Driving down the road you may have noticed a lot of dead trees pockmarked in the mountains or along streams. There’s a good chance those are dead and dying ash trees that are being hammered hard by the emerald ash borer right now.
Emerald ash borer (Agrilus planipennis) is a shiny green beetle that feeds exclusively on ash trees. It’s an invasive insect brought in accidentally from Asia, probably in wooden pallet material. It started out in southern Michigan in 2002 and reached our area around 2010.
Ash trees are killed by the feeding activity of the larvae (flat white grubs) of the borer. They feed on wood just beneath the bark, destroying cells responsible for transporting sugars produced by the leaves to the rest of the tree. So as the larvae feed they slowly destroy the tree’s ability to feed itself. In the beginning when borer populations were low it took 2 or 3 years for a tree to die from attack. But bug populations have exploded over the last couple of years and trees can be overwhelmed in one growing season.
Trees under attack start out with yellowing leaves at the end of twigs, which gradually turn into a lot dead branches. During advanced infestation the tree will have a lot of sucker sprouts on the trunk where the tree is desperately trying to grow back lost leaf canopy. You might see splotches of bark appear a lighter color (called blonding) where woodpeckers are knocking off the outer bark to feed on the borer larvae. Eventually D shaped holes will appear on the trunk, which are exit holes for the adult beetles after they have pupated.
Infested trees can be chemically treated and saved if the infestation is caught early enough, and uninfected trees can be treated as a preventative measure For large trees the best way is to use a systemic insecticide which is poured on the ground beneath the tree and the chemical is taken up by the roots to kill the borer larvae internally. Insecticides that have the active ingredient Imidacloprid seem to be most effective. A common insecticide found locally is Bayer Advanced Tree and Shrub Insect Control. Be sure to follow label directions.
The borer can be spread through the transportation of infested nursery stock, ash logs, or firewood, the latter being most likely. Lots of folks bring firewood with them when camping, and if borer eggs are in the wood they can hatch out perhaps hundreds of miles from their origin. So national and state parks and all forestry agencies have a big time education campaign urging people to not transport firewood from home, but collect or buy it locally where they are camping.
Ash is a common tree around here that can be found in most forests. We have several species of ash that grow here, the most common two being white and green. I have found blue ash along river sites, but it’s uncommon. Ash is a popular landscape tree to plant, and ash wood is valued for its strength and light weight. For questions on emerald ash borer, contact your local forestry agency or agriculture extension service.
Steve Roark is the area forester in Tazewell, Tennessee, for the Tennessee Department of Agriculture, Forestry Division.