WILMINGTON, Del. -

Three conservation groups and a butterfly expert are asking the U.S. Fish & Wildlife Service to list the monarch butterfly as a threatened species.

The News-Journal of Wilmington, Del. reports the petitioners blamed farming practices Tuesday for a loss of milkweed, a plant the butterflies rely on for feeding and breeding.

The petitioners are the Tucson, Arizona-based Center for Biological Diversity, the Washington-based Center for Food Safety, the Xerces Society of Portland, Oregon, and monarch scientist Lincoln Brower of Sweet Briar College in Virginia.

Threatened-species status would allow federal officials more latitude to help the populations rebuild, but would not be as restrictive as endangered species status.