Maryland Boy Scouts kicked off their annual spring exposition at the Gaithersburg Fairgrounds Saturday afternoon by launching Operation Gypsy Moth, a service project aimed at limiting the spread of the insect pests in the state.

An estimated 13,000 people, including scouts, their families and friends, attended the day-long exposition, which included games, climbing activities, and demonstrations of cooking and camp craft. Everyone seemed to have a good time, despite the overcast sky and intermittent drizzle.

The National Capital Area Council of the Boy Scouts of America, which includes 65,000 scouts in part of Maryland, Virginia and the District of Columbia, has begun a major drive to fight the spread of gypsy moths. With the help of the U.S. Department of Agriculture, Boy Scouts, Cub Scouts and Explorers have planned conservation projects to educate the public and encourage homeowners to purchase and use gypsy moth traps.

Boy Scout official James S. Culp kicked off Operation Gypsy Moth by presenting Montgomery County Executive Charles W. Gilchrist with a moth trap to place in the yard of his home in Rockville. Gilchrist thanked the scouts on behalf of the entire community for putting together the gypsy moth program, saying, "Nowhere in the U.S. has an organization undertaken a mass education project to deal with such a specific problem."

At the expo, Scouts sold gypsy moth traps scented with sex lures to attract male moths and interfere with breeding. The traps are a new scientific development, according to Don G. Fornoff, director of development for the National Capital Area Council, who helped organize the expo. The scouts are selling the traps for $5, with proceeds from the sales to be used for scouting activities.

U.S. Forest Service official David A. Graham said the scouts are working in a coordinated basis to serve the Maryland metropolitan area.

The Maryland Department of Agriculture predicts that the gypsy moths will affect 60,000 to 80,000 acres of land in the state this year. About 9,000 acres are said to have been affected in 1981. The moths devastated 13.1 million acres of forest in the Northeast last year and are most destructive when their caterpillars hatch in April or May. The state has spent more than $370,000 to spray 37,000 acres of forest in 1982, and $13 million has been spent nationwide this year on aerial spraying in attempts to reduce the gypsy moth population, Graham said.

Explorer Post 14 of Potomac researched the gypsy moth problem for a large information display at the expo and has been asked to show parts of its display to other scouting groups. Post 14 Explorer Kristi Hagen said her coed scout unit has asked local radio and television stations to air public service announcements because "people don't know how big the gypsy moth problem is."

Mary Wilcox, public relations director for the National Capital Area Boy Scouts Council, said Giant Food stores are aiding the scouts' effort by putting the Operation Gypsy Moth logo on milk cartons.

Graham said that after three years of gypsy moth infestation, 25 percent of the trees in an area might be destroyed. To remove each infected tree costs between $400 and $1,000, he added. Graham said the number of moths attracted to the traps can indicate the extent of local infestation and help determine whether spraying is necessary. The traps can be used to supplement spraying in heavily infested areas.

John Kimbrough, 13, of Bethesda, said his Troop 240 plans to assemble traps and tie burlap around trees to catch gypsy moth caterpillars for their neighbors.

But the expo wasn't all business. There were tire bridges to climb, go-carts to race and various tests of scouting skills spread among the tents, barns and booths at the fairgrounds.

It was also a great place to learn about camping and survival techniques. Lessons included how to cook outdoors using orange peels, green peppers and other foods in place of utensils, the proper way to set up a campsite safely and details such as using tree-bark place cards to brighten up a camp table.

Members of the coveted Order of the Arrow, one of scouting's highest honors, provided a display of Indian dancing.

David Liebow, 16, of Potomac, the senior patrol leader of Troop 1449, said he joined the Boy Scouts 10 years ago because of the challenge. Beside making new friends and learning new things, the best thing about scouting, he said, is "you have to earn everything. Nothing is handed to you."