Haleakalā: Fighting Avian Malaria to Save Maui's Honeycreepers

The kiwikiu lives only in cloud forest high on the slopes of Haleakalā Volcano, Maui. Warmer air is letting disease-carrying mosquitoes invade its mountain stronghold — a threat to the species' survival. Photograph by C. Warren (NPS / Maui Forest Bird Recovery Project).

Hawaiʻi is home to the Hawaiian honeycreepers, forest birds found nowhere else in the world. The birdsong that once filled Hawaiian forests from mauka to makai — from the mountains to the sea — has grown quiet, because of habitat loss and avian malaria, which arrived with human help. Of at least 50 species of Hawaiian honeycreepers, only 17 remain, and most are highly endangered.

A disease moving uphill

Avian malaria, the deadliest threat to the honeycreepers, spread through the islands in the early 1900s after disease-carrying mosquitoes arrived. With habitat loss, it drove honeycreepers out of lower-elevation forests entirely.

• Kauaʻi, an older, more eroded island, gave its birds little refuge: all but the flat tops of its highest mountains have long been within mosquitoes' reach.
• Maui, younger and taller, kept high-elevation strongholds: the band of cloud forest on Haleakalā known as Wao Akua, which Native Hawaiians consider sacred.

Now warming temperatures are pushing the mosquitoes' range upward through that forest. Managers also worry that climate change could push the cloud forest's upper edge downward, through its effect on the trade wind inversion — a stable boundary, at about 7,000 feet above sea level, common over tropical oceans between moist lower air and very dry upper air.

Cloud forest on Haleakalā's windward, north-facing slope gives way to high-elevation shrubland and grassland. The sharp, roughly horizontal boundary reflects the climate break caused by the trade wind inversion. Photograph by G. Schuurman.

Honeycreepers in crisis

Caught in a band of cloud forest that may be shrinking and is now filling with mosquitoes, Maui's remaining forest birds are in immediate danger.

• Kiwikiu: fewer than 200 remain in the wild, down from about 500 that had held on from the 1980s into the 2000s.
• ʻAkikiki, found only on Kauaʻi, fell from about 450 birds in 2018 to 5 in 2023. As of 2024 a single ʻakikiki was known in the wild, leaving the species functionally extinct there; a small captive population had been set up a few years earlier.

An infected female mosquito's bite kills upwards of 95% of these birds, as soon as nine days later.

Two lines of defence

Haleakalā National Park is working to resist honeycreeper extinction in two ways.

1. The traditional approach: find where the most endangered birds are concentrated and focus conservation there. Decades of management, including control of hoofed animals and weeds, have protected native forest — but with bird numbers plunging from a new stressor, that was not enough.
2. Going after the mosquitoes. The National Park Service, the Hawaiʻi Department of Land and Natural Resources, The Nature Conservancy, the American Bird Conservancy and others — together called Birds, Not Mosquitoes — set out to develop and apply ways of suppressing mosquito populations to control avian malaria.

The southern house mosquito (Culex quinquefasciatus), introduced to Hawaiʻi in 1826, carries the avian malaria that has devastated honeycreepers on Kauaʻi and much of Maui. Photograph by C. Warren (NPS).

The incompatible insect technique

The most promising method was the Wolbachia incompatible insect technique (IIT).

• Wolbachia is a bacterium that lives naturally in most insects — roughly 60% of the world's insects carry it. Two mosquitoes can produce offspring only if they carry the same or compatible strains, or neither carries it. If a male and a female carry incompatible strains, the eggs do not hatch.
• How it is used. Southern house mosquitoes are reared in a lab with a strain incompatible with Maui's wild mosquitoes. Only the males, which never bite, are released. A female mates only once, so a wild female that mates with a released male lays eggs that will not hatch, and the population falls.
• Not genetic engineering. Neither the mosquitoes' nor Wolbachia's genome is changed; it works more like birth control. Unlike the sterile insect technique, the males can breed — their offspring simply do not hatch.

Similar methods have been tested for decades in more than a dozen countries and are used worldwide to protect people from dengue, Zika and chikungunya. This is the first time IIT has been used specifically for conservation.

"The only thing that's more tragic is if [the birds] went extinct and we didn't try. You can't not try," said Chris Warren, the park's Forest Bird Program Coordinator.

Releases began in November 2023

In November 2023 the park and its partners began releasing batches of incompatible male mosquitoes, planning deliveries twice a week by drone or helicopter in biodegradable capsules — like large coffee pods — each holding thousands of males. The partners work regardless of boundaries: the NPS lets its collaborators monitor and release mosquitoes inside the park, and the same groups carry the work beyond the park and beyond Maui. "The birds don't care where the boundaries are," Warren said.

Common questions

• Why are the birds declining? Mainly avian malaria carried by invasive southern house mosquitoes. Both are non-native, and many native birds — above all the honeycreepers — have lost whatever resistance their mainland ancestors may have had. High areas once too cold for mosquitoes were refuges, but as the climate warmed, mosquitoes spread across high-elevation Maui. Habitat loss and non-native predators have also contributed.
• Will fewer mosquitoes harm Hawaiʻi? No. Mosquitoes arrived roughly 200 years ago and have not replaced any key pollinator or food source.
• Does it affect people? The release areas are mostly too far from where people live or visit for them to notice fewer mosquitoes; released males do not bite; and IIT has been used in cities for years to protect human health.
• Is Wolbachia safe? It has lived alongside other organisms for millions of years and survives only inside insect cells. There are no reports of people or animals being harmed by it, and laboratory and field research has found mosquitoes carrying it safe for people, animals and the environment.

Resist, accept, direct

Park managers face growing change as climate change combines with land-use change, rising visitation, pollution and non-native species, pushing ecosystems and park infrastructure beyond their historical range. The resist–accept–direct (RAD) framework helps them choose deliberately among three strategies:

Sources

• Kaylin Thomas, NPS Climate Change Response Program, 2024, "Haleakalā National Park: Resisting climate-enhanced threats of avian malaria," National Park Service. https://www.nps.gov/articles/000/hale-rad.htm — with information from Chris Warren and Paul Hosten of Haleakalā National Park.
• The second half of the article (the incompatible insect technique, the releases and the questions) and the mosquito photograph were missing from the imported page and are taken from the article as published.
• Rewritten in hubnx's own words.