An environmental group has alleged in a federal lawsuit that the U.S. Environmental Protection Agency ignored the dangers of a widely used herbicide that resulted in the demise of the monarch butterfly population in the United States.

The lawsuit filed Friday by the Natural Resources Defense Council (NRDC) in the federal district court in New York, argues that glyphosate, an ingredient in the herbicide, has had devastating effects on monarch butterflies, causing the population to halve in seven years.

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Glyphosate destroys milkweed, the only food consumed by monarch butterflies in their migration. The lawsuit states that federal law requires the Environmental Protection Agency (EPA) to ensure the pesticides it approves will not cause "unreasonable adverse effects on the environment, including wildlife."

The suit alleges the agency never considered glyphosate's impacts on monarchs.

"There are multiple factors that may be affecting monarchs including loss of habitat, weather and pesticides," the EPA said in its defense.

The orange-and-black spotted monarchs are renowned for migrating several thousand miles across the U.S., Canada, and Mexico. The Center for Biological Diversity said their numbers have fallen dramatically in recent years.

The lawsuit states the population was tallied at one billion in 1997 and this winter was down to 56.5 million butterflies, the second-lowest number ever measured.

Since 1997, the population of monarch butterfly has shown a decline of about 950 million butterflies.

TagsEPA, environmental protection agency, endangered species, Monarch butterfly, monarchs, Natural Resources Defense Council, Monarch butterfly demise