iÀ>Ê*}Ã]ÊÌÀ`ÕVi`Ê ÃµÕÌiÃ]Ê >`ÊÌ iÊ iViÊvÊ>Ü>¼½ÃÊ >ÌÛiÊ À`Ã
The introduction of mosquitoes, avian pox, and avian malaria to the Hawaiian Islands has had a profound effect on the geographical distribution and population number of highly susceptible Hawaiian honeycreepers, and likely contributed to the extinction of several species. While the mosquito vector (disease-carrier) is most closely associated with human activity, in remote Hawaiian rain forests, feral pigs may be pivotally impor- tant to the disease system. Since 1991, USGS scientists have taken a leadership position in identifying the role these diseases continue to play in the decline and extinction of native Hawaiian forest birds and in finding ways to mitigate their impacts.
>Ü>>Ê À`ÃÊ«iÀi` on continents worldwide, these bird diseases were absent from isolated islands where mos- Although introduced avian disease may quitoes did not occur. In the extreme isolation have contributed to the early extinctions of of the Hawaiian Islands, birds evolved in the many Hawaiian birds, it is the Hawaiian hon- absence of mosquito-transmitted disease and eycreepers or Drepanidinae that appear most lost their natural immunity to these common sensitive to disease. All fifty-seven known continental pathogens. species are believed to have evolved from a Avian pox is a virus that can be efficiently single finch-like ancestor. Much like Darwin’s transmitted by mosquito bite. It causes large finches of the Galápagos Islands, honeycreep- nodules or lesions, most commonly on the ers are an amazing example of adaptive radia- head or feet, that may interfere with feeding or tion—the evolution of related species adapted predator avoidance. Avian malaria is similar to different ecological niches. to human malaria but only affects some bird Honeycreepers were once found in the for- species. It is caused by a single-celled parasite ests from the coast to the timberline and were that depends on a mosquito for transmis- so abundant that one early naturalist described sion to the next bird. In the bird, the parasite the sound of their singing in coastal forest destroys red blood cells thereby weakening as deafening. Unfortunately, more than half the bird. Acutely infected birds are inactive, of the honeycreeper species are now extinct. do not feed, and probably succumb to starva- Some disappeared before the arrival of Eu- tion, cold weather, or predation. For some bird ropeans, but most extinctions have occurred species, like the ‘i‘iwi (Vestiaria coccinea), since that time. Loss of forest habitat and malaria infection is almost always fatal. the introduction of small mammal predators, Avian pox may have arrived in the Hawai- like the black rat, were probably responsible ian Islands with migratory birds but avian for extinctions during the 1800s. During the malaria most likely arrived with the importa- last century, however, introduced mosquito- tion of exotic game and cage birds during transmitted bird diseases are thought to have the first half of the last century. Neither of contributed to the extinction of several more these diseases, however, could have species. become so widely established Today, these diseases are considered if mosquitoes had not been one of the most important factors limiting accidentally introduced to the Hawaiian honeycreeper populations. The Hawaiian Islands. greatest density and species diversity of Hawaiian honeycreepers is now found in the highest, coolest, and driest forests where mosquitoes, and avian disease, seldom occur. }ÕÀiÊ£°Ê/ iÊiÝÌ>ÌÊëiViÃÊvÊ>Ü>>Ê iÞVÀii«iÀÃÊvÕ`ÊÊÌ iÊÜiÌÊvÀiÃÌÃÊÊ>Ü>¼Ê ÌÀ`ÕVi`ÊÛ>Ê*ÝÊ>`ÊÊ Ã>`°Ê\Ê>Ü>¼Ê Àii«iÀ]Ê"ÀiÞÃÌÃÊ>>Ê Û>Ê >>À> i`>}iÀi`®ÆÊ \ʼi«>]Ê Ý«ÃÊVVViÕÃÊ i`>}iÀi`®ÆÊ \Ê«>«>i]Ê>ÌiÊÃ>}Õ Avian pox Avipoxvirus and avian malaria i>ÆÊ \Ê>Ü>¼Ê¼> ]Êi}>Ì ÕÃÊÛÀiÃÆÊ \Ê Plasmodium relictum are mosquito-transmitted ¼¼Ü]Ê6iÃÌ>À>ÊVVVi>ÆÊ>`Ê\ʼ>«>¼>Õ]Ê diseases that affect native Hawaiian birds. i}>Ì ÕÃÊÕÀÊi`>}iÀi`®°Ê* ÌÃÊ^Ê Although common in wild bird populations >VÊivvÀiÞ®
