(AP) — Nebraska has begun a voluntary firewood restriction as part of its effort to stem the spread of an insect pest that’s killing ash trees.

The Nebraska Game and Parks Commission says the spread of the emerald ash borer can be prevented if park users acquire firewood within 50 miles of their destination and burn all their wood. Signs will be posted at all parks.

Park users bringing firewood from other states to Niobrara, Ponca, Eugene T. Mahoney and Indian Cave state parks, as well as Pawnee State Recreation Area, will be asked to exchange their firewood for locally acquired firewood.

The Asian insect has killed more than 50 million trees since it was first detected in Michigan in 2002. The ash borer has been found in northeast Iowa as well.