Ashes were long a tried-and-true workhorse as an urban shade tree until the introduction of the emerald ash borer circa 2000. Ashes have since been largely eliminated from the natural and landscaped environment in s.e. Michigan and much of the midwest.
The introduction of EAB was truly an environmental disaster. Ashes are (or were!) part of, or dominant in, many forest types. They are easy to propagate and very tolerant of urban conditions. However, none of the major species native to Michigan have shown significant tolerance to emerald ash borer.
The emerald ash borer larvae kill trees by eating the phloem under the bark. This leads to the starvation and decline of the tree roots.
- Green ash dies more quickly than white ash. Some, but not many, white ash specimens in prime condition survived the first wave of borers. The uncommon blue ash has a better survival rate, but is usually disfigured.
- Control treatments are more successful with isolated specimens.
- Commonly the first sign of insect attack is the activity of downy woodpeckers, who seek out the larvae under the bark. Yum!
- Small d-shaped exit holes are diagnostic, but when these are found on the main trunk at eye level, the tree is probably beyond saving.
The emerald ash borer is an aggressive insect, and aggressive control measures are required to keep specimen trees healthy. Control options include:
- spraying ash trees several times in late spring and early summer;
- injecting an insecticide into the soil near the base of the tree, and
- injecting one of several products directly into tree trunks.
For small trees, any one of these methods will likely work.
Large trees are a challenge. Annually-applied trunk injections with formulations of imidicloprid have worked for GreenStreet on average-sized shade trees. “Tree-age” (emamectin benzoate) is a newer formulation that we are now using exclusively. Not only is it more immediately effective, it remains so for at least two years. Unfortunately the product itself is quite expensive, and it is time-consuming to apply.
With great interest we are closely watching to see to what extent ash trees return to the landscape. Many that died to the ground have re-emerged from stump sprouts, and millions of seedlings have sprouted in the woods and along fence-rows. And then there are the occasional specimens that were able to withstand the initial pest onslaught.
For the historical record, we at GreenStreet were the first to notice the decline of ash trees in western Wayne County and subsequently report it to authorities at Michigan State. It took a while before it was determined that the culprit was an insect. In the spring, we collected adult specimens that were hovering around an ash tree in Plymouth, tried to match it against pictures of similar insects in our manuals, and determined that it was a close relative of several other species of borers that are found locally — but not an exact match. We called the entomologists at MSU, who had themselves just made the same discovery and had furthermore determined that it was a new pest introduced from Asia.
For more on the current status of ash in Michigan, you can refer to this earlier post.