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}ÕÀiÊÓ°ÊÊÃÕÌ iÀÊ ÕÃiÊõÕÌÊ ÕiÝʵյÕiv>ÃV>ÌÕîÊLÌiÃÊ>Ê outbreaks of avian malaria. Adult mosquito populations at the highest ¼>> °Ê7 iÊVÌiÌ>ÊLÀ`ÃÊÌÞ«V>ÞÊ`ivi`ÊÌ iÃiÛiÃÊ>}>ÃÌÊ elevations (≥1800 meters) are infrequent, reduced in number, short- L`vii`}ÊõÕÌiÃ]Ê>Ü>>ÊLÀ`ÃÊ>ÀiÊÀiÊÌiÀ>ÌÊvÊLÌ}Ê lived, and do not appear to transmit avian malaria. Mosquitoes and õÕÌiÃÊ>`ÊÀiÊiÞÊÌÊLiViÊviVÌi`ÊÜÌ Ê`Ãi>Ãi°Ê ÕiÝÊ disease are limited at high elevations by cool year-round temperatures õÕÌiÃÊvii`ÊÃÌÊ>VÌÛiÞÊ>ÌÊ} Ì]ÊÜ iÊLÀ`ÃÊ>ÀiÊÀÃÌ}° that prolong larval, and inhibit parasite, development. Mosquito larvae are found in ditches, catch basins, livestock *>À>`ÃiÊ ÃÌ\Ê/ iÊ-ÕÌ iÀÊÕÃiÊ ÃµÕÌÊÊ>Ü>¼ troughs, and any number of artificial containers or water impound- ments typical of urban and rural communities. High densities of adult Though common in most continental areas, mosquitoes are not mosquitoes can be produced from the rainwater-filled refuse and native to many oceanic islands. The southern house mosquito Culex poorly-maintained infrastructure common to most areas of human quinquefasciatus occurs throughout the subtropical and tropical regions activity. In Hawaiian forests, however, suitable larval mosquito habitat of the world. It is often associated with manmade habitats and was is more limited than might be imagined. Larvae are seldom found in spread widely through commerce. It was the first mosquito to arrive ground pools, bogs or flowing streams but do occur in tree fern cavities in Hawai‘i, having been introduced in 1826 when, presumably, it was and the rock holes and pools of intermittent stream beds. brought ashore in water casks from a ship recently sailed from Mexico. Today, six species of biting mosquitoes can be found in the Hawaiian Islands but only Culex carries avian malaria and, most likely, avian pox. iÀ>Ê*}ÃÊVÀi>ÃiÊ ÃµÕÌÊLÕ`>Vi Field and laboratory research by USGS scientists and collabora- Although a considerable amount of precipitation falls on the slopes tors has increased our understanding of the basic biology and vector of Mauna Loa, very little water is retained on the surface of its young, potential of this mosquito in the Hawaiian Islands. The southern house volcanic substrates. In these wet forests, mosquito larvae are most mosquito is the most widely distributed mosquito in the Hawaiian Is- commonly found in the cavities of tree fern trunks. Small cavities form lands and can be found from the coast to 6000 feet. Typically, females naturally through the action of decay but the larger cavities, occupied lay eggs in small, calm bodies of water that are rich in organic nutri- by mosquito larvae, are created by feral pigs (Sus scrofa). Feral pigs ents. The filter feeding larvae can complete their development in seven knock over tree ferns, tear through the fibrous and woody outer layers, days under optimal nutrition and temperature but may take months to and eat the starchy pith leaving behind a bowl- or trough-shaped cavity develop under less favorable conditions. that fills up with rainwater. In forests where tree ferns and pigs occur Although the majority of research on the dynamics of introduced together, these cavities are the most abundant and productive habitat avian disease in Hawai‘i has been conducted on Hawai‘i Island, the for larval mosquitoes. Feral pigs increase the numbers of mosquitoes results are believed to be applicable to other parts of the archipelago. found in the forest. On the windward side of the island of Hawai‘i, adult mosquitoes are Feral pigs have been eliminated from much of the Department of present year-round at low elevations where adequate rainfall and opti- Interior (DOI) land in the Hawaiian Islands, but this is not necessarily mal temperatures allow for multiple generations. There is evidence of so on adjacent state or private land. Mosquitoes may disperse up to 2 year-round disease transmission in these lowlands. At mid-elevations miles (3 km) in native forests. Because of this surprising dispersal abil- (around 900–1200 meters), adult mosquitoes are seasonally abundant ity of mosquitoes, small and/or fragmented refuges and parks cannot in the late summer. This late summer peak coincides with annual expect to manage avian disease through feral pig elimination without
