INSECTS : Northwest Set to Take to Air in War on Moth : Some fear spraying will affect more people than Asian caterpillars.

SEATTLE — Beginning Thursday, weather permitting, low-flying helicopters are scheduled to begin spraying a specialized, caterpillar-killing bacteria over Tacoma, Wash., and parts of Portland, Ore., in an aerial war against a West Coast infestation of the dreaded Asian gypsy moth. Spraying is to begin in the Vancouver area of British Columbia on Wednesday. The issues at a glance:

WHAT THE MOTH IS LIKE: Voracious, prolific and fast-spreading, the foliage-devouring Asian moths are exotic ugly cousins of the European gypsy moths that annually strip leaves off 4 million acres of hardwood trees in the Northeast. Two traits in particular make the Asian moth especially feared: In the European strain, the fertile female is flightless. Her Asian sister, however, does fly, perhaps long distances, and lays more eggs. Females lay egg masses in the autumn. The emerging caterpillars, which are the destructive stage of the insect, begin appearing in April. They are black and mottled and about two inches long, quite hairy, and distinguished by a row of red and blue bumps on the back.

HOW THEY GOT HERE: Asian moths were first noticed in the Western Hemisphere last fall. They were believed to have arrived at the West Coast ports aboard freighters from Russia, where lights used for round-the-clock unloading of grain are believed to attract the moths onto the ships. Researchers have found that the moths have already traveled 20 miles from dockside.

FEARED IMPACT: The government forecasts that uncontrolled spread of the Asian moth over the next 40 years could lead to:

* Timber losses from 15% to 25%--or $35 billion in the best case to $58 billion in the worst.

* Up to $821 million to suppress the insect once it is established.

* Profound effects on wildlife habitat, including the loss of valley oak forests in California and their conversion into grasslands. Loss of California valley oaks would hit deer, woodpeckers and squirrels particularly hard.

* Tourism and recreation losses of up to $2 billion due to public land spoilage and rehabilitation requirements.

* Destruction of watershed foliage, resulting in loss of stream life.

Most immediately threatened are the conifer forests in the Northwest, which are economically and environmentally critical to the region.

QUOTE: “It’s potentially the worst insect threat we’ve seen in the U.S.”

--Washington state Agriculture Director Alan Pettibone.

WAR ON THE MOTH: State and federal officials pronounced the Northwest in a state of emergency and announced a $19-million eradication plan. Bacteria known as BT--Bacillus thuringiensis--is to be sprayed in a solution with water at three intervals until June over a 130,000-acre area of western Puget Sound, including the cities of Tacoma, Kent and Auburn. Other spraying is planned for smaller areas around Portland and Vancouver, B.C.

BT produces a toxin that interferes with food digestion during the moth’s caterpillar stage. The bacteria has been approved for application to food products, is used by organic gardeners (because it is naturally occurring), and is said to be harmless to mammals. But Washington state officials say people should stay indoors during spraying to avoid skin and eye irritation. People susceptible to infections, such as those with leukemia, AIDS or other immune deficiencies, should stay indoors for 30 minutes after spraying.

QUOTE: “BT has been studied extensively and has a strong safety record for people, pets and wildlife.”

--Washington state Health Secretary Kristine M. Gebbie.

OPPOSITION: The declaration of emergency has short-circuited public input about the spraying. In Oregon and British Columbia, activist groups are challenging the spraying plans in court. Ground spraying is the preferred alternative, says the group Bug Off in Portland. The Washington Toxics Coalition calls it a “total overreaction” to spray a population of more than 378,000 people after finding only 10 moths.

An Enemy Invasion

The government forecasts that uncontrolled spread of the Asian moth during the next 40 years could lead to major timber losses and destruction of wildlife habitat:

A Voracious Eater

* Caterpillar feeds ravenously on green foliage.

* Oak is one of the insect’s favorite foods.

* Officials fear the moth may develop an appetite for the conifers of the Northwest.