Mosquitoes on Maui: Invaders Driving Native Forest Birds Toward Extinction

A Maui forest bird. NPS / Jill Peters.

More than a nuisance

On Maui, the buzz and bite of invasive mosquitoes is more than an annoyance: the insects are doing catastrophic damage to the island's ecosystems. The State of Hawaiʻi has eight invasive mosquito species — six bite people and two feed only on plants. Two biters are most common on Maui.

The Asian tiger mosquito. CDC.

The southern house mosquito. CDC.

How they got here

"Hawaiʻi's isolation meant that the archipelago was among the few places on the planet without mosquitoes," says Chris Warren, forest bird biologist at Haleakalā National Park. A whaling ship brought them in 1826, and in the warm climate they spread across the islands — "just a blink in the eye of evolutionary history." Since then, climate change has warmed Hawaiʻi, letting mosquitoes move into high-elevation forests once too cool for them, and carry disease to vulnerable birds island-wide.

How they multiply

A female stores a male's sperm for life, fertilizes her eggs when she's ready, and lays them in or near water. The life cycle has four stages — egg, larva, pupa, adult — and takes two to four weeks, or as little as one week in heat. Faster cycles mean bigger populations.

Southern house mosquito pupae in standing water. CDC.

The damage

Where they're native, mosquitoes can feed other animals or pollinate plants. In Hawaiʻi they do no ecological good. The southern house mosquito, the first species to arrive, does the most harm: it spreads avian malaria, a deadly blood parasite that is the main cause of the decline of Maui's forest birds, which have little or no defense against it. Four species are predicted to go extinct within two to ten years, with others close behind.

"Many of our native birds evolved in isolation for millions of years and now their very existence is threatened by mosquitoes. These birds are already proven survivors — weathering a hurricane of human-caused threats (habitat loss, mammalian predators), but they are at the end of their rope with mosquitoes and avian malaria." — Chris Warren

Fighting back

All Hawaiian mosquitoes are invasive. In the past, new species reached Hawaiʻi about once every 10,000 years, Warren notes, giving evolution time to absorb them; now they arrive about once a month — faster than native ecosystems can adapt. "Even if we cannot stop all the new invaders," he says, "we can address the impacts of the most destructive species that are already here."

So the National Park Service, with the State of Hawaiʻi and other agencies, is working to suppress Maui's mosquitoes.

• The old ways don't fit: broad pesticides must be sprayed over huge areas and hit other species, and draining breeding habitat destroys habitat for others too, including native invertebrates.
• The new way: the Incompatible Insect Technique (IIT) keeps mosquitoes from ever biting or spreading disease by making females lay eggs that never hatch. It uses Wolbachia, a natural bacterium living in insect cells, including Maui's southern house mosquitoes. Mosquitoes with different Wolbachia strains can't reproduce together, so scientists raise male mosquitoes — which don't bite — with one strain and release them where wild mosquitoes carry another. Wild females that mate with them lay eggs that don't hatch.

Without action, mosquitoes will keep upsetting the island's balance and, within the next decade, drive several of Maui's remaining native forest birds extinct. More on Maui's honeycreepers and how to save them.

Sources

Based on "Mosquitoes on Maui," Haleakalā National Park, National Park Service; a work of the United States government in the public domain.