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Although introduced avian disease may 

have contributed to the early extinctions of 

many Hawaiian birds, it is the Hawaiian honeycreepers or Drepanidinae that appear most 

sensitive to disease. All fifty-seven known 

species are believed to have evolved from a 

single finch-like ancestor. Much like Darwin’s 

finches of the Galápagos Islands, honeycreepers are an amazing example of adaptive radiation—the evolution of related species adapted 

to different ecological niches. 

Honeycreepers were once found in the forests from the coast to the timberline and were 

so abundant that one early naturalist described 

the sound of their singing in coastal forest 

as deafening. Unfortunately, more than half 

of the honeycreeper species are now extinct. 

Some disappeared before the arrival of Europeans, but most extinctions have occurred 

since that time. Loss of forest habitat and 

the introduction of small mammal predators, 

like the black rat, were probably responsible 

for extinctions during the 1800s. During the 

last century, however, introduced mosquitotransmitted bird diseases are thought to have 

contributed to the extinction of several more 

species. 

Today, these diseases are considered 

one of the most important factors limiting 

Hawaiian honeycreeper populations. The 

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.

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Avian pox Avipoxvirus and avian malaria 

Plasmodium relictum are mosquito-transmitted 

diseases that affect native Hawaiian birds. 

Although common in wild bird populations 

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on continents worldwide, these bird diseases 

were absent from isolated islands where mosquitoes did not occur. In the extreme isolation 

of the Hawaiian Islands, birds evolved in the 

absence of mosquito-transmitted disease and 

lost their natural immunity to these common 

continental pathogens. 

Avian pox is a virus that can be efficiently 

transmitted by mosquito bite. It causes large 

nodules or lesions, most commonly on the 

head or feet, that may interfere with feeding or 

predator avoidance. Avian malaria is similar 

to human malaria but only affects some bird 

species. It is caused by a single-celled parasite 

that depends on a mosquito for transmission to the next bird. In the bird, the parasite 

destroys red blood cells thereby weakening 

the bird. Acutely infected birds are inactive, 

do not feed, and probably succumb to starvation, cold weather, or predation. For some bird 

species, like the ‘i‘iwi (Vestiaria coccinea), 

malaria infection is almost always fatal. 

Avian pox may have arrived in the Hawaiian Islands with migratory birds but avian 

malaria most likely arrived with the importation of exotic game and cage birds during 

the first half of the last century. Neither of 

these diseases, however, could have 

become so widely established 

if mosquitoes had not been 

accidentally introduced to the 

Hawaiian Islands. 

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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 important 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.
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Though common in most continental areas, mosquitoes are not 

native to many oceanic islands. The southern house mosquito Culex 

quinquefasciatus occurs throughout the subtropical and tropical regions 

of the world. It is often associated with manmade habitats and was 

spread widely through commerce. It was the first mosquito to arrive 

in Hawai‘i, having been introduced in 1826 when, presumably, it was 

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.

Field and laboratory research by USGS scientists and collaborators has increased our understanding of the basic biology and vector 

potential of this mosquito in the Hawaiian Islands. The southern house 

mosquito is the most widely distributed mosquito in the Hawaiian Islands and can be found from the coast to 6000 feet. Typically, females 

lay eggs in small, calm bodies of water that are rich in organic nutrients. The filter feeding larvae can complete their development in seven 

days under optimal nutrition and temperature but may take months to 

develop under less favorable conditions.

 Although the majority of research on the dynamics of introduced 

avian disease in Hawai‘i has been conducted on Hawai‘i Island, the 

results are believed to be applicable to other parts of the archipelago. 

On the windward side of the island of Hawai‘i, adult mosquitoes are 

present year-round at low elevations where adequate rainfall and optimal temperatures allow for multiple generations. There is evidence of 

year-round disease transmission in these lowlands. At mid-elevations 

(around 900–1200 meters), adult mosquitoes are seasonally abundant 

in the late summer. This late summer peak coincides with annual 

outbreaks of avian malaria. Adult mosquito populations at the highest 

elevations (≥1800 meters) are infrequent, reduced in number, shortlived, and do not appear to transmit avian malaria. Mosquitoes and 

disease are limited at high elevations by cool year-round temperatures 

that prolong larval, and inhibit parasite, development.

