WASHINGTON - A prized caterpillar fungus that is more valuable than gold and is nicknamed "Himalayan Viagra" in Asia, where it is seen as a wonder drug, is becoming harder to find due to climate change, researchers said on Monday.

The elusive fungus, dongchongxiacao, is known formally as Ophiocordyceps sinensis.

Although it has no scientifically proven benefits, people who boil dongchongxiacao in water to make tea or add it to soups and stews believe it cures everything from impotence to cancer.

It is "one of the world's most valuable biological commodities, providing a crucial source of income for hundreds of thousands of collectors," said the report in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.

In recent decades, it has skyrocketed in popularity and prices have soared - it can fetch up to three times the price of gold in Beijing, researchers said.

While many have suspected over-harvesting was the reason for its scarcity, researchers wanted to find out more.

So they interviewed around four dozen harvesters, collectors and traders of the prized fungus.

They also scoured previously published scientific literature, including interviews with more than 800 people in Nepal, Bhutan, India and China, to understand its apparent decline.

Weather patterns, geographic factors and environmental conditions were also analyzed to create a map of dongchongxiacao production in the region.

"Using data spanning nearly two decades and four countries, (we) revealed that caterpillar fungus production is declining throughout much of its range," said the report.

"While collectors increasingly attribute the decline in caterpillar fungus to over-harvesting, habitat and production modeling suggest that climate change is also likely playing a role."

Particular temperatures

The cone-shaped fungus is only found above an elevation of 3,500 meters, and forms when the parasitic fungus lodges itself in a caterpillar, slowly killing it.

To grow, it needs a specific climate that is frigid - with winter temperatures below 0 C - but where the soil is not permanently frozen.

"Such conditions are typically present at the margin of permafrost areas," said the PNAS report, led by researchers at Stanford University.

"Given that winter temperatures have warmed significantly from 1979 to 2013 across much of its range, and especially in Bhutan, its populations are likely to have been negatively affected."

The warming trend has particularly affected Bhutan, with average winter temperatures "increasing by 3.5-4 C across most of its predicted habitat (+1.1 C per decade, on average)", added the study.

Researchers also found that vegetation on the Tibetan Plateau "did not shift upward in response to climate warming in 2000-14," suggesting that the caterpillar fungus will not be able to simply move up the mountain to colder habitats as the climate warms.

This spells trouble for harvesters who sell the fungus in order to survive, "underscoring the need for alternative livelihood options in the communities that depend on this niche commodity", researchers warned.

Agence France-presse