New Hampshire is strengthening a ban on out-of-state firewood because of concern over non-native invasive insects, Boston.com reported.

Two years ago, a joint state-federal regulation restricted firewood movement on state reservation lands and national forest lands within New Hampshire, which includes campgrounds.

The new ban starting Monday expands those restrictions to prohibit out-of-state firewood movement to all lands in New Hampshire. Only approved heat-treated firewood that’s properly packaged and labeled or firewood transported with a compliance agreement from select counties in Maine, Vermont and Massachusetts may enter the state.

Pre-approved counties in Maine are Franklin, Oxford, Androscoggin, Cumberland, and York. All counties in Vermont are approved. In Massachusetts, Essex, Middlesex and Franklin counties are approved.

Forest officials are concerned about importing insects such as the emerald ash borer and the Asian long-horned beetle, which have harmed trees in other states. In some cases, infestations have been attributed to the movement of camp firewood.

More than half of all forests in New Hampshire are susceptible to the insects.

“It’s surprising how much and how far people are willing to transport firewood,” said New Hampshire Forest Entomologist Kyle Lombard. “We surveyed firewood use at campgrounds and found that 40 percent of the out-of-state campers brought it from home. Many of those folks were from as far away as Florida, North Carolina and California. We have seen that every stick of firewood taken from these campers had insects, to the tune of about 30 per piece.”

Violators of the ban are subject to a fine.