Monarch butterflies may warrant U.S. Endangered Species Act protection because of farm-related habitat loss blamed for sharp declines in cross-country migrations of the orange-and-black insects, the U.S. Fish and Wildlife Service said. Monarch populations are estimated to have fallen by as much as 90 percent during the past two decades because of destruction of milkweed plants they depend on to lay their eggs and nourish hatching larvae, according to the Xerces Society for Invertebrate Conservation. The loss of the plant is tied to factors such as increased cultivation of crops genetically engineered to withstand herbicides that kill native vegetation, including milkweed, the conservation group says. Monarchs, unique among butterflies for the regularity and breadth of their annual migration, are also threatened by widespread pesticide use and logging of mountain forests in central Mexico and coastal California where some of them winter, said biologist Karen Oberhauser at the University of Minnesota. The Fish and Wildlife Service said on Monday a petition requesting federal protections for monarchs -- filed by the Xerces Society and others -- "presents substantial information indicating that listing may be warranted." The agency's initial review will take about a year to complete.