As recently as a decade ago, farms in the Midwest were commonly marred - at least as a farmer would view it - by unruly patches of milkweed amid the neat rows of emerging corn or soybeans.
Not anymore. Fields are now planted with genetically modified corn and soybeans resistant to the herbicide Roundup, allowing farmers to spray the chemical to eradicate weeds, including milkweed.
And while that sounds like good news for the farmers, a growing number of scientists fear it is imperiling the monarch butterfly, whose spectacular migrations make it one of the most beloved of insects - “the Bambi of the insect world,” as an entomologist once put it.
Monarchs lay their eggs on milkweed, and their larvae eat it. While the evidence is still preliminary and disputed, experts like Chip Taylor say the growing use of genetically modified crops is threatening the orange-and-black butterfly by depriving it of habitat.
“This milkweed has disappeared from at least 100 million acres of these row crops,” said Dr. Taylor, an insect ecologist at the University of Kansas and director of the research and conservation program Monarch Watch. “Your milkweed is virtually gone.”
The primary evidence that monarch populations are in decline comes from a new study showing a drop over the last 17 years of the area occupied by monarchs in central Mexico, where many of them spend the winter. The amount of land occupied by the monarchs is thought to be a proxy for their population size.
“This is the first time we have the data that we can analyze statistically that shows there’s a downward trend,” said Ernest H. Williams, a professor of biology at Hamilton College and an author of the study along with Dr. Taylor and others.
The paper, published online by the journal Insect Conservation and Diversity, attributes the decrease partly to the loss of milkweed from use of “Roundup Ready” crops. Other causes, it says, are the loss of milkweed to land development, illegal logging at the wintering sites in Mexico, and severe weather.
The study does not suggest the monarch will become extinct. But it questions whether the annual migration, the impetus for butterfly festivals around the United States and waves of tourism to Mexico, is sustainable.
Still, the paper does not present any data backing its contention that genetically engineered crops are reducing monarch populations. Some experts dispute that the monarch populations are declining at all, and say it is unclear whether the biotech crops are having an effect.
Andrew K. Davis, an assistant research scientist at the University of Georgia, said censuses of adult monarchs taken each fall at Cape May, N.J., and Peninsula Point, Mich., did not show any decline.
It could be that “even though the overwintering population is getting smaller and smaller, once they come northward in the spring they are able to recoup the numbers,” Dr. Davis said. His paper disputing that there has been a decline in the monarch population was published online by the same journal.
Leslie Ries, a research professor at the University of Maryland, said other butterfly counts she had examined also did not show a decline, but rather year-to-year fluctuations. Since milkweed populations are not likely to fluctuate as much, the milkweed is probably not the major determinant of butterfly populations, she said.
But two other researchers, Karen Oberhauser of the University of Minnesota and John M. Pleasants of Iowa State, cite other evidence for a decline: the number of monarch eggs in the fields of the Midwest.
“Monarch production has decreased significantly” Dr. Pleasants said. “The reduction is caused by loss of milkweed resources available to them.”
The two scientists have submitted a paper to a scientific journal and said they did not want to discuss their data before publication.
Roundup Ready crops contain a bacterial gene that allows them to withstand Roundup or its generic equivalent, glyphosate, allowing farmers to kill the weeds without harming the crop.
Because they make weed control much easier, the crops have been widely adopted by farmers. This year, 94 percent of the soybeans and 72 percent of the corn being grown in the United States are herbicide-tolerant, according to the Department of Agriculture.
That in turn had led to an explosion in the use of glyphosate, according to the Environmental Protection Agency. About five times as much of the weed killer was used on farmland in 2007 as in 1997, a year after the Roundup Ready crops were introduced, and roughly 10 times as much as in 1993.
