Haleakalā National Park: Resisting climate-enhanced threats of avian malaria

By Kaylin Thomas, NPS Climate Change Response Program, 2024

The kiwikiu occurs only in cloud forests on the upper slopes of Haleakalā Volcano on Maui. Climate change-driven increases in air temperatures are allowing non-native, disease-carrying mosquitos to invade the species’ mountain stronghold, representing an existential threat to the species.

Photo by C. Warren (NPS/Maui Forest Bird Recovery Project)

Hawaiʻi is home to some of the most diverse and unique wildlife on the planet, including the Hawaiian honeycreepers, a group of forest birds found nowhere else in the world. Unfortunately, the chorus of native birds that once filled Hawaiian forests from mauka to makai (from the mountains to the sea) has quieted due to the human-assisted arrival of avian malaria, as well as habitat loss. Today, only 17 of at least 50 species of Hawaiian honeycreepers remain, and most of them are highly endangered.

Avian malaria, the deadliest of the threats to Hawaiian honeycreepers, spread through the islands in the early 1900s after the arrival of disease-carrying mosquitoes. Driven by habitat loss and disease, honeycreepers disappeared entirely from lower-elevation forests. Birds on older, more eroded Hawaiian islands like Kauaʻi have been at a particular geographical disadvantage because all but the flat tops of the tallest mountains have long been within mosquitoes’ reach. In comparison, birds on Maui, a younger and taller island, have high-elevation strongholds—the band of cloud forest known as Wao Akua that Native Hawaiians consider sacred—high up on the slopes of Haleakalā Volcano. Now, warming temperatures are expanding the mosquitoes’ range upward through the forest. At the same time, managers worry that climate change may also drive the upper edge of the cloud forest downward through its effects on the trade wind inversion, a stable boundary (at about 7000 feet above sea level) common over tropical oceans between a moisture-rich lower air mass and an extremely dry upper air mass.

A band of cloud forest that blankets the windward (north-facing) slope of Haleakalā Volcano gives way to high-elevation shrubland and grassland above it. The sharp, roughly horizontal ecotone reflects a strong elevational climatic discontinuity caused by the trade wind inversion.

Photo by G. Schuurman

Trapped in a potentially shrinking band of cloud forest now starting to swarm with deadly mosquitoes, Maui’s remaining forest birds are in imminent danger. Haleakalā National Park’s staff and collaborators are racing against a wave of avian malaria to keep these native species alive. Fewer than 200 kiwikiu, for example, are left in the wild, down from a population of roughly 500 that had persisted from the 1980s into the 2000s. Meanwhile, Kauaʻi has already seen the impact of warming temperatures on an island-endemic Hawaiian honeycreeper called the ‘akikiki, which dropped from approximately 450 individuals in 2018 to just 5 individuals in 2023. As of 2024, a single ‘akikiki is known to be in the wild, rendering the species functionally extinct in the wild (fortunately, a small captive population was established a few years ago). With upwards of 95% mortality as soon as nine days after a bite from an infected female mosquito—and the population of Maui’s kiwikiu dwindling—Haleakalā’s birds needed an emergency rescue to avoid the same fate.

How can environmental stewards protect wildlife against an expanding aerial plague? Haleakalā National Park is resisting Hawaiian honeycreeper extinction with two efforts, one traditional and one novel. The first has been to locate the population centers of the most endangered forest birds and focus conservation efforts there. Several decades of management — including ungulate and weed control — have successfully protected native forest habitat, but with the number of birds plummeting due to a pervasive new stressor moving through the forest, managers recognized that this knowledge and the traditional conservation approaches it supports were insufficient. Haleakalā National Park had to expand their strategy with a second, more novel effort: taking on the new stressor—the mosquitoes—directly. The National Park Service, in collaboration with the Hawaiʻi Department of Land and Natural Resources, The Nature Conservancy, the American Bird Conservancy, and others (collectively known as Birds, Not Mosquitoes) began working together to develop and apply methods to suppress mosquito populations as a means of controlling avian malaria.

The southern house mosquito (Culex quinquefasciatus) is a non-native mosquito, introduced to the Hawaiian Islands in 1826. These mosquitoes carry avian malaria, a disease responsible for decimating Hawaiian honeycreeper populations across Kauaʻi and much of Maui.

Photo by C. Warren (NPS)

The most promising candidate for that management was the Wolbachia incompatible insect technique (IIT). Unlike genetic modification strategies, IIT relies on Wolbachia—a bacterium that lives naturally within most insects and prevents their insect host from producing viable offspring with partners that have even a slightly different strain of Wolbachia. Unlike sterile insect techniques (SIT), mosquitoes with mismatched Wolbachia can still breed, but their resulting eggs will not hatch. As a quirk of mosquito reproductive biology, a female mosquito mates only once and will not mate again. This aspect of mosquito natural history presents an opportunity to create a safe method to reduce mosquito reproductive output by inoculating lab-bred male mosquitoes (which do not bite) with an incompatible strain of Wolbachia and releasing these males on Maui where the wild females they mate with produce inviable offspring, causing the mosquito population to decline. This effort therefore uses a natural process to try to protect forest birds without harming the environment, humans, or other animals.

