Tue Jan 27, 2015 11:21pm GMT

A tally of monarch butterflies wintering in Mexico rose to 56. 5 million this year from a record low of 34 million last year but conservationists said on Tuesday the increase was too slight to reduce the threat of extinction facing the insect.

The U.S. Fish and Wildlife Service said last month the popular orange-and-black butterfly may warrant federal Endangered Species Act protections tied to declines in cross-country migrations because of farm-related habitat loss.

Sarina Jepsen, endangered species director for the Xerces Society for Invertebrate Conservation, said scientists had predicted that favorable weather in the butterfly's breeding grounds in the U.S. Midwest and elsewhere would result in more monarchs migrating to Mexico this year than the 56.5 million estimated this week by the Mexican government and that country's chapter of the World Wildlife Fund.

Jepsen said the latest population estimate for monarchs is the second-lowest since surveys began in 1993 and that the butterflies faced the possibility of extinction without habitat and other protections that would come with adding the insects to the U.S. federal endangered and threatened species list.

"We're happy to see numbers higher this year than last winter. But we're still seeing a decline of more than 90 percent compared to the 1 billion monarchs that migrated to Mexico in the mid-1990s," she said.

Population losses suffered by the charismatic butterfly stem from destruction of milkweed plants they depend on to lay their eggs and nourish hatching larvae. The plant's decline is tied to factors such as increased cultivation of crops genetically engineered to withstand herbicides that kill native vegetation like milkweed, conservationists say.

Monarchs, singular among butterflies for the regularity and breadth of their annual migration, also are threatened by widespread pesticide use, logging in the mountains of central Mexico and development in coastal California where some of them winter, according to University of Minnesota biologist Karen Oberhauser.

The butterflies, celebrated for their striking beauty and for emerging from a jade green chrysalis decorated with gold stitching, are roughly divided into two populations in the United States according to fall migration patterns.

Monarchs from east of the Continental Divide fly 3,000 miles (4,800 km) to Mexico, while those from west of the Divide – including states like Idaho – make a relatively shorter journey to California.