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milkweed

Timothy Bargar / USGS

Monarch butterflies need more to eat. That's the conclusion of a new study from the U.S. Geological Survey.

The only thing monarch butterfly caterpillars can eat is milkweed.

Wayne Thogmartin is a quantitative ecologist with the USGS. He says the butterfly population has dropped by about 80% since the mid-90s. The population has rebounded a little bit in the last three years, but Thogmartin says it's not a huge improvement.

photo of a monarch butterfly
user Jim, the Photographer / Flickr - http://michrad.io/1LXrdJM

This week, two environmental groups, the Center for Biological Diversity and the Center for Food Safety, put the U.S. Fish and Wildlife Service on notice.

They’re planning to sue the agency because they say it’s dragging its feet on protecting the monarch butterfly.

Tiago J. G. Fernandes / Wikimedia Commons

Monarch butterflies are declining.

The U.S. Fish and Wildlife Service is considering whether to add the monarch butterfly as a threatened species under the Endangered Species Act. The agency just launched a one-year review of the butterfly’s status.