’ essential food supply is all but destroyed.

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