Lehua Island is a 115-hectare (280 acre) extinct volcanic island located 1.2 km (0.75 miles) off the northern shore of Niihau in the Hawaiian Islands. Lehua is one of Hawaii’s most important seabird breeding islands, because of its size and the fact that it rises to an elevation of 700 ft. (215 m.) above sea level. Most Hawaiian seabird breeding islands are low elevation atolls, extremely vulnerable to sea-level rise, which will inevitably occur with climate change, inundating the islands and extirpating the breeding seabirds. Lehua Island is federally owned by the US Coast Guard, and is already protected as a state-designated seabird sanctuary managed by the Hawaii Department of Land and Natural Resources (DLNR). The island has been plagued with introduced Pacific rats (Rattus exulans) that have preyed on seabird eggs, nestlings and nesting adult seabirds, limiting the success of the 50,000 seabirds attempting to breed there each year.
The Lehua Island Ecosystem Restoration Project (LIERP) is supported by a partnership of stakeholders who have been working on the project for many years. Some have been involved as far back as 2005. In 2012, the partnership was restructured to include the Robinson Family and the Niihau Community as partners in the review of past actions and planning of current and future actions at Lehua island. Through these partnering efforts, the interests of State, Federal, private, and public entities were addressed and those most impacted were brought in to the planning process.
The Lehua Island Ecosystem Restoration Project partnership formed a steering committee which includes members from the Hawaii Department of Land and Natural Resources, U.S. Fish and Wildlife Service, U.S. Department of Agriculture-APHIS-WS National Wildlife Research Center, Island Conservation, U.S. Coast Guard, National Tropical Botanical Garden, the Robinson Family, and the Kauai Endangered Seabird Recovery Project. These members meet on a regular basis (currently monthly) in order to review ongoing monitoring on Lehua Island and plan for its continued restoration into future.
Why aerial applications of diphacinone?
When restoration efforts were proposed for Lehua Island in 2005, two criteria were agreed upon as key to making the restoration a success: 1) the eradication of rabbits and 2) the eradication of rats would be needed for a long-term restoration effort to be effective. The plans for both eradications were proposed at the same time. The rabbits were eradicated using mainly a hunting effort carried out with the assistance of dogs in 2005/2006.
The effort to eradicate the rats has been more complex given the more diverse terrain that rats can inhabit on Lehua, of which humans can access only a portion due to steep inclines. An attempt to eradicate rats in 2009 using aerial applications of Diphacinone 50 did not succeed, and rats repopulated the island. In a 2015 feasibility study, aerial application of a rodenticide bait was described as the only option that could reasonably meet the goal of rodent eradication in all the available habitats on the island without putting staff at serious risk of injury.
Around the world, anticoagulent rodenticides have been successfully used to eradicate rodents from islands. Anticoagulant rodenticides come in two main categories: 1) first generation anticoagulants that require multiple feedings to provide a lethal dose, and 2) second generation anticoagulants that usually require only one feeding to provide a lethal dose. Single feed rodenticides, such as brodifacoum, have been used in many successful rodent eradications throughout the world. The active ingredients in these single-feed rodenticides, however, can be persistent in an ecosystem longer and have the potential to harm more non-target animals because of their single-feed mode of activity. In contrast, multi-feed rodenticides, such as diphacinone, have a lower number of documented successes in eradication projects, but carry lower risk for non-target impacts.
After weighing the risks and benefits associated with rodenticide choice, the LIERP selected the use of a multi-feed anticoagulant, diphacinone, as the preferred rodenticide. Learning from the previous attempt in 2009, important modifications were made to increase the likelihood of success: 1) application of bait during the summer months when the island provides less alternative food sources; 2) use of a new and more palatable bait matrix to increase the likelihood of bait consumption by rats; and 3) dispersing bait to all potential rat territories with no exclusion zones. Since diphacinone has not been used as extensively in eradications, the Environmental Assessment built in a backup option for the use of brodifacoum, should diphacinone fail. Currently, brodifacoum, is registered by the EPA but is not licensed by the Hawaii Department of Agriculture for use in the State of Hawaii.
Why DITRAC D-50 pellets?
Although past efforts at rodent control/eradication in Hawaii had used a pelleted bait product called Diphacinone 50, palatability and effectiveness trials of the bait had shown that in several trials rats did not prefer the Diphacinone 50 baits over standard lab rodent chow. This made using Diphacinone 50 significantly less effective since the rats consumed much less of it.
