Wednesday, March 5, 2008

Air drops eradicate Hawaii islet rat population

State and federal agencies believe they have wiped out the rat population on a small island off north Moloka'i.

(Images: Helicopter dropping rat bait on Mōkapu Island. Heather Eijzenga photos.)

The rat control program is aimed at allowing the native species of Mōkapu Island to recover from decades of rat predation. Rats are not native to the Hawaiian Islands.

Similar rat eradication efforts have been or are being conducted at several locations throughout the Hawaiian archipelago, after the recognition that rats are among the major predators of native species, eating chicks and eggs of native birds, as well as seeds and small native plants.

Among the native species on the island are three seabird species: wedge-tailed shearwaters, red tailed tropicbirds, and white-tailed tropicbirds. The island also has most of the state's remaining wild specimens of the plant hoawa, or Pittosporum halophilum, and a small group of the rare Hawaiian fan palm, loulu lelo, or Pritchardia hillebrandii.

The steep-sided little island is a challenging location for conducting an invasive species program, and the program involved helicopter drops of fish-flavored pellets of a rodenticide called diphacinone.

“This aerial application of the rodenticide diphacinone is the first of its kind for offshore

islands in Hawai‘i,” said Chris Swenson, Pacific Islands Coastal Program Coordinator for the

U.S. Fish and Wildlife Service. “Similar rat eradication methods have been successfully used in other parts of the world since the early 1990s.”

The Mōkapu program was a joint effort of the service, along with the state Department of Land and Natural Resources' Division of Forestry and Wildlife, and the U.S. Department of Agriculture's APHIS-Wildlife Services agency. Others involved included Hacco Inc., Moloka‘i/Maui Invasive Species Committee, the Moloka'i branch of The Nature Conservancy of Hawai'i and United Agriproducts

Black rats, also known as roof rats, are the primary problem in many forest ecosystems in Hawai'i, but on Mōkapu, the predator of native seabirds and rare plants was the Polynesian rat, Rattus exulans.

Two aerial drops of diphacinone were used to be sure the entire island was covered, and to ensure the bait was available to rats in adequate doses.

Diphacinone is particularly effective for rats, but not as dangerous to birds and other species that might come into contact with it. Water samples and tests from marine life will be made to ensure the compound has not spread in the environment.

“The success of the Mokapu Island rat eradication project will provide natural resource

managers throughout the state access to a proven method of bait application using a less toxic

and more environmentally friendly rodenticide,” said the eradication program's project manager, Peter Dunlevy.

The wildlife agencies said they believe the eradication effort was successful, but that they will monitor the island for a two-year period to be sure. They will also keep an eye on seabirds and the native plants of the island to attempt to confirm their recovery from the years of rat presence.

The program was conducted after an environmental assessment, and after public meetings on Moloka'i about the project. The results of testing on water and marine life is to be offered to Moloka'i media for publication.

A copy of the environmental assessment is available on the web at

© 2007 Jan W. TenBruggencate