A very tiny invader has potentially usurped another destructive pest four times its size for the latest tree-destroying bug of which one should beware.

It’s the emerald ash borer, which is a slender 0.33 to 0.55 inches long — but a creature with a mighty devastating and long interstate rap sheet. It’s responsible for the death and decline of tens of millions of ash trees, according to the Connecticut Agricultural Experiment Station. This invasive insect, which has been found in 20 states — as far west as Kansas and as far north as New Hampshire — was detected for the first time in Fairfield County last month in the county’s northernmost town — Sherman.

A single emerald ash borer was outed by one of its enemies in the wild — a native wasp called Ceceris fumipennis, which scientists say hunts beetles in the ash borer’s Buprestride family.

The identification of ash borer was confirmed by the federal regulatory officials in the U.S.D.A. Animal and Plant Health Inspection Service, Plant Protection and Quarantine. The Fairfield County detection, like that of the first New Haven County detection last summer, was made via a program the Connecticut Agricultural Experiment Station calls “Cerceris wasp biosurveillance.”

Call it a case of bug on bug crime mixed with espionage orchestrated by humans. The experiment station called this wasp “an efficient and effective ‘biosurveillance’ survey tool” which “does not sting people or pets.” The station reported that more than 300 detection traps have been set across the state by the University of Connecticut Cooperative Extension System as part of the surveillance. Three of them have been in Wilton for at least three years, according to Director of Environmental Affairs Pat Sesto. “They are purple and look like small box kites up in a tree.”

The emerald ash borer, or Agrilus planipennis, has now been found in two of the state’s counties and 12 towns.

To protect state and town forestry, state and local environmental officials have resorted to a quarantine that controls the movement of ash logs, ash materials, ash nursery stock and hardwood firewood from within the affected New Haven County to any area outside that county. The detection of the emerald ash borer (EAB) in Fairfield County will extend the state and federal quarantines in the state, the experiment station reported late last month.

“This latest detection and those in an expanding area of New Haven County show how invasive this insect can be — putting more of our ash trees at risk,” said State Entomologist Kirby C. Stafford III. “Not moving firewood or ash is the best way to help slow the spread of EAB.”

“Now that EAB has been detected in another Connecticut county, it is more important than ever to curb its spread and the most effective way to do that is preventing the movement of wood products out of affected areas,” Department of Energy and Environmental Protection Commissioner Daniel C. Esty said. “We will continue to work closely with the Connecticut Agricultural Experiment Station, the Department of Transportation, and other state and local agencies to put effective strategies in place to limit the spread of EAB and minimize the impact this invasive beetle has on Connecticut’s ash trees.”

“Yes, from what I have read, Fairfield County can expect a quarantine,” Ms. Sesto confirmed in an email message to The Bulletin last week.

Information about the quarantine, including its expansion, and the firewood regulations may be found at www.ct.gov/deep/eab or www.ct.gov/caes.

Ash trees make up about 4% to 15% of Connecticut’s forests and is a common urban tree, according to the experiment station.

“We do have a strong number of ash trees in our forests and as part of our residential landscapes,” Ms. Sesto said. “Both green and white ash are native to this area and white ash have been steadily in decline due to an ash blight. This has prompted many homeowners to removed sick trees. Even with this decline, the news of the EAB in the county is tough and an obvious threat to the ash trees we have left.”

For residents looking for information on protecting their ash trees, Ms. Sesto noted the website emeraldashborer.info “is a pretty good source of information for homeowners. It does speak of proactively treating ash trees with insecticides and the parameters to make that decision.”

In recent years it has been the Asian long-horned beetle, an exotic pest measuring 0.75 to 1.5 inches long, that has been threatening a wide variety of hardwood trees in vast regions from Chicago to Cape Cod.

Because this beetle happens to find such very common trees — maples, willows and elms — to be very good hosts, local and state conservation officials, arborists, scientists and university entomology labs have joined in public education campaigns to spread the word and heighten awareness of this invasive insect with very long black and white banded antennas.

The Asian long-horned beetle’s development and exit holes weaken the integrity of infested trees that can eventually result in death if the infestation becomes severe, according to the University of Vermont’s Entomology Research Laboratory. Larvae are considered to be the most dangerous because they tunnel in the delicate vascular tissue areas of wood. Larvae feeding reduces wood quality. After a tree has been occupied by generations of the beetles, larval feeding can disrupt the tree’s tissues, spur fungal growth, and lead to structural weakness. Any of these debilitating developments could kill a tree.

Wilton’s Tree Committee with Tree Warden Paul Young and Ms. Sesto have been working for several years “on educating the public on the Asian long-horned beetle as that insect seemed more likely to be the first threat,” Ms. Sesto noted. “We have also tried to get the word out regarding the hazards in moving firewood.”

Wilton’s Conservation Commission through its newsletter has put out the word in the past on the importance of firewood purchasing precautions and directives, pointing out that transporting firewood is considered to be “the leading cause in spreading harmful insects and disease among our forests.”

Fifty miles is generally regarded as too far to transport wood, and the optimal range is 10 miles or less. Residents should ask their wood provider where the wood comes from and be diligent.

Fallen limbs and brush on property can be left to rot, chipped into mulch, burned in fire pits or taken to nearby landfills.

When storing wood outdoors, it is important to keep supplies away from homes to avoid ants, termites and mice. Outdoor wood should be kept in a dry area, preferably stacked under a tarp or in a shed.

A task force was formed to protect the health of state forestry, and a complete list of recommendations are available on the DEEP website (ct.gov/dep).

The regional Tree Festival, for which Wilton is a sponsor and the seventh edition of which was held in the spring in Norwalk’s Cranbury Park, has become a fun, family-oriented event with contests, exhibits and demonstrations by experts. It is also an educational venue for “a number of tree enthusiasts to get the word out” on issues like invasive insects, quarantines and other steps to protect trees, Ms. Sesto noted.

Other insects in the state that can be harmful to forestry include the Asian gypsy moth, the nun moth and the pine shoot beetle. Diseases include oak wilt, bacterial leaf scorch and plum pox.