The project worked with Bell Labs in their development of another diphacinone bait that used the same active ingredient content (50 parts per million diphacinone) but a different bait matrix. Lab trials were done with the new bait and the rats known to be on Lehua Island, Pacific rats (Rattus exulans). The new bait both outperformed the lab chow and effectively killed all the rats within the trial period.
Based on these favorable results, the project utilized Bell Labs’ new bait for the 2017 Lehua project, which is now registered and labeled as DITRAC D-50 pellets. This product was applied aerially to the island three times, in August and September of 2017.
The rationale for the use of rodenticides to eradicate invasive rodents from islands is that the potential short-term contaminant inputs into the environment is offset by the long-term ecological and societal benefits of invasive rodent removal (e.g., Jones et al. 2015, Le Corre et al. 2015, Russell and Broome 2016). The maintenance of this rationale requires that we continue to test assumptions about the actual primary and secondary adverse impacts of rodenticide use following eradication operations.
Before, during and after the rodenticide applications on Lehua Island, the United States Department of Agriculture (USDA) collected samples from three sites (Fig. 1): once prior to bait application (to establish a pre-treatment baseline); again, one to four days following each of the three applications; and finally, approximately two weeks after the last bait application. Samples collected included sweater, soil, crabs, limpets, and fish.
Furthermore, Lehua team members collected any non-target organisms found dead (species other than rats) and submitted them for diphacinone residue analysis to assess whether the organism had been exposed to rodenticide (with birds being the primary taxa of concern). Bird carcasses that were relatively fresh when discovered (N=14) were sent to the Animal Industry Division of the Hawaii Department of Agriculture, where they were necropsied by state veterinarians to evaluate evidence as to whether the mortality was associated with rodenticide ingestion.
Diphacinone residues were assayed and quantified by liquid chromatography and mass spectrometry (LC-MS/MS) at the USDA APHIS WS NWRC Chemistry Lab Unit in Fort Collins, Colorado. Diphacinone concentrations are reported as ng/g (mass, wet weight) or ng/mL (volume), both of which can also be expressed as parts per billion (ppb). Fish and bird samples were analyzed for diphacinone residues in both muscle and liver tissue.
The results from the analytical tests performed for the detection of diphacinone residues in samples from the 2017 rat eradication operation on Lehua corroborate the assumption that the potential short-term environmental contamination risks would be offset by the long-term benefits of successful rodent removal from islands.
• No detectable levels of diphacinone were found in any sample of seawater and soil;
• Three out of twelve samples of crabs contained detectable levels of diphacinone immediately following rodenticide applications, however, no diphacinone residues were detected two weeks after the final bait application (N= 3);
• Only three out of 46 fish muscle tissue had detectable levels of diphacinone immediately following the rodenticide applications and there were no detectable diphacinone residues in any fish muscle sample (N= 29) two weeks after the last application;
• Twelve out of sixteen fish liver samples (all Triggerfish - Melichthys vidua, Melichthys niger, Sufflamen bursa) had detectable levels of diphacinone immediately following rodenticide applications. However, by two weeks after the last bait operation, only two of 26 fish livers had detectable diphacinone residues;
• Lehua is used by local communities for the collection of limpets (Cellana sp.) known as Opihiand there was concern that the rodenticide use on Lehua could contaminate these invertebrates. No detectable diphacinone residues were found in any of the Opihi samples collected (N=15).
• As detailed in the Environmental Assessment (EA), we expected shorebirds to be at higher risk of primary exposure given their feeding behavior. Since they are migratory birds, the project mitigated exposure of shorebirds to the rodenticide by timing the operation before most of migrant birds arrive to Lehua in the Fall. Of all bird carcasses collected by the Lehua project team, two Ruddy Turnstones (Arenaria interpres) and one Pacific Golden Plover (Pluvialis fulva) exhibited internal biomarker fluorescence and gross pathology (hemorrhaging) consistent with direct bait consumption.
• As expected, of all seabird carcasses recovered throughout the operation, none showed conclusive signs of anticoagulant exposure.
