A drastic drop in the number of western Monarch butterflies, likely due to a virus, has resulted in combined efforts by fish and game departments in the West to look for solutions to support the species that winters in California.

Bill Van Pelt, the grasslands coordinator for the Western Association of Fish and Wildlife Agencies (WAFWA), which includes Idaho Fish and Game, said proposals include planting milkwood on state and federal land across the Monarch’s range to ensure the viability of the plant that Monarchs require for their eggs and offspring.

Van Pelt, who is monitoring the effort from Arizona, said the Western Monarch Butterfly Conservation Plan is designed to secure the future of the species range-wide west of the Rockies.

“We need to come up with conservation strategies for this species, while maintaining working landscapes,” Van Pelt said.

Monarch counts two years ago showed an estimated 170,000 of the migrating insects that overwinter in California’s Central Valley. A year later, 28,000 were estimated to have survived a virus.

The drop is just the latest in a population decline that has been documented for more than a decade.

“Over the past 20 years, the monarch butterfly population has declined by more than 80 percent throughout much of its range,” Van Pelt said.

The western butterfly’s range includes western states from Washington and Idaho south to California. Unlike Monarchs in the Midwest that migrate to overwintering sites in Mexico, Idaho’s western Monarchs overwinter in California, where efforts are underway to create an additional 50,000 acres of habitat in the Central Valley. The area contains about half of all known western Monarch overwintering sites.

The Monarch is the only butterfly that, like birds, makes a two-way migration. Unlike other butterflies, Monarchs and their eggs and caterpillars are unable to survive northern winters and use a combination of air currents and thermals to travel long distances — sometimes as far as 3,000 miles, according to researchers.

North and central Idaho have at least three milkweed species that Monarchs rely on for survival. Much of the milkweed is found in road ditches, and on public land that isn’t used for agriculture. Milkweed often grows in areas disturbed by fire or other natural disturbances, Van Pelt said. The group of fish and game agencies hopes to achieve a five-year running average by 2029 of 500,000 monarch butterflies counted at 75 sites, Van Pelt said.

“That would represent a near doubling of the 2017 population estimate,” he said.

An online site called Milkweed Mapper, shows the distribution of milkweed and Monarchs in the Western U.S.