Jan Smith, a 28-year-old intelligence officer, thinks the problem could be gypsy moths, and she has stopped by the "Neighborhood Plant Clinic" table at a local library to find out.

Smith wears a Mickey Mouse T-shirt and sandals. She lives in a Falls Church town house development. "The moths got in my tree, and I only have one tree," she says. "Just that one."

It happened last summer -- big, gauzy nests. "You could see the caterpillars crawling around inside. It was awful. It was just so awful." Smith does not want it to happen this year.

The library she has come to is Fairfax County's Tysons Pimmit Regional, on Rte. 7. Staffing the "Neighborhood Plant Clinic" table are two volunteers: Lenna Vasur, 44, a landscape architect, and Joe McKinney, 62, a retired Air Force pilot.

Between them, they have a Swiss Army knife (for scraping and probing plant samples), two magnifying glasses (for examining plant spores and "counting legs") and reference books, including: "Identifying The Diseases of Vegetables," "Woody Ornamentals," and the fifth edition of "Diseases & Pests of Ornamental Plants."

McKinney also has brought several live specimens -- a jar of asparagus beetles captured in his McLean garden, and a fresh-picked asparagus stalk covered with beetle eggs.

This is the 10th year the Fairfax County branch of the Virginia Cooperative Extension Service, in cooperation with other groups, has offered free neighborhood plant clinics.

There are 16 clinics scattered throughout the county, and they are staffed by 150 volunteers. It is the first year a clinic has been offered at Tysons Pimmit.

Before Smith came along with her gypsy moth question, McKinney and Vasur answered several others, including one from a woman who stopped to ask about an ailing birch.

"It has three little dead leaves at the bottom," she said. "Should I just be patient with it, or what?

"I'm praying over it," she added. "I'm praying, and my husband keeps saying, 'It's dead, it's dead, it's dead.' "

Her problem, McKinney and Vasur determined, was improper watering. One cannot just drown a tree. Water must be applied correctly, weekly.

Collectively, their knowledge is strong. They know about leaf hoppers and European corn borers, azalea petal blight and cabbage loopers.

McKinney has taken horticulture courses at the Northern Virginia Community College. At home, he has a vegetable garden -- "1,000 square feet planted in one thing or another."

For Vasur, working at the clinic is a way of "getting back to plants" after taking time out to raise three children. Like McKinney, she also has had extension service training.

Last summer, more than 4,000 people showed up at Fairfax's neighborhood clinics with Baggies of garden soil, dead leaves wrapped in tinfoil, envelopes of grass clippings.

"A lot of diseases are very tough to identify," McKinney said.

"Viruses," Vasur interjected.

"Oh, yes -- viruses are very, VERY hard," McKinney agreed.

Problems that can't immediately be diagnosed are sent to the main extension service laboratory, where people like Mary Bean can look at a spray of white-blotched hemlock and say: "That's woolly aphid."

Questions come and go with gardening cycles. Right now, people are interested in pine sawflies and clover mites. "Blossom end rot -- we'll be getting a lot of that soon," Bean said.

Regardless of the season, everyone is interested in gypsy moths, including Jan Smith. She presents the symptoms -- ugly caterpillars, big tents spun of white, chewed leaves. "Ugh," she says. "Awful."

McKinney peers at her through black-rimmed glasses. He reaches under a pile of books for "Insects That Feed On Trees and Shrubs," and flips immediately to Plate 62.

"The Eastern Tent Caterpillar," he says. "Yes, I think you're seeing the Eastern Tent Caterpillar."