From: Yale Environment 360 and the Star Tribune
Published March 20, 2012 06:37 AM

Monarch Butterfly decline linked to genetically modified crops

A new study suggests that the increased use of genetically modified (GM) crops across the Midwestern U.S. may be causing a decline in monarch butterfly populations. From 1999 to 2010, a period when GM crops became more common on U.S. farms, the number of monarch eggs in the Midwest declined by 81 percent, according to researchers from the University of Minnesota and Iowa State University. 



The reason, according to the study, is the near-disappearance of milkweed, an important host plant for monarch eggs and caterpillars. The researchers attribute sharp declines in milkweed to widespread use of genetically modified corn and soybeans that are resistant to the herbicide, Roundup, which is then sprayed on fields, killing milkweed. Other experts say it is too early to link GM crops to population declines, suggesting that other causes, including damage to the butterflies’ wintering grounds in Mexico, may be a factor. In a separate study, U.S. researchers say early snowmelt in the Colorado Rocky Mountains may be causing a decline in populations of the Mormon Fritillary butterfly because the advanced melting is triggering a decline in the insect’s preferred flower species.

Earlier studies suggested that monarch caterpillars would die if they ate milkweed dusted with pollen from another kind of engineered seed known as BT corn. It contains a gene that produces a toxin that kills corn-eating pests.

That theory was disproved, but it led scientists to take a hard look at milkweed plants in corn and soybean fields, said Pleasants. "Surprisingly, monarchs use those milkweeds more heavily than milkweed outside [farm fields]," he said. The butterflies lay nearly four times as many eggs on farm field plants as on those in pastures or on roadsides, the researchers said.

More important, they also found "that milkweed in the fields was disappearing," he said. That's because more farmers are using a new kind of genetically modified seed developed by Monsanto, Roundup-ready corn and soybeans, that contain a gene allowing the plants to withstand Roundup, or glyphosate. That allows farmers to spray their fields without harming the crop.

Monarch Butterfly via Shutterstock

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