WORCESTER � With the dreaded emerald ash borer pest spreading across the state, a statewide quarantine was instituted Monday, prohibiting the transportation of all hardwood firewood, ash nursery stock and ash lumber.

The emerald ash borer, a beetle that targets ash trees, has spread from Berkshire County to Essex County and most recently Suffolk County, where the tiny insects were spotted at the Arnold Arboretum in Jamaica Plain in Boston. Experts say little can stop the infestation at this point, though a quarantine is expected to slow it down.

State Department of Conservation and Recreation officials say ash products that have not been treated must stay within county boundaries. Proper treatments for wood include removal of the bark and half an inch of wood, dry kiln sterilization and fumigation or heat treatments. DCR advises a "buy local, burn local" firewood measure as a precaution for campfires and fireplace use.

In Worcester County, residents are reminded that ash trees also remain one of the 13 species of "host trees" susceptible to Asian longhorned beetles. A 110-square mile area in Worcester County is regulated in an effort to eradicate Asian longhorned beetles. Host trees or wood from those species may not be taken out of the regulated area, which covers Worcester, Shrewsbury, West Boylston, Boylston, and parts of Holden and Auburn.

Massachusetts is one of 23 states where emerald ash borers have been found. The invasive insect is a tiny metallic green beetle, native primarily to China and Korea, which was first found in the U.S. in Detroit in 2002. The beetle was found in Dalton in the Berkshires in 2012 and then in North Andover in 2013.

Kenneth Gooch, DCR forest health program director, said in the Midwest, the bugs have swept through forests and destroyed all the ash trees. In Indiana, for example, nearly all of the state's living 150 million ash trees have died or are dying, according to the Associated Press.

Mr. Gooch said the statewide quarantine on the transportation of firewood was necessary because so many Bay State towns with the beetles border other counties, including Hampshire and Middlesex counties. Worcester County, he noted, is the only one not threatened with infestation at this point.

"There's no way it's going to stop the spread of the insect. It made more sense to put our efforts into slowing the spread of the insect for where it is right now," he said. "We're doing more outreach, more education and the biggest thing that people should know is we recommend not moving firewood. Keep it local."

Mr. Gooch said the state has also worked to release a natural biocontrol to combat the emerald ash borers: two types of predatory wasps, Tetrastichus planipennisi and Oobius agrili. Last year, thousands of the wasps were released, and this year another batch has been dispersed. Mr. Gooch estimated it could take five or 10 years, though, to see the full effects of those non-stinging wasps.

"The eventual hope is to recreate the balance of how it is in (the beetles') native land," he said. "The emerald ash borer lives with all these other insects (in its native countries) which keep it in check."

For now, Mr. Gooch said the destruction of local ash trees � which make up about 2 to 10 percent of Massachusetts forests � will have a major ecological impact in the state.

"We all want healthy forests," he said. "Healthy forests equal oxygen, good water quality, and no healthy forest affects climate change. � When we lose a tree species, that's a big deal. You want there to be diversity � that's what makes up a healthy planet."

Contact Samantha Allen at samantha.allen@telegram.com. Follow her on Twitter @SAllen_89