Mosquito larvae are found in ditches, catch basins, livestock 

troughs, and any number of artificial containers or water impoundments typical of urban and rural communities. High densities of adult 

mosquitoes can be produced from the rainwater-filled refuse and 

poorly-maintained infrastructure common to most areas of human 

activity. In Hawaiian forests, however, suitable larval mosquito habitat 

is more limited than might be imagined. Larvae are seldom found in 

ground pools, bogs or flowing streams but do occur in tree fern cavities 

and the rock holes and pools of intermittent stream beds. 

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Although a considerable amount of precipitation falls on the slopes 

of Mauna Loa, very little water is retained on the surface of its young, 

volcanic substrates. In these wet forests, mosquito larvae are most 

commonly found in the cavities of tree fern trunks. Small cavities form 

naturally through the action of decay but the larger cavities, occupied 

by mosquito larvae, are created by feral pigs (Sus scrofa). Feral pigs 

knock over tree ferns, tear through the fibrous and woody outer layers, 

and eat the starchy pith leaving behind a bowl- or trough-shaped cavity 

that fills up with rainwater. In forests where tree ferns and pigs occur 

together, these cavities are the most abundant and productive habitat 

for larval mosquitoes. Feral pigs increase the numbers of mosquitoes 

found in the forest. 

Feral pigs have been eliminated from much of the Department of 

Interior (DOI) land in the Hawaiian Islands, but this is not necessarily 

so on adjacent state or private land. Mosquitoes may disperse up to 2 

miles (3 km) in native forests. Because of this surprising dispersal ability of mosquitoes, small and/or fragmented refuges and parks cannot 

expect to manage avian disease through feral pig elimination without 

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extending control into neighboring lands. There are many obstacles, 

however, to landscape level feral pig control. Feral pig elimination is 

expensive and, in many cases, competes with the mandates of state 

agencies to provide game for recreational hunting.

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The infrastructure of DOI lands in Hawai‘i is not often designed or 

maintained with mosquito control in mind, and can contribute significantly to mosquito production. Neighboring residential communities 

and agricultural operations may also produce mosquitoes that will 

immigrate to parks and refuges. Removal of refuse and unnecessary 

structures where water accumulates can mitigate much of the problem 

but requires commitment and vigilance from individual land owners, 

communities, NGOs, as well as DOI land managers to be successful. 

Some DOI lands, like Hawai‘i Volcanoes National Park, may face 

additional challenges when considering historical structures that are 

protected by law. In a recent collaborative project between USGS scientists and NPS resource managers, mosquito numbers at the historic 

Ainahou Ranch were successfully reduced through identification of 

artificial habitat and its subsequent removal, modification, or treatment 

with a selective biopesticide.

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New mosquito-borne diseases, like West Nile Virus (WNV), may be 

introduced to the Hawaiian Islands in the future. While infective migratory birds might initiate an outbreak, an even more likely scenario 

would be the arrival of an infective mosquito on commercial aircraft or 

cargo ship. Naïve native and non-native birds and abundant competent mosquito vectors (Culex quinquefasciatus and Aedes albopictus) 

would ensure rapid establishment and spread of WNV throughout the 

island chain. New mosquito species can still be introduced as well. In 

2003, the first temperate mosquito species, Aedes japonicus, became 

established on the island of Hawai‘i. Its ability to endure cooler 

temperatures may alter the present altitudinal distribution of avian pox 

thereby jeopardizing endangered honeycreepers inhabiting high elevation forests. 


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Native Hawaiian birds have been decimated by introduced, mosquitotransmitted avian malaria and pox. In native forests, introduced feral 

pigs create habitat for larval mosquitoes by feeding on tree fern trunks. 

