The monarch butterfly is in line for possible protection under the Endangered Species Act.
The U.S. Fish and Wildlife Service announced Monday it is launching a year-long status review of the monarch population in response to a request from conservation groups.
The iconic butterflies face threats from pesticide use and habitat loss – particularly from the loss of milkweed plants, which are the sole food source for monarch caterpillars.
Many monarch butterflies migrate 3,000 miles between Mexico and Canada, so their habitat extends across most of U.S. They are known to breed in Western states including Oregon, Washington and California in the spring, summer and fall. However, they're much more prevalent in the Midwest and Northeast than the Pacific Northwest.
According to the insect conservation group The Xerces Society – one of the groups that petitioned for Endangered Species listing – monarch butterfly numbers have seen a 90 percent decline since the 1990s. Population estimates have dropped from more than a billion in the 1990s to around 33 million today.
The Xerces Society has connected the decline to an increase in the use of the herbicide Roundup, which is often used on genetically modified corn and soybean crops.
"The use of Roundup has really skyrocketed in the past couple decades along with the increased planting of Roundup-ready corn and soybeans," said Sarina Jepsen of the Xerces Society. "So that allows these plants to be sprayed with larger amounts of Roundup where milkweed historically grew in these fields."
Jepsen said milkweed on cropland is important habitat for monarchs.
"We know from research that monarchs lay four times more eggs on milkweed plants growing in cropland than other areas," she said. "So that milkweed is especially important, and it's just disappearing at an unprecedented rate."
Jepsen said homeowners can help by planting milkweed in their yards, but they should use this guide to find local species in their home states.
Fish and Wildlife Service spokesman Brett Lawrence said his agency's status review will determine whether the threats to monarch butterflies are serious enough to warrant federal protection for the species. The agency will be collecting information from the public over the next two months.
"What we need now is more information from the public and scientists before we can make a decision," he said. "We'll be looking at all the possible threats from loss of habitat to changes in migration patterns or populations."
The agency will be looking for new information about the species' range and habitat requirements, historical and current population levels and conservation measures for the species and its habitat.