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The monarch butterfly population has declined by more than 90 percent over a 20 year period.

The monarch butterfly is in trouble. The Tucson-based Center for Biological Diversity has filed a petition to protect the insect under the Endangered Species Act as a threatened species.

The butterfly population has declined by more than 90 percent over a 20 year period. Tierra Curry is a senior scientist with the center. She says one reason for the drop is the use of glyphosate, an herbicide that kills milkweed.

"Monarchs only lay their eggs on milkweed and the caterpillars can only eat milkweed, so any time you have glyphosate, or roundup, being applied to milkweed, you’re taking away the monarchs only food source," Curry said.

Curry says native milkweed grows in Arizona’s cotton fields. The Center encourages people to plant milkweed that hasn’t been treated with pesticides.