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immigrate to parks and refuges. Removal of refuse and unnecessary structures where water accumulates can mitigate much of the problem }ÕÀiÊÈ°Ê ÃµÕÌÊi}}Ê but requires commitment and vigilance from individual land owners, À>vÌÃÊ>`Ê>ÀÛ>iÊÜiÀiÊ communities, NGOs, as well as DOI land managers to be successful. ViVÌi`ÊvÀÊÌÀiiÊ Some DOI lands, like Hawai‘i Volcanoes National Park, may face viÀÊV>ÛÌiÃÊvÕ`Ê additional challenges when considering historical structures that are >ÌÊÃiÛiÀ>Ê`vviÀiÌÊ protected by law. In a recent collaborative project between USGS sci- vÀiÃÌÊÃÌiðÊ/ÀiiÊviÀÊ entists and NPS resource managers, mosquito numbers at the historic V>ÛÌiÃÊ>ÞÊVÌ>Ê Ainahou Ranch were successfully reduced through identification of ÃiÛiÀ>Ê Õ`Ài`Ê>À artificial habitat and its subsequent removal, modification, or treatment Û>iÊÊiÃÃÊÌ >Ê>ÊÌiÀÊ with a selective biopesticide. µÕ>ÀÌ®ÊvÊÜ>ÌiÀ° ÕÌÕÀiÊ/ Ài>Ìà New mosquito-borne diseases, like West Nile Virus (WNV), may be introduced to the Hawaiian Islands in the future. While infective migra- }ÕÀiÊÇ°Ê ÃµÕÌiÃÊ tory birds might initiate an outbreak, an even more likely scenario Ài>Ài`ÊÊÌ iÊ>LÊ would be the arrival of an infective mosquito on commercial aircraft or ÜiÀiÊ>Ài`ÊÜÌ Ê cargo ship. Naïve native and non-native birds and abundant compe- yÕÀiÃViÌÊ«Ü`iÀ]Ê tent mosquito vectors (Culex quinquefasciatus and Aedes albopictus) Àii>Ãi`ÊÊÌ iÊvÀiÃÌ]Ê would ensure rapid establishment and spread of WNV throughout the >`ÊV>«ÌÕÀi`ÊÊÌÀ>«ÃÊ island chain. New mosquito species can still be introduced as well. In ÃiÌÊÕ«ÊÊ>Ê}À`Ê>ÀÕ`Ê 2003, the first temperate mosquito species, Aedes japonicus, became Ì iÊÀii>ÃiÊÃÌi°Ê/ iÃiÊ established on the island of Hawai‘i. Its ability to endure cooler ÃÌÕ`iÃÊ i«ÊÌÊ`iÌiÀ temperatures may alter the present altitudinal distribution of avian pox iÊÌ iÊ>ÝÕÊ thereby jeopardizing endangered honeycreepers inhabiting high eleva- >`Ê>ÛiÀ>}iÊ`ÃÌ>ViÃÊ tion forests. Ì >ÌÊ ÕiÝÊõÕÌiÃÊ ÜÊÌÀ>Ûi° VÕÃà extending control into neighboring lands. There are many obstacles, Native Hawaiian birds have been decimated by introduced, mosquito- however, to landscape level feral pig control. Feral pig elimination is transmitted avian malaria and pox. In native forests, introduced feral expensive and, in many cases, competes with the mandates of state pigs create habitat for larval mosquitoes by feeding on tree fern trunks. agencies to provide game for recreational hunting. Human structures provide habitat for larval mosquitoes in rural and ur- ban areas. USGS research has shown that reducing artificial larval mos- -ÕÀViÊ,i`ÕVÌÊvÊ > >`iÊÊ quito habitat and eliminating feral pigs effectively reduces mosquito populations in the surrounding area. Future research should be directed >ÀÛ>Ê ÃµÕÌÊ>LÌ>Ì at determining whether reducing feral pig populations can control avian The infrastructure of DOI lands in Hawai‘i is not often designed or disease in native forests. Reducing larval mosquito habitat is our best maintained with mosquito control in mind, and can contribute signifi- hope for limiting disease transmission to native birds. cantly to mosquito production. Neighboring residential communities Avian disease management is essential to the preservation of native and agricultural operations may also produce mosquitoes that will Hawaiian birds and with sound research, and commitment from land 7 >ÌÊÌ iÊ*ÕLVÊ >Ê Everyone can help to reduce the number of mosquitoes in • Change the water in cemetery flower vases regularly or use their community, work place, and in forests by taking the fol- arrangements that do not require water. lowing actions: • Keep ornamental ponds clean of debris and stock with fish • Dispose of, or turn over, any containers collecting water (but don’t put fish in natural waterways). around the yard. Abandoned tires are particularly notorious for harboring mosquitoes and should be disposed of properly. • Fill or drain depressions in lawns or fields that regularly fill with rainwater. • Cut drainage holes in old tires used around farms or play- grounds so that they cannot collect water. • Cover or turn over boats and canoes when not in use. • Regularly flush out birdbaths, pet water dishes, and water- • Hunt feral pigs. ing troughs. • When other means of reducing larval mosquito habitat are • Keep rain gutters free flowing and close, or screen off, all not possible, use monthly applications of Bacillus thuring- openings to cisterns. iensis israelensis in strict accordance with the label.
