RICHMOND, Va. - Agriculture officials are hoping to stop the eastward spread of the emerald ash borer beetle, an invasive, hard-to-control insect that has killed more than 20 million trees in the Midwest and Canada and is heading toward Virginia.

"It's not here yet but it can be transported easily in firewood, so we're asking people to buy firewood at your destination," said Elaine Lidholm, a spokeswoman for the Virginia Department of Agriculture and Consumer Services. "Don't take it with you."

The beetle is indigenous to eastern Russia, northern China, Japan and Korea and was initially found in southeastern Michigan in 2002, likely arriving in ash wood used in packing material. Because the insect has no native predators in North America, the infestation has continued to spread through parts of Michigan, Illinois, Indiana, Ohio and Maryland - areas now under a federal quarantine.

Fewer than 300 damaged nursery trees were found in Virginia in 2003, and no further infestation has been found after they were destroyed, according to Debbie Martin, the invasive species coordinator for the state agriculture department's office of plant and pest services.

But state and federal agriculture officials want to continue to inform people about the insects, even designating May 20-26 as national Emerald Ash Borer Awareness Week.

In the Shenandoah National Park, where ash trees make up about 65 percent of the acreage, officials occasionally are checking with visitors to see if they've brought firewood in from quarantined areas. If so, park employees recommend that visitors immediately burn the wood or put it in sealed plastic bags to prevent the insects' spread.

The emerald ash borer is a metallic-green beetle between 3/8-inch to 5/8-inch long. Its larvae feed on the layer of wood just beneath the bark of the ash tree, cutting off water and nutrients. Infested trees have D-shaped holes in the bark, thinned canopies and often sprouts growing from the trunks and roots. Once infested, the trees die within two to four years, according to researchers.

The economic impact "would be devastating" if the infestation spread from currently affected regions into the northeastern forests. About $25.1 billion worth of ash timber is grown in the eastern United States, the U.S. Department of Agriculture said in a report.

The emerald ash borer could cost a potential urban loss of $20 billion to $60 billion nationally, according to the USDA report. If not contained and eradicated, the report said, the infestation could result in about $7 million in additional costs to state and local governments and landowners to replace dead and dying ash trees in urban and suburban areas over the next 25 years.

In Virginia, the infested trees were purchased originally by a Maryland nursery in spring 2003 from a Michigan nursery. Some of these trees were purchased by a landscaper who planted them in Vienna, Va., that July.

A nursery inspector discovered the infested trees in August 2003 at the Maryland nursery, and the Vienna trees were subsequently removed and destroyed, Martin said. Further eradication was done in early 2004 in the area where the original trees had been planted.