Mosquitoes to the Rescue! The Last-Ditch Effort to Save Kaua‘i’s Endangered Birds

A modern twist on controversial biocontrol methods aims to make disease-carrying mosquitoes in Hawaii turn on themselves.
A grid displays photos of eight birds—all of the remaining Kaua'i forest birds. They come in shades of yellow, red, and brown.

Kauaʻi’s forest birds aren’t exactly easy to know. Eight remaining species live in woodlands in the middle of the Pacific Ocean, many only on this island, where threats encroach from all sides. Lisa “Cali” Crampton is one of the rare people who has become acquainted with the elusive avians after spending more than a decade hiking through unforgiving terrain to study them.

Ask which is her favorite, and she names ‘Anianiau, a bright yellow honeycreeper that weighs less than four pennies. But all the forest birds have quirks. “They’re entirely charming and fascinating birds,” she says. ‘Apapane and ‘Iʻiwi are crimson red, nectar-drinking honeycreepers, the former distinguishable by its “white underpants,” she says, and the latter by a striking curved bill. The endangered ‘Akeke’e pries open leaf and flower buds with a unique cross-tipped beak. And acrobatic ‘Akikiki are just as likely to perch upside-down on a branch as right-side up. “You can’t underestimate their spiritual and cultural value,” Crampton says. “These birds have always had Hawaiian names.”

Treasured as the birds are, several species are also in serious trouble. Three live only on Kauaʻi and are critically endangered, including ‘Akeke’e and ‘Akikiki, with estimated populations of less than 1,000 and 500 individuals, respectively. Four other species were deemed extinct by the U.S. Fish and Wildlife Service last month. The same forces that spurred their declines, including deforestation, invasive predators, and introduced mosquitoes carrying disease, have already driven many of Kauaʻi’s forest birds to extinction in the past 50 years. While the forests once held at least 13 different species, today only eight remain, and several are critically endangered. 

Elsewhere, birds evolved alongside native mosquitoes and developed some immunity to avian pox and avian malaria. But until humans inadvertently introduced the insects to the islands in the 19th century, Hawaiian birds lived in a mosquito-free environment. They have no innate defenses against the pests. Unable to fend off disease, the birds retreated to mountainside areas too cold for the mosquitoes’ survival. Now even that haven is threatened: Climate change has brought warmer temperatures, allowing mosquitoes to expand upslope into habitats once cool enough to keep the birds safe. Crampton, leader of the Kauaʻi Forest Bird Recovery Project, which is a member of the Birds, Not Mosquitoes partnership, is focused on combatting mosquitoes, and she’s employing a powerful weapon to rid Kauaʻi of these pests: bacterial birth control, an approach that manipulates the mosquitoes’ gut bacteria to cause their population to crash.

A tropical forest landscape, with lush ferns, trees, and other plants, extends back to the horizon with its blue sky. It looks endless, but is actually one of the last wildernesses on Kaua'i.

Of the six invasive mosquitoes on Hawaii, Crampton is most interested in Culex quinquefasciatus, a nocturnal bug known as the southern house mosquito that’s been here since 1826 and acts as a vector for avian malaria and avian pox. Both diseases weaken and kill endangered ‘Akikiki and ʻAkeke’e. The first step to squashing their populations, somewhat ironically, is producing more of the blood-suckers.

“They stink, I’m warning you,” Crampton says as she places a black mosquito trap on her office desk in Hanapepe, Kauaʻi. She flips a switch and a battery-powered fan blows air laced with carbon dioxide and a dank smell approximating a locker room. When she sets the trap in the field, female mosquitoes seeking a blood meal approach and it sucks them inside. Crampton then transports her quarry to her lab, where she sends them to be bred at labs on the mainland. Then, a strain of bacteria will be introduced to the male mosquitoes that effectively sterilizes them: If those males mate with wild females, the resulting eggs never hatch.

If scientists can release enough lab-altered males to overwhelm the wild ones, they could dramatically reduce the bugs’ population—and protect birds from mosquito-borne diseases.

A bright circle, produced by a microscope lens, illuminates a mosquito and other insects in close-up. The mosquito has big back eyes, long legs, and a proboscis extending from its head that's used to pierce skin and find blood vessels.

It’s a time-intensive measure, and it falls under the umbrella of “biocontrol”—a controversial practice wherein new species are introduced to an ecosystem in hopes that they can control established invasives. Crampton and others know how spectacularly such measures have failed in the past; thus they’re approaching their project with extreme caution. Yet they feel like such a dramatic strike might be the only option to give the birds a fighting chance.

Intentionally and inintentionally, humans have ferried non-native species around the world for as long as we've traveled. But in the 1800s, the experimental practice of biocontrol set new standards for importing invasive species, often with disastrous results. In 1883, owners of sugar cane plantations in Hawaii introduced mongoose to the islands of Maui, Molokaʻi, and Oʻahu to control rats. They failed to realize that not only are diurnal mongoose asleep while nocturnal rats are active, but also that mongoose eat insects, birds, eggs, and plants—notably, not rats. Today, mongoose continue to prey on Hawaii’s endangered birds and cause millions of dollars in damages annually. Fortunately they’re not present on Kauaʻi.

Scientists and wildlife workers today have absorbed this and other disastrous lessons. They still experiment with biocontrol, but typically undertake stringent testing before releasing any organisms to the wild, especially predators that could have ecosystem-wide impacts.