*>ÀÌiÀÃÊ>`Ê >LÀ>ÌÀÃ • US Fish and Wildlife Service • National Park Service • The Nature Conservancy • Smithsonian Institution • National Science Foundation
• Hawai‘i Department of Land and Natural Resources* ÌÊ^Ê>VÊivvÀiÞ • Hawai‘i Department of Health }ÕÀiÊ°Ê >>`iÊÜ>ÌiÀÊ«Õ`iÌÃÊÃÕV Ê>ÃÊ ÀÃiÊÌÀÕ} îÊÜiÀiÊ • University of Hawai‘i at Manoa,ˉ Pacific Cooperative Studies Unit Ì iÊ>Ê>ÀÛ>ÊõÕÌÊ >LÌ>ÌÊ>ÌÊ> ÕÊ,>V °Ê7 iÊÃÌ>`}ÊÜ> ÌiÀÊVÕ`ÊÌÊLiÊV«iÌiÞÊi>Ìi`]ÊõÕÌÊ>ÀÛ>iÊÜiÀiÊÀi`ÕVi`Ê • University of Hawai‘i at Hilo, Hawai‘i Cooperative Studies Unit ÜÌ ÊÌ ÞÊ>««V>ÌÃÊvÊÌ iÊL«iÃÌV`iÊ >VÕÃÊÌ ÕÀ}iÃÃÊ ÃÀ>iiÃð
management agencies and the general public, the impact of disease ,iVi`i`Ê,i>`} can be minimized. Control of mosquito-borne disease, however, must Ahumada, J.A., D.A. LaPointe and M.D. Samuel. 2004. Modeling the be coordinated over broad landscapes and will necessitate the coopera- population dynamics of Culex quinquefasciatus (Diptera: Culici- tion of individuals, communities, and governmental agencies. dae), along an elevational gradient in Hawai‘i. Journal of Medical Resources should be directed into integrative mosquito control and Entomology 41: 1157–1170. the development of long-term disease management strategies. Source reduction and mosquito-proof design of infrastructure should be con- Atkinson, C.T., K.L. Woods, R.J. Dusek, L.S. Sileo and W.M. Iko. sidered in avian disease management. Lands acquired for conservation 1995. Wildlife disease and conservation in Hawai‘i. Pathogenicity should focus on inholdings and adjacent lands to minimize edge effects of avian malaria (Plasmodium relictum) in experimentally-infected of unmanaged land. ‘I‘iwi (Vestiaria coccinea). Parasitology 11: S59–S69 Direct source reduction through removal and modification of artificial containers and impoundments, and indirect source reduc- Jacobi, J.D. and C.T. Atkinson. 1995. Hawai‘i’s endemic birds. IN: tion through the elimination of feral pigs, can successfully reduce and Our Living Resources: a Report to the Nation on the Distribution, potentially eliminate mosquito populations. Abundance and Health of U.S. Plants, Animal, and Ecosystems. E.T. LaRoe, G.S. Farris, C.E. Puckett, P.D. Doran and M.J. Mac, eds. U.S. Department of Interior, National Biological Service, Washington, D.C. pp. 376–381. ÀÊ>``Ì>ÊvÀ>ÌÊVÌ>VÌ\ Kilpatrick, A. M., Y. Gluzberg, J. Burgett, and P. Daszak. 2004. Quan- À°Ê iÃÊ°Ê >*Ìi]Ê V}ÃÌ titative risk assessment of the pathways by which West Nile virus * i\Ênän®ÊÈÇn££ could reach Hawai‘i. EcoHealth 1(2): 205–209. >\`iÃÚ>«ÌiJÕÃ}ð}Û LaPointe, D.A., M.L. Goff and C.T. Atkinson. 2005. Comparative * ÌÊ Ài`ÌÃ\ susceptibility of introduced forest-dwelling mosquitoes in Hawai‘i to avian malaria, Plasmodium relictum. Journal of Parasitology 1--ÊwiÊ« Ì}À>« Ã]ÊÕiÃÃÊÌ iÀÜÃiÊÌi`° 91: 843–849.