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May 18, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Mosquito and pig: cautionary tale of two alien species

Mosquitoes are not native to the Hawaiian Islands. Anyone driven away from an outdoor activity or rudely awakened by a biting mosquito would agree that the islands of old were indeed a paradise. That bit of paradise was lost in the early 1800s, when the southern house mosquito arrived, allegedly aboard the sailing ship Wellington. This mosquito has evolved in long and close association with humans. It utilizes any standing water but is particularly adapted to sewage-polluted water associated with people and livestock. By 1900, this mosquito was well established throughout the islands, wherever human activities created the stagnant water habitat necessary for its immature stages.

Most people realize that some mosquitoes transmit serious human disease, but few are aware of the many diseases mosquitoes transmit to domestic and wild animals. Fortunately, no known human diseases transmitted by this mosquito occur in Hawai`i. Unfortunately, two mosquito-transmitted bird diseases have become established.

The date of entry for avian pox and malaria remains a mystery, but the devastating impact of the two diseases on lowland populations of native birds has long been recognized. By 1900, some of the earliest naturalists had observed pox lesions on increasingly rare bird species. Avian malaria and pox, are no longer confined to lowlands, however; both diseases are now prevalent in mid-elevation forests.

How could the transmitting mosquito, so dependent on stagnant water around human settlements, become established in the large remaining tracts of Hawaiian forest? This question was particularly baffling on the windward slope of Mauna Loa, where the porous nature of young volcanic soil prevents most standing water. Enter alien species number two, the feral pig, unwittingly paving the way for mosquitoes into the forest.

European domestic pigs were among the earliest introduced species to arrive in Hawai`i. Without predators or herbivore competitors, these animals adapted well to life in the wet forest and rapidly established large feral populations. The starchy core of native tree ferns is among the pigs' favorite foods. Foraging pigs greedily consume this starch, leaving behind cavities that quickly collect rainwater and fallen leaves. As the tree fern starch and leaves decompose, the water becomes rich with bacteria upon which larval mosquitoes feed. Chemicals from decomposing plant material are attractive to female mosquitoes, ensuring that eggs are laid in a suitable habitat. So, the activity of one alien species provided the habitat needed by another. The mosquito figuratively piggy-backed its way into the forest. Many tree fern cavities are small, containing little more than a cup of water; some are larger and hold 3-4 liters (quarts). Even a small cavity can support development of hundreds of mosquito larvae. Consequently a typical acre of wet forest may produce thousands of mosquitoes that can spread the two bird diseases and have a profound effect on bird populations.

Every year between September and December, the Kilauea Field Station receives dead or dying birds from the Volcano area. Many are infected with avian malaria. Each year fewer `i`iwi, the magnificent red honeycreeper, are seen in the mid-elevation forests, and vast tracts of wet forest are now largely devoid of bird life.

Can we control the diseases that threaten our native forest birds? In the few successful campaigns against mosquito-transmitted human disease (malaria on the mainland and in Europe, dengue in Hawai`i), elimination of the larval mosquito habitat was the winning factor. In the windward forests of Mauna Loa, reduction of feral pig numbers would reduce habitat for mosquitoes, ultimately disrupt disease transmission, and help enable forest-bird populations to rebound.

Mosquitoes would still be produced in agricultural and residential areas near forests, so we should practice mosquito control there as well. Everyone can do their part by eliminating standing water on their property. Cleaning a gutter or turning over a pail in the garden may one day be rewarded by a glimpse of an `i`iwi in your own backyard.

Eruption Update

Eruptive activity of Kilauea Volcano continued during the past week. Lava erupts from Pu`u `O`o and flows through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. A large skylight in the main tube is visible at night on Pulama pali. Surface flows are intermittently active on the coastal flat between Royal Gardens subdivision and the coast. Lava is entering the ocean at three sites near Waha`ula. Remember that ocean-entry areas are extremely hazardous; explosions occur unpredictably or accompany sudden collapses of new land into the water. The active lava flows are hot and have very thin crust. The steam clouds at the entries are highly acidic and laced with glass particles. Use common sense around hot lava and steam.

No earthquakes were reported felt during the week ending on May 18.

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Updated: 18 May 2000