The monarch butterflies' numbers have been plummeting in recent years, and a new study has pointed to the likely main culprit: loss of its summer habitat as a result of genetically modified crops.
The findings from researchers with the University of Guelph were published Wednesday in the Journal of Animal Ecology.
A report issued earlier this year from the World Wildlife Fund and Mexico’s National Commission for Protected Areas pointed to three main factors threatening the pollinators: deforestation and forest degradation in monarch reserves that serve as their winter habitat in Mexico, habitat loss due to land use changes and the loss of its larval food plant—milkweed—as a result of the widespread use of the herbicide glyphosate, and weather extremes.
The new study, however, puts the main cause of the crisis squarely on its summer habitat loss in the United States.
The researchers' projection model showed that disturbances in their breeding grounds affected the butterflies' number to a greater degree than disturbances to their wintering grounds. Those breeding grounds need to have milkweed, the only host plant for the monarch caterpillars.
But the number of milkweed plants has been plummeting—21 percent between 1995 and 2013—especially in the Corn Belt, home to widespread planting of crops that have been genetically modified to be resistant to glyphosate, the main ingredient in Monsanto's herbicide Roundup. The region is also "monarch corridor," as Monarch Watch Director Chip Ward has described, because it serves as critical summer and spring breeding grounds for a large proportion of monarchs
“Our work provides the first evidence that monarch butterfly numbers in eastern North America are most sensitive to changes in the availability of milkweed on breeding grounds, particularly in the Corn Belt region of the United States,” stated Ryan Norris, a professor in Guelph’s Department of Integrative Biology and study co-author.
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