The three-year exemption by the U.S. Environmental Protection Agency allows nurseries, property owners and government agencies to use the chemical commonly known as hydrated lime to exterminate the tiny, noisy frogs.

Citric acid was already on a list of exempted chemicals that could be used for pest control. However, citric acid is much more expensive to use than hydrated lime.

Sandra Lee Kunimoto, chairwoman of the state Board of Agriculture, said obtaining the EPA exemption is a "major development" in the fight against coqui infestations, which are most prevalent on the Big Island and on Maui.

"This product is as effective as citric acid, but much cheaper and readily available as it is commonly used by farmers and home gardeners to enrich the soil," Kunimoto said.

The EPA told the state that this was the first time that any agency has sought the use of hydrated lime for such purpose under the emergency program of the Federal Insecticide, Fungicide and Rodenticide Act.

Anyone wishing to use hydrated lime for coqui control will be required to submit to the dealer selling the product an "Applicator Authorization Form," which may be obtained from the dealer or the Agriculture Department's Web site.

Coqui frogs are beloved in Puerto Rico as a symbol of the island, and likely ended up in Hawai'i in shipments of tropical plants. Since about 1990, they have been interrupting Hawai'i's previously quiet nights with their "ko-KEE" mating call.