As I have taken daily walks in the neighborhood this summer, I’ve noticed many ash trees are dying due to the emerald ash borer (EAB), a small insect that hitched a ride probably in packing crates from China (the crates sometimes are made of ash and the larvae could have already been inside the wood). It’s been seen as a menace since 2002. 60 million ash trees have been affected and the estimated cost of this destruction is $20 billion just for landscape trees over the next ten years!
The EAB actually has 4 larval stages; it is bigger in each stage. After the adult lays its eggs in the folds of the bark, the larvae bore into the inner part of the bark, called the phloem, which they feed on. The larvae, as they feed, create “galleries” or tracks within the phloem and grow larger in size. The phloem is essential to the tree’s health, because it is through the phloem that nutrients are transported upward toward the branches and leaves.
Apparently, the EAB will begin at the top half of the tree.
It’s so sad to see so many trees that are dying from this pest. Ash trees were popular shade trees
which were planted to replace elm trees that had been nearly wiped out by Dutch elm disease. The EAB has migrated to more than 20 U.S. states and Canadian provinces. It started in Michigan, apparently arriving in a shipping container that unloaded in Detroit. Originally it was thought that quarantine could contain the EAB’s spread, but this has not been effective. I remember hearing that it arrived in the Chicago area in the Ravenswood neighborhood, before we saw any sign of it here in the suburbs.
On our street, there are mostly maples, so no trees have had to be cut down, but some streets around here have many ash trees. The infected trees are marked with green paint, meaning that the tree will be cut down. I have seen ash trees that are already a pile of logs with green paint on them.
Scientists have been looking for a biological solution – that is, finding a natural enemy of the EAB – to eradicate or control the infestation. They have studied some types of parasitic wasps that are not harmful to people, first a native one, which didn’t work out because its life cycle didn’t correlate with the EAB’s. Now they have discovered 3 types of parasitic wasps that only target EAB. They have done tests and found that these parasitic wasps attack EAB at the egg and larval stages. One kind of wasp lays its egg directly on an EAB egg, killing the EAB egg. Two other types lay their eggs on EAB larvae, where they hatch and grow, consuming the EAB larvae.
Scientists have also discovered that there would be very little impact on our native environments in the release of these parasitic wasps. (They are very small – much smaller than most wasps). They have targeted release of parasitic wasps in several states, and Illinois is one of the most recent to receive these wasps.
One article states that in order to have a major effect, most of the trees around the epicenter of release should be in the early stages of infestation – if most are already at stage 4 or 5 (dead), chances are that the EAB will not be very numerous and the technique of releasing the parasitic wasps will not work.
Ways to tell if an ash tree is infected:
Epicormic shoots – these are shoots of growth near the base of the tree – I’ve seen lots of these. It’s the tree’s attempt to compensate for loss of energy at the top of the tree where there is little or no foliage. The nutrients are unable to be transported to the top of the tree due to damage by EAB galleries.
Woodpecker damage – certain woodpeckers prey on the EAB, and if there is a lot of woodpecker damage to the bark, chances are it is infected.
Bark splits or “callous” – the bark of the ash tree will weaken and split in places along the trunk of the tree if infested.
D-shaped holes where the adult EAB emerges – these are smaller than a fingertip, so they’re hard to detect.
Thinning or brown foliage, or absence of foliage – especially at the top of the tree, the leaves will thin out. I’ve seen a lot of this too – it seems as though the tree dies from the top down: at the lower level, the foliage is still full and there are many trees with epicormic shoots. (This trees is at about stage 3.) I’ve also seen the stage 5 – dead – trees that are completely bare, in the middle of summer.
The second article I read, and downloaded part of it, not only describes this entire process, but also gives detailed instructions on how to identify EAB eggs, how to identify parasitic-wasp eggs, how to collect samples, how to attract the wasps to the tree, etc. That web site address is as follows:
I did see a healthy ash tree (or it seems to be) yesterday, on a street I don’t often walk on. I wondered how it could be healthy when so many trees around it are dying. However, today an acquaintance told me they have an ash tree that they have treated every two years for $250 each time. The person doing this service injects something into the phloem to make it taste bad; thus the EAB don’t want to eat it.
I started this post over a month ago. Two days ago, I took a walk down a street that had been lined with shady ash trees. I counted 28 trees that had been removed in a four-block stretch of that street alone; on a parallel street, I counted 9 more removals, but on that street, I also saw 4 healthy ash trees! Three of these were very young, but the 4th was at least 20 years old, judging by the size of its trunk. I looked up and could see no sign that this tree was infected. Either these trees have been treated with the chemical that repels the EAB or the parasitic wasps have finally been released in this area! That would be good news; unfortunately, it’s too late for the majority of the ash trees around here.