HILO, Hawai'i — The state has begun its biggest effort to date to get rid of annoying coqui frogs in a forest on the southwest side of the Big Island.

The state has hired a private helicopter to spray citric acid on six acres of the Manuka Natural Area Reserve.

The Department of Land and Natural Resources also is sending crews along the trails of Manuka State Park to spray hydrated lime.

The state did a test spraying of one acre in the area in May, which resulted in an 89 percent reduction in the number of frogs.

But surveys showed that the frogs were back within a month, so this week's spraying will be repeated during the week of Nov. 7.

The park and the reserve were both closed to the public this week as a condition set by the U.S. Environmental Protection Agency for using hydrated lime.

The state had done some test spraying in other areas before, but officials said this is the first concentrated effort to control the frog population in a natural reserve area.

"These animals should not be in there," said Roger Imoto, head of the Big Island branch of the state Division of Forestry and Wildlife.

State officials talk about control because eradication is difficult once a frog population is established in an area.

Earlier this month, Peter Young, director of the state Department of Land and Natural Resources said it is unlikely the coquis could be eradicated from the Big Island or Maui because the infestations on those islands are so bad.

O'ahu and Kaua'i, which have recorded only small infestations, have had better results in getting rid of the frogs.

Although the frogs have spread outside of the Manuka area being sprayed this week, state officials hope the project will restrict the population.

"I think this will take care of the concentration," Imoto said.

Federal authorities praised the state's effort.

"They're taking the bull by the horns," said Will Pitt, head of the Hilo field station for the National Wildlife Research Center. "The state really has been proactive in controlling this pest in a natural area reserve."

Beloved in their native Puerto Rico, coqui frogs are reviled in Hawai'i for their loud shrieking.

With no natural predators, such as snakes, to keep coqui numbers under control, the frogs and their loud "ko-KEE" mating calls have multiplied exponentially.

Hawai'i's year-round temperate weather and open space also provide the coqui with an ideal environment in which to reproduce. The first frog — or frogs — are believed to have hitched a ride to Hawai'i in a plant shipped from Puerto Rico or Florida in the 1990s.