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Open Access Publications from the University of California

Nontarget Risk and Environmental Fate of the Broadcast Application of a Diphacinone Rodenticide at Mōkapu and Lehua Islands, Hawai'i


Invasive commensal rodents such as Rattus spp. and Mus spp. imperil many threatened and endangered native species including plants, invertebrates and birds within Hawai'i and U.S. territories and possessions in the Pacific. In some cases, the eradication or control of invasive rodents could allow natural recovery and active restoration of native species and ecosystems negatively impacted. The broad scale application of rodenticides is a necessary management tool for this purpose, but it is highly controversial to the public and regulatory agencies. There is great perceived and actual risk of nontarget mortality and environmental contamination. One of the conservation uses of rodenticides registered in Hawai'i is the aerial broadcast of rodenticide bait over large areas of native ecosystems on the main Hawai'ian islands, repeated periodically to maintain reduced rodent population levels. Recognizing that the success of this program depends on public and regulatory support, a coalition of state and federal agencies and private landowners have carefully designed Hawai'i’s rodent control program to minimize short- and longterm environmental impacts. In the early 1990s, diphacinone was selected as the primary rodenticide for conservation uses in Hawai'i because of its long track record of safe and effective use in agriculture worldwide. Hawai'i’s program has 5 components: research on efficacy and environmental impacts, regulatory compliance, developing and using local technical expertise, monitoring of rodenticide impacts and native species recovery and public outreach and engagement, particularly at the community level. After many years of generating the efficacy and safety data in support of regulatory approval by the U.S. Environmental Protection Agency (EPA) and the State of Hawai'i, a diphacinone product (Diphacinone–50, Hacco, Inc., Randolph, WI) was approved in 2007 for conservation uses in the U.S. Subsequently, rodenticide pellets containing the active ingredient diphacinone at 0.005% (50 ppm) were broadcast by helicopter in February 2008 on Mōkapu and in January 2009 on Lehua. Mōkapu was the first island in the world where the aerial broadcast of this less hazardous active ingredient was used to eradicate rats. Island eradications in other parts of the world have usually used broad-spectrum active ingredients that are far more persistent and bioaccumulative, thus imparting a much higher risk to nontarget species and the environment. Monitoring of nontarget and environmental effects on Mōkapu and Lehua did not detect diphacinone residues. A number of factors, including state of Hawai'i restrictions on bait entering the ocean, led to rats surviving on Lehua. These projects demonstrate that the aerial broadcast of a rodenticide containing diphacinone can be an effective and safer tool for conservation. Hawai'i is using the results from Mōkapu and Lehua to plan future rat eradication and control projects and continue development of a long-term program.

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