WASHINGTON – Call them creepy little creatures if you like, but insect-munching bats are so valuable to US agriculture that their deaths could cost the economy billions of dollars per year, experts said Thursday.

A fungal disease known as white nose syndrome, combined with the rise in wind turbines which can ensnare the dark fliers, have killed off more than a million of the bug predators in North America since 2006.

Their deaths mean the elimination of an important natural pesticide which is worth at least 3.7 billion dollars per year to farmers, said the study by US and South African researchers in the journal Science.

"Without bats, crop yields are affected. Pesticide applications go up," said Gary McCracken, head of the Department of Ecology and Evolutionary Biology at the University of Tennessee, Knoxville.

"Even if our estimates were quartered, they clearly show how bats have enormous potential to influence the economics of agriculture and forestry."

The analysis was based on "published estimates of the value of pest suppression services provided by bats," the study said.

The cost ranges "from about $12 to $173/acre (with a most likely scenario of $74/acre) in a cotton-dominated agricultural landscape in south-central Texas."

Extending those estimates across the United States as a whole, they found "the value of bats may be as low as $3.7 billion/year and as high as $53 billion/year."

McCracken's co-authors were Justin Boyles of the University of Pretoria in South Africa, Paul Cryan of the US Geological Survey and Thomas Kunz of Boston University.

The study says that more than a million bats in North America have died due to fungal diseases in the past five years, and that some projections show that "by 2020, wind turbines will have killed 33,000 to 111,000 annually in the Mid-Atlantic Highlands alone."

The cost analysis focused on the expense of pesticides but did not include the effects of pesticides on the environment or human and animal health.

"Not acting is not an option because the life histories of these flying, nocturnal mammals -- characterized by long generation times and low reproductive rates -- mean that population recovery is unlikely for decades or even centuries, if at all," said McCracken.