The Vermont Department of Forest, Parks, and Recreation is reminding people that the best way to prevent the spread of an invasive insect that kills ash trees is to burn only local firewood. The emerald ash borer is an invasive pest from China first detected in this country in 2002. Since then, it has been discovered in 19 states, including New York and Massachusetts, as well as the province of Quebec. Earlier this week officials in New Hampshire said they had found infested trees in the Concord area. Vermont officials have set up a monitoring system to look for the pest. Infested areas are subject to state and federal quarantines.