The United States’ effort to fight climate change by producing corn ethanol is driving the iconic monarch butterfly to extinction, according to a report released Thursday by the nonprofit Center for Food Safety.

A massive increase in the cultivation of genetically engineered corn and soybeans in the Midwest farm belt has resulted in the elimination of milkweed, the sole source of food for monarch caterpillars, the study states. The population of monarch butterflies, meanwhile, has plummeted 90 percent over the past 20 years, from 1 billion to 35 million.

“Without strong action to restore milkweed to Midwest crop fields, the spectacular migration of the monarch butterfly may be relegated to biological history,” wrote report authors Bill Freese and Martha Crouch.

Thanks to a federal subsides designed to increase the use of low-carbon biofuels, 43 percent of all corn is now grown to make ethanol, according to the U.S. Department of Agriculture.

The report is likely to fuel efforts to win endangered species protection for the monarch. The U.S. Fish and Wildlife Service this past December concluded such a move may be warranted. If the government ultimately declares the butterfly threatened or endangered, it must take action to minimize threats to the monarch’s survival.

“This might involve restrictions on the planting of G.E. crops resistant to herbicides,” Freese, a science policy analyst at the Center for Food Safety in Washington, D.C., said in an email.

Monarch numbers began to fall in the mid-1990s with the introduction of corn and soybeans genetically altered to withstand herbicides. That trigged a huge spike in the use of Monsanto’s Roundup weed killer. Old-fashioned herbicides killed milkweed above the ground but left its roots intact, allowing the plant to regrow. But Roundup Ready, composed of the chemical compound glyphosate, is the Terminator of herbicides, eradicating roots and all.

(Maps: Courtesy Center of Food Safety)

Between 1995, when so-called Roundup Ready crops were introduced, and 2013, the number of acres treated with glyphosate jumped from 17 million to 157 million. Some 205 million pounds of Roundup Ready are sprayed on crops annually, up from 10 million pounds two decades ago, according to the report. About 80 percent of corn grown in the U.S. is Roundup Ready, as is 90 percent of soybeans.

So while milkweed was found in half of Iowa’s corn and soybean fields as recently as 1999, the plant appeared in only 8 percent of fields a decade later, the researchers wrote. It’s no coincidence that the monarch population has fallen 88 percent in the Midwest, a crucial breeding ground for the butterfly, which undertakes a 2,500-mile annual migration between the northern United States and Mexico.

2013 corn and soybean production. Green represents corn, blue represents soybean. (Map: Courtesy Center for Food Safety)

“Milkweed does grow outside of cropland, but there is too little habitat to support a viable monarch population,” the report’s authors wrote.

Even that land is rapidly being converted to grow corn by farmers cashing in on the ethanol boom. Since 1996, 17 million acres of fields that once might have fed monarchs have been taken over for corn cultivation, according to the report.

Now there’s a new threat: next-generation herbicides for G.E. crops that could kill wildflowers that produce the nectar adult monarchs eat.

“Because herbicides are used more frequently, at higher rates, and later in the season with resistant crops, they will be even more likely to harm sensitive nectar plants,” Freese and Crouch wrote. “The resulting reduction in nectar resources may lead to poorly nourished monarchs that lay fewer eggs, or do not survive migration and overwintering.”

Monsanto, not surprisingly, disputed the report’s conclusion. “Experts that study butterflies say there are a number of factors that are contributing to fewer monarchs migrating from the U.S. to Mexico including logging of overwintering sites in Mexico, weather events (e.g., freezing temperatures and drought) and the availability of host plants and nectar sources,” Monsanto spokesperson Charla Marie Lord wrote in an email.

The Center for Food Safety called on the federal government to ban herbicide-resistant crops and end subsides for ethanol that encourage farmers to grow corn.

So far, the plea has fallen on deaf ears. Just last October, the U.S. Environmental Protection Agency approved a powerful—and highly toxic—new weed killer called Enlist Duo.