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.

Anticoagulant rodenticides have been detected in many species of wildlife worldwide; yet the origins, exposure pathways, and effects of this exposure are not well understood. Furthermore, to accurately characterize the risks from rodenticide use, information is needed on what proportion of populations are being exposed, what proportion of the exposed individuals are affected, and in what ways. The relationship between anticoagulant rodenticide concentrations found in wildlife and the rate of mortality or illness is the subject of much current research. Residue levels observed in liver and whole body analyses vary, and overlap extensively among apparently healthy asymptomatic individuals and sublethal and lethal cases. Results from laboratory studies also show there can be wide variability in lethal and sublethal effects among and within taxonomic groups. Correlating the sublethal and reproductive effects observed in laboratory studies with realistic exposure scenarios and effects in the wild is needed to improve risk assessments. For species with limited numbers/declining populations, a critical question is whether the rodenticide exposure documented in individual animals inhibit population growth or contribute to population declines by lowering survival and reproductive success. This information is essential to the regulatory agencies that must weigh the risks and benefits of rodenticide uses and identify restrictions that are effective in reducing risks to wildlife. A primary objective of this symposium was to facilitate communication between regulators and researchers. Current research on many of these topics was presented, and was followed by discussions on how to improve our understanding of what factors lead to wildlife exposure and improve our ability to assess the effects of exposure on individuals and populations. A collaborative approach will be developed to design studies that provide regulatory and wildlife management agencies with additional science on which to base their decisions.

In the early 1990s, a coalition of federal and state agencies, NGOs, and private landowners in Hawaii agreed to pursue a Special Local Needs pesticide registration [24(c) FIFRA] for the aerial broadcast of a 0.005% diphacinone rodenticide for the control of rodents in native ecosystems. While there was recognition of the important role introduced rodents play in the decline and extinction of native species, there were concerns expressed about the potential non-target impacts of this technique. Over the next 10 years, numerous studies were undertaken to address specific non-target issues. This research, along with other published and unpublished research on diphacinone and its human pharmaceutical counterpart, Dipaxin, was compiled and analyzed in 4 hazard assessments (human dietary and drinking water consumption, aquatic and terrestrial non-target species) that comprise the foundation of Hawaii’s registration application. Hazards to humans and other non-target terrestrial organisms were evaluated in terms of dietary intake of contaminated food or water required before lethal or sublethal effects might be anticipated. Hazard to aquatic organisms was assessed according to traditional risk quotient methods employed by the U.S. Environmental Protection Agency. These assessments indicate the greatest human health hazard is to pregnant women drinking untreated stream water; however, even this risk is low. With a few exceptions, such as the Hawaiian crow, the ecological assessments indicate the acute risks to terrestrial or aquatic non-target species are minimal, even under the most conservative risk scenarios. However, there could be detectable physiological effects in birds exposed at sublethal levels. We believe that under proper supervision, this technique can be safely used in Hawaii, and elsewhere, to protect native species from the impacts of introduced rodents.

One of the goals of this Symposium was to bring together agricultural and conservation users of rodenticides to discuss the impacts of rodenticides on the environment, examine the current regulatory climate governing their use, and identify ways that users can reduce or eliminate these impacts. Some of the presentations in today’s symposium highlighted specific impacts, and the preceding talk described the scenario of what can happen if an issue related to pesticide impacts ends up in the courts. The three agencies that were represented on this panel (USDA, USFWS, and EPA) have regulatory oversight and enforcement authority for the use of rodenticides and/or the adverse effects resulting from the use of rodenticides. In addition, USDA and FWS are the primary federal users of rodenticides for agriculture and conservation. USDA holds the registrations for a number of rodenticide products for agricultural and conservation purposes. Panelists were asked to describe how Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), Migratory Bird Treaty Act (MBTA), Endangered Species Act (ESA), National Environmental Policy Act (NEPA), the Animal Damage Control Act, and the administration of USDA’s pesticide labels apply to rodenticide adverse effects. Panelists were then asked to bring up an issue within the scope of their agency that they view as problematic for conducting eradication projects. Panelists were also asked to suggest proactive measures that the rodent eradication community can undertake to improve future rodent eradication efforts. Finally, the floor was opened to audience members for questions and comments.