On 5 September 2017, Lehua-based project personnel visited the large tide pool within the east arm of the caldera where they found 45 small dead mullet-type fish and two dead immature boobies in the pool. All fish and birds were collected for analysis. All samples were in advanced stage of tissue degradation. Three samples in best condition were sent to the NWRC Chemistry Lab Unit for diphacinone residue analysis. No liver or other organ tissues could be salvaged from the three mullet samples. Diphacinone residues were not detected in two of the three mullet muscle tissue samples. The third sample had detectable levels of diphacinone, but contamination from exposure to the elements could not be ruled out. Signs of anticoagulant exposure could not be identified and the pattern of diffuse pyranine biomarker throughout tissues (indicated by fluorescence under ultraviolet light) was not what would be expected to result from ingestion of diphacinone bait pellets. The fish and bird carcasses were found with a large amount of organic debris including seaweed that appeared to have been washed ashore by an unusually high tidal event.
The DLNR and all the partners of the Lehua rat eradication project take the potential environmental risks of the use of rodenticides for rodent eradications very seriously. To our knowledge, this is the most comprehensive and methodologically sensitive marine sampling effort associated with any aerial rodenticide application to date. There were no substantial nontarget mortality events clearly associated with this operation, apart from plausible implication in the deaths of three shorebirds. Put into context, Lehua Island supports several tens of thousands of seabirds of which a significant percentage of their eggs, chicks, and adults were preyed upon by rats annually in the past. By comparison, no new rat predations of birds have been found since the rat eradication operation in September 2017. Although our results demonstrated a low pulse of diphacinone concentrations in crab, fish, and some bird tissues, our detection window of residues was relatively short-lived, with very few detections persisting beyond two weeks after the last bait application. Animals that survive direct exposure quickly metabolize the majority of diphacinone ingested (Yu et al. 1982), and residual levels following operations such as the one on Lehua are not likely to be biologically significant.
Jones, H. P., N. D. Holmes, S. H. M. Butchart, B. R. Tershy, P. J. Kappes, I. Corkery, A. Aguirre-Muñoz, D. P. Armstrong, E. Bonnaud, A. A. Burbidge, K. C. Campbell, F. Courchamp, P. E. Cowan, R. J. Cuthbert, S. Ebbert, P. Genovesi, G. R. Howald, B. S. Keitt, S. W. Kress, C. M. Miskelly, S. Oppel, S. Poncetu, M. Rauzon, G. Rocamora, J. C. Russell, A. Samaniego-Herrera, P. J. Seddona, D. R. Spatz, D. R. Town, and D. A. Crolle. 2015. Invasive mammal eradication on islands results in substantial conservation gains. Proceedigns of the National Academy of Sciences 113(15):4033–4038.
Le Corre, M., D. K. Danckwerts, D. Ringler, M. Bastien, S. Orlowski, C. Morey Rubio, D. Pinaud, and T. Micol. 2015. Seabird recovery and vegetation dynamics after Norway rat eradication at Tromelin Island, western Indian Ocean. Biological Conservation 185:85–94
Russell, J. C., and K. G. Broome. 2016. Fifty years of rodent eradications in New Zealand: another decade of advances. New Zealand Journal of Ecology 40(2):197–204.
Yu, C. C., Y. H. Atallah, and D. M. Whitacre. 1982. Metabolism and disposition of diphacinone in rats and mice. Drug Metabolism and Disposition, 10(6):645-648.
Since the bait application in 2017, the LIERP has deployed a wide array of monitoring devices across the island (see map). Of these devices (traps, tracking tunnels, chew cards, cameras, bait stations) only the cameras have been able to capture any indications of what appears to be a very small number of remnant rats. Currently our monitoring has returned less than ten camera images (which may be showing the same individuals multiple times) in what appears to be four localized areas. In those areas where cameras have shown rat activity, “spot treatment” bait station grids and kill traps have been deployed to respond to the sightings.
The LIERP is currently using the same DITRAC D-50 bait used for the eradication operation in the bait stations but is currently exploring other diphacinone bait options in order to provide the remaining rats with variety. The project is also exploring the use of brodifacoum applied by hand broadcast and/or in bait stations. This option would require licensing of brodifacoum by the Hawaii Department of Agriculture (HDOA).
The LIERP is committed to continue its current level of monthly monitoring and “spot treatment” response until November 2018. At that point, based on the information collected from the monitoring effort, and in line with the Environmental Assessment, legal requirements, and budgetary constraints, the partnership will select the best course of action to pursue the best solution to eradicate or control the rat population on Lehua Island. These may include continued monitoring and spot treatment until the eradication is declared successful; mounting an additional aerial eradication attempt; or implementation of control efforts in perpetuity to maintain the population at low levels.