Human structures provide habitat for larval mosquitoes in rural and urban areas. USGS research has shown that reducing artificial larval mosquito habitat and eliminating feral pigs effectively reduces mosquito 

populations in the surrounding area. Future research should be directed 

at determining whether reducing feral pig populations can control avian 

disease in native forests. Reducing larval mosquito habitat is our best 

hope for limiting disease transmission to native birds.

Avian disease management is essential to the preservation of native 

Hawaiian birds and with sound research, and commitment from land 

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management agencies and the general public, the impact of disease 

can be minimized. Control of mosquito-borne disease, however, must 

be coordinated over broad landscapes and will necessitate the cooperation of individuals, communities, and governmental agencies. 

Resources should be directed into integrative mosquito control and 

the development of long-term disease management strategies. Source 

reduction and mosquito-proof design of infrastructure should be considered in avian disease management. Lands acquired for conservation 

should focus on inholdings and adjacent lands to minimize edge effects 

of unmanaged land. 

Direct source reduction through removal and modification of 

artificial containers and impoundments, and indirect source reduction through the elimination of feral pigs, can successfully reduce and 

potentially eliminate mosquito populations. 

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,iVi`i`Ê,i>`}

Ahumada, J.A., D.A. LaPointe and M.D. Samuel. 2004. Modeling the 

population dynamics of Culex quinquefasciatus (Diptera: Culicidae), along an elevational gradient in Hawai‘i. Journal of Medical 

Entomology 41: 1157–1170.

Atkinson, C.T., K.L. Woods, R.J. Dusek, L.S. Sileo and W.M. Iko. 

1995. Wildlife disease and conservation in Hawai‘i. Pathogenicity 

of avian malaria (Plasmodium relictum) in experimentally-infected 

‘I‘iwi (Vestiaria coccinea). Parasitology 11: S59–S69

Jacobi, J.D. and C.T. Atkinson. 1995. Hawai‘i’s endemic birds. IN: 

Our Living Resources: a Report to the Nation on the Distribution, 

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.

Kilpatrick, A. M., Y. Gluzberg, J. Burgett, and P. Daszak. 2004. Quantitative risk assessment of the pathways by which West Nile virus 

could reach Hawai‘i. EcoHealth 1(2): 205–209.

LaPointe, D.A., M.L. Goff and C.T. Atkinson. 2005. Comparative 

susceptibility of introduced forest-dwelling mosquitoes in Hawai‘i 

to avian malaria, Plasmodium relictum. Journal of Parasitology

91: 843–849.

Everyone can help to reduce the number of mosquitoes in 

their community, work place, and in forests by taking the following actions:

• Dispose of, or turn over, any containers collecting water 

around the yard. Abandoned tires are particularly notorious 

for harboring mosquitoes and should be disposed of properly.

• Cut drainage holes in old tires used around farms or playgrounds so that they cannot collect water.

• Regularly flush out birdbaths, pet water dishes, and watering troughs.

• Keep rain gutters free flowing and close, or screen off, all 

openings to cisterns.

• Change the water in cemetery flower vases regularly or use 

arrangements that do not require water. 

• Keep ornamental ponds clean of debris and stock with fish 

(but don’t put fish in natural waterways).

• Fill or drain depressions in lawns or fields that regularly fill 

with rainwater.

• Cover or turn over boats and canoes when not in use.

• Hunt feral pigs.

• When other means of reducing larval mosquito habitat are 

not possible, use monthly applications of Bacillus thuringiensis israelensis in strict accordance with the label. 

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• US Fish and Wildlife Service

• National Park Service 

• The Nature Conservancy

• Smithsonian Institution

• National Science Foundation

• Hawai‘i Department of Land and Natural Resources

• Hawai‘i Department of Health

• University of Hawai‘i at Manoa, Paci ˉ fic Cooperative Studies Unit

• University of Hawai‘i at Hilo, Hawai‘i Cooperative Studies Unit

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