Take the hemlock woolly adelgid. The sap-drinking insect, native to Asia, has no natural predators in the Eastern United States, giving it free range to devastate hemlock forests and drive declines in avian inhabitants like Acadian Flycatcher and Hermit Thrush. To restore hemlocks, government scientists tested whether releasing two insect-eating beetle species from British Columbia would cause any unintended consequences. After they proved that both beetles preyed on hemlock woolly adelgids without causing problems for native bugs, they let loose thousands of them, starting with Japanese ladybeetles in 1995 and then Laricobius nigrinus in 2003. But so far, the invaders’ populations haven’t become established enough to crush the adelgids and save the trees.

University of Minnesota entomologist George Heimpel is taking a more targeted approach to exterminate an invasive fly that poses one of the greatest threats to Galápagos birds. Philornis downsi, which arrived to the islands in the 1960s, parasitizes at least 18 endemic and native bird species. Fly larvae creep into the nostrils of defenseless baby birds, including the native Galápagos Finch, and chew away at their nasal lining, killing chicks or leaving them with deformed beaks. Heimpel’s team has identified two wasp species that parasitize Philornis downsi exclusively, which they predict will protect the Galápagos’ native insects from potential side effects. “If we do our work right, it won’t be killing other organisms,” says Hempel, who has not yet released the wasps.

A cylindrical device about the size of a toolbox, attached to a carbon dioxide canister, sits in the tropical forest beneath a tarp.

Crampton’s approach to controlling Kauaʻi’s mosquitoes goes fully microscopic. Instead of targeting Culex quinquefasciatus itself, she is working with a team to modify bacteria in the insects’ reproductive tissues. These mosquitoes, and as many as 65 percent of all terrestrial insect species, carry strains of a bacteria called Wolbachia. By swapping out the specific strain of Wolbachia in lab-raised Culex males with one that is different from that carried by wild females, any females that couple with the altered males will produce infertile eggs. Over time, this should reduce the overall mosquito population on Kauaʻi, and, if it works, also the rate of avian disease. The end goal is to reduce the mosquito population enough that the endangered birds will be safe from disease and make a comeback.

This Wolbachia method is already in use in the United States, for example in Miami, to bring down Aedes aegypti mosquito populations that transmit disease to humans. Because Aedes mosquitoes don’t carry Wolbachia, scientists expose the bugs to the bacteria; the male-female mismatch results in infertile eggs. In 2018 scientists released more than 6.8 million Wolbachia-infected males over a six-month period across a 150-acre area of the city, hoping that reduced mosquito populations would lead to fewer cases of Zika. A follow-up study showed a 75 percent reduction in area Aedes aegypti mosquitoes because eggs failed to hatch.

If Crampton released altered mosquitoes in Kauaʻi, it would be the first time the Wolbachia technique is used for avian conservation. Scientists are optimistic it could work without causing unintentional harm to other species.

“It’s not the sort of traditional biocontrol where you release a specific organism to target another organism,” says Eben Paxton, a U.S. Geological Survey ecologist in Hawaii who is currently working on an avian malaria genome project. “It seems to be the least risky in terms of anything unintended going wrong.”

It will be years before any mosquitoes are released. There are still regulatory hurdles to clear, including environmental assessments and public hearings. “If everything went smoothly, maybe we could do field trials in one to two years and then actual full-on landscape release in three to four years,” says Teya Penniman, who coordinates the Hawaiʻi Landscape-scale Mosquito Project at the non-profit American Bird Conservancy. They don’t yet know how many mosquitoes they’d have to release to overwhelm the wild population with Wolbachia birth control.

A woman in a white T-shirt holds a small yellow bird. She's smiling while looking at it.

Crampton is not relying solely on this biocontrol strategy to save the birds. This year the team began dosing forest streams with bacteria called BTI (Bacillus thuringiensis israelensis), which kill mosquito larvae; outside of also killing a few midge species, BTI has no other known ecological impacts. It’s a short-term fix at best, since it only targets mosquitoes in small, localized areas. The Wolbachia approach, on the other hand, would sterilize mosquito eggs more widely in critical forest bird habitats. But Crampton says that they can’t sit by until the biocontrol route is approved; the most endangered birds might not have a couple years left.

“We’re doing emergency meetings this month with all the partners across the state that can influence forest-bird conservation to determine if we need to take more aggressive measures,” Crampton says. They could launch captive-rearing programs for ‘Akikiki and ʻAkekeʻe, or relocate some birds to the island of Hawaiʻi where mosquitoes are less of a problem.

“It has been rough watching these birds decline, but we cannot give up,” she says. “Not taking action is a choice, too.”

Before I leave Kauaʻi, Crampton directs me to a trail that will take me into the Alakai Swamp so I can see some of her beloved birds myself. The sun is out when I arrive at one of the wettest places on the planet, which has an average annual rainfall of 200 inches. The rich, red earth forming the path includes inclines so steep, I have to crawl with my hands. Years of heavy rainfall have produced deep grooves around the clay. After two miles of hike-scrambling, the dirt transitions into a boardwalk—and that’s when I hear birdsong. Feathered brown heads peek out from the trees—they’re Kauaʻi Elepaio, the most numerous of the island’s songbirds.

None of the three endangered species show themselves to me, but as I’m leaving the swamp a red dart shoots by. Maybe it’s an ‘Apapane or ʻIʻiwi, two dazzling honeycreeper species, both declining in number. Seeing it, I understand better why Crampton has dedicated herself to a battle that may only ever be a stalemate. Even if she manages to rid Kaua’i of mosquitoes, she’ll still have to contend with rats and habitat loss and invasive plants and climate change. But if she can reduce this one threat, it may create enough of an opening for the forest birds to make a comeback.

Correction, November 10, 2021: An earlier version of this story incorrectly described minor details of the Wolbachia introduction process. Audubon regrets the error.