“That’s the pressure we’re feeling with this. Any delays are distressing. We are not jumping in too fast—we have done the requisite small-scale studies, and now the rubber is meeting the road. We are doing it, and then we’re learning how we’re going to do it better as we do it,” said Chris Warren, the Forest Bird Program Coordinator for Haleakalā National Park. “The only thing that’s more tragic is if [the birds] went extinct and we didn’t try. You can’t not try.”

In November 2023, Haleakalā National Park and its partners began releasing batches of Wolbachia-incompatible male mosquitoes into the wild. The group plans to make these deliveries twice a week, using drones or helicopters to drop capsules that resemble large, biodegradable Keurig pods filled with thousands of male mosquitoes.

Reflecting on this project in early October, Chris Warren stressed the importance of cooperation during the adaptation process.

“One of the best things about this whole project is how collaborative it’s been," he said. In meeting with each other, HALE and its partners aimed to, as much as possible, work on this project irrespective of boundaries. The NPS empowered their collaborators to conduct monitoring and mosquito releases within Haleakalā, and the same groups are extending their efforts outside of park boundaries—and outside of Maui—to rescue Hawaiian honeycreepers from extinction. “The birds don’t care where the boundaries are.”

Thank you to Chris Warren and Paul Hosten at Haleakalā National Park for providing information for this article.

FAQ

In a nutshell, why are the birds declining?

Avian malaria, carried by invasive southern house mosquitoes, is the primary cause of decline. Both the disease and the vector are non-native and many native birds — most prominently the honeycreepers — have lost whatever resistance to mosquito-borne diseases their mainland ancestors may have had. High-elevation areas that were previously too cold for mosquitoes and malaria were once refuges for some forest birds—but as the climate has warmed, mosquitoes have spread undeterred across high-elevation Maui. Other factors, such as habitat loss and predation by non-native species, have also contributed to the longer-term decline of forest birds. 

 

How does the Incompatible insect technique (IIT) on Maui work?

Roughly 60% of the world’s insects carry a naturally occurring intracellular (within cells) bacterium called Wolbachia. It is generally harmless to insects, and different strains have evolved in different mosquito populations. Mosquitoes can produce offspring with one another if they either have the same or compatible Wolbachia strains or if they both do not have Wolbachia. If a male and female with different, incompatible strains of the bacterium try to mate, the eggs will not hatch. The incompatible insect technique (IIT) works by rearing southern house mosquitoes in a lab environment with a strain of Wolbachia that is incompatible with the one found in Maui’s mosquitoes. The male and female mosquitoes are separated, and only the males (which never bite) are released into the wild to mate with wild female mosquitoes. Because of the incompatibility of the different Wolbachia strains, the eggs of these wild females will not hatch, safely and effectively reducing the mosquito population. 

How will it affect Hawaiʻi if there are fewer mosquitoes?

Mosquitoes are a non-native species in Hawaiʻi. They arrived roughly 200 years ago and have not replaced any key pollinators or food sources in Hawaiian ecosystems since then. It would only benefit Hawaiʻi to have fewer mosquitoes.

How does releasing more mosquitoes affect people?

The areas where IIT is currently being conducted are largely too far away from areas where people live or recreate for people to experience the benefits of reduced populations of the southern house mosquito (which does bite people). However, because male mosquitoes do not bite, people would not be directly affected by released mosquitoes, should they encounter them. In fact, IIT has been conducted in urban areas to keep humans healthy for years; the difference, in this case, is simply that the technique is being used to save forest birds. 

Are there any downsides to using Wolbachia?

Wolbachia has coexisted with organisms in the natural environment for millions of years and is only able to survive within insect cells, meaning that it cannot exist freely outside of its host. Despite continuous, natural exposure to Wolbachia through contact with insects, there are no reports that humans or animals have been harmed by Wolbachia, and comprehensive laboratory and field-based research has confirmed that mosquitoes carrying the bacterium are safe for humans, animals, and the environment. 

Is IIT genetic engineering?

No. There is no change in the genome of mosquitoes or Wolbachia. The technique is more like mosquito birth control because the female mosquitoes simply lay eggs that do not hatch. 

Has this been done before?

Yes, similar mosquito control methods have undergone decades of testing in over a dozen countries, and Wolbachia is used worldwide to protect humans from mosquitoes and mosquito-borne diseases such as dengue, Zika, and chikungunya. This is the first time that IIT has used launched specifically as a conservation tool.