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.