With populations of monarch butterflies falling 90 percent from the levels seen in 1990, conservation groups have petitioned the U.S. government to list them as "threatened" under the Endangered Species Act.

In a petition sent to the U.S. Fish and Wildlife Service, the groups blamed the plummeting of the butterfly population mostly on the destruction of milkweed, a plant vital for their feeding and breeding.

Petitioners blame spray herbicides used on genetically modified corn and soybeans in Midwestern U.S. farmland the monarchs pass through on their migrations between southern Canada and the winter habitats in central Mexico.

From around a billion of the orange and black butterflies in the early and mid 1990s, numbers have dropped to about 35 million, conservationists say.

"Monarchs are in a deadly free fall and the threats they face are now so large in scale that Endangered Species Act protection is needed sooner rather than later, while there is still time to reverse the severe decline in the heart of their range," says monarch expert and conservationist Lincoln Brower.

In the last 20 years around 160 million acres of monarch habitat, including almost a third of the butterflies' summer breeding areas, have disappeared, the petition said.

The petition asks the federal government to designate "critical" areas across the U.S. to give the butterflies places for feeding and breeding.

Although climate change, drought and illegal deforestation in their Mexican wintering grounds are factors in their decline, herbicides have been singled out as the greatest threat.

Increasingly since 1990, corn and soybean farmers have been planting crops genetically engineered to resist herbicides such as Monsanto's Roundup, which allows them to spray their fields in order to control weeds damaging to their crops.

Unfortunately, that spraying also destroys milkweed, the only food source for the butterfly larvae.

And the butterflies are not alone in facing the threat, conservationists say; other "pollinators" including bees, birds and bats have experienced population declines in past decades.

That makes the need for listing critical, especially for the monarchs, the petitioning groups said in a statement.

"Listing will make it illegal to intentionally kill monarchs or modify their habitat without a permit," the Center for Biological Diversity and the Center for Food Safety said.

In response to the petition, the USFWS will issue a "90-day finding" to determine if it presents sufficient information to warrant federal protections.

If it makes that determination, the USFWS would conduct a year-long status review and seek additional information to decide if the monarch should be given threatened species status.

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