Six hundred little cardboard traps, baited with sticky female gypsy moth pheromones and hung in trees along the Wasatch Front, caught 2,200 male gypsy moths in 1989. Last year, more than 5,000 traps hung in six counties caught only about 500 male moths.

That's good, say representatives of the U.S. Department of Agriculture and the U.S. Forest Service. Fewer moths in traps mean fewer moths in general. The biological spraying campaigns mounted over the past two years are killing the destructive insects.But nearly 300 of the moths caught last year were found in traps outside the sprayed areas. So the two federal agencies, in cooperation with the Bureau of Land Management and state forestry and fire agencies, want to enlarge the spraying program to include 30,000 acres in four counties in 1991.

Forest Service and agriculture department officials have held hearings in Salt Lake, Davis and Provo counties to allow the public to comment on the spraying plan.

In Bountiful Thursday night, just one resident attended the hearing held at City Hall as part of a required environmental assessment process.

Forest Service representative John Anhold said this year the agencies will again use bacillus thuringiensis, also known as B.t., to kill the gypsy moths by attacking their digestive systems.

The insecticide is a bacterium that occurs naturally in the soil, and is actually a primitive form of plant that forms a substance toxic only to moths and butterflies, according to forest service and agriculture department officials.

The agencies will use helicopters to drop the insecticide on the target areas at the rate of 1/2 gallon per acre. The sprayings will begin as early in the spring as the weather permits, and proceed into the summer until all the target areas have received three insecticide applications.

The target areas include Mueller Park in Davis County; Burr Fork in Emigration Canyon; upper Olympus Cove and Mill Creek; the area between Big and Little Cottonwood canyons; the suburban Salt Lake foothills; and areas in lower Provo Canyon and northwestern Summit County.

Officials said they will notify the public as far in advance as possible before spraying commences, but said the advance notice could be limited because the operation is weather-dependent.

The gypsy moth, which has few natural enemies, has been defoliating trees and shrubs in the eastern United States since 1869, when moths imported from Europe escaped after a Massachusetts naturalist failed in his attempt to cross the moths with silkworms.

The first gypsy moth eradication program began in 1889 in Massachusetts. But over the past century, the eradication programs have mostly failed. Efforts to control the gypsy moth in the East now concentrate on a very few areas, such as parks, where caterpillar infestations are particularly bothersome.

A female gypsy moth lays as many as 1,000 eggs at a time. The moths have voracious appetites and feed on 200 species of shrubs and trees, including evergreens but excluding ash, tulip poplar, locust and sycamore trees.

Six western states - California, Oregon, Washington, Idaho, Colorado and Utah - have reported spot infestations of the moths.