· Michael Blair
The emerald ash borer is a small metallic-green beetle from Asia whose larvae feed under the bark of ash trees, cutting off the flow of water and nutrients until the tree dies. It reached Tennessee more than a decade ago and it is established across Middle Tennessee now. An untreated ash tree in its path dies, typically within a few years of the first symptoms. For a homeowner with an ash in the yard, the question is not whether the tree is at risk. It is whether this tree is worth treating for the rest of its life, or should come down while it is still safe to take down.
First, is it an ash
Ash trees have opposite branching: twigs and buds grow in pairs directly across from each other, which is uncommon among large trees here. The leaves are compound, with five to nine leaflets on a central stem. Mature bark has a tight, regular diamond pattern of ridges. Green ash and white ash are the common species in Nashville yards, often planted as street trees in the 1970s through the 1990s, and some neighborhoods have a lot of them.
Maples also branch opposite but have simple lobed leaves; hickory and walnut have compound leaves but alternate branching. If you are unsure, an arborist will identify it in a minute, and it matters, because the borer attacks only ash.
What an infested tree looks like
The canopy thins from the top down, with dieback in the upper branches first. Sprouts appear on the trunk and the base as the tree tries to compensate. Woodpeckers work the bark heavily, flecking it light in patches, because they are after the larvae. Under the bark, if a piece comes away, the larvae leave winding S-shaped galleries. The adults leave a small D-shaped exit hole, a few millimeters across, which is the definitive sign but hard to spot on a tall tree.
By the time the canopy is noticeably thin, the tree has usually been infested for a few years. A tree that has lost more than about half its canopy is generally past saving; treatment works on trees that still look mostly healthy.
- Thinning at the top of the canopy
- Sprouts from the trunk or the base
- Heavy woodpecker damage on the bark
- D-shaped exit holes and S-shaped galleries under bark
Treat or remove
Systemic insecticide treatment, applied by trunk injection or soil drench by a licensed applicator, protects an ash reliably when it is started early and repeated on the product’s schedule, typically every one to three years, for the rest of the tree’s life. It makes sense for a healthy, well-placed, valuable tree: a large ash shading the house or the one specimen tree in the yard. The cost is ongoing, and stopping it means the tree becomes vulnerable again.
Removal makes sense for a tree that is already declining, poorly placed, one of several ashes, or simply not worth a recurring bill. The important thing about removal is timing. Dead ash becomes brittle quickly and is dangerous to climb, so a tree removed while it still has live wood is a routine job, and the same tree a couple of years after death is a hazard removal with a crane and a higher price.
What waiting costs
An untreated ash in an area where the borer is present will be infested; the only variable is when. Deciding nothing is deciding on removal, on the tree’s schedule rather than yours, at the point when it has become the most expensive and hazardous to take down. Dead ashes shed large limbs without warning, and one over a driveway, a play area or the neighbor’s roof is a liability.
If you have an ash, have an arborist look at it this year. A healthy tree can be put on a treatment plan; a declining one can be scheduled for removal while it is still safe; and either way the decision is made with the tree standing rather than after it has dropped a limb.