We have had many discussions with customers about the frequency of ash borer injections. Although we know from our experience that injection of the trees every two years gives excellent control of ash borers with no evidence of new damage from borer infestation, the customers who have to pay for the injections wonder when we might decrease the frequency of the injections.
One of our customers recently inquired specifically about this subject, contacting MSU extension, and received the following reply (copy and pasted directly from their email reply)
Here's the response to your question:
Here is a synopsis of recommendations by Drs. David Smitley, MSU Landscape Entomologist, and Deb McCullough, MSU Forest Entomologist. Both researchers have worked extensively on EAB since it was discovered in Michigan.
Since we’re at the point where most of the untreated ash trees in Midland are dead, you can extend the treatments to a 4 or 5-year treatment plan. Here are the strategies to consider:
1. Emamectin benzoate trunk injection at the highest label rate once every five years. If you see new bark splits and limb dieback in the future, treat a year or two early, then go back to the 5-year schedule.
2. If the tree is under 14 inches DBH (trunk diameter in inches at breast height), apply emamectin benzoate at a moderate rate, e.g, 5 ml per diameter at DBH inch. (This option is a consideration since the high rate can be difficult to get into trees. Plus, the treatment doesn’t cost as much.) Trees should be treated at either 4 or 5-year intervals. For trees with a DBH over 14-16 inches, then the high rate might be better.
Note that if you follow a 4 or 5-year treatment plan, it would be wise for someone to examine a few green ash trees in August. Those are likely to be colonized before any white ash trees. If new woodpecks, declining branches or any other new signs of EAB are observed in green ash, then the EAB density might be starting to build at least a bit. At that point, it might be worth shortening the injection frequency – e.g., switching from 5 to 4 years, for example.
Hope this information helps you make your decision.
Mary Wilson, MSU Extension, Horticulture Educator
Here is my reply email to this customer:
This email response read about like I anticipated that it would. They suggest higher than the usual rate and they admit that it can be difficult to get it into the tree.
One way to get the higher rate would be to drill the tree, inject it with the usual rate, then return in one week and inject the same rate again (since the first dose would have gone up the tree and the tree would accept another dose. The cost to the customer would be about twice the regular rate because of using twice the amount of chemical and making two visits to the property.
And, of course, they suggest monitoring the trees closely for re-infestation.
They are also stating a questionable "fact" that most of the untreated ashes in Midland are dead. Depending on the definition of "most", it may or may not be true, but what is true is that there are hundreds to thousands of ash trees in the city of Midland and Midland county that are still alive and harboring ash borers.
I don't know what the response would be from our customers if we offered a four year treatment at twice the cost or nearly twice the cost, but with the caveat that we need to return in August of each year to check for signs of re-infestation.
Another thing to consider is that there may be a charge for the re-evaluation visits, so the ultimate expense may be more.
ALSO, the label of the product used for ash borer injections still reads: "up to two years of control with one dose", so if we recommended less frequent intervals, we would be at odds with the label (not considered to be ethical or legal practice.)
We have treated thousands of ash trees and have excellent success going with the rates and frequency on the product label. So that is still our recommendation. And for valuable ash trees, we consider that to be the best approach.
However, if a customer would like to try less frequent, but higher doses of TREE-Age, that is their option and we would honor that request.
UPDATE! A NEW ARTICLE recently published by Dr. McCullough and others, shows three year satisfactory results on smaller trees, using the low dose of chemical, which is the dose that would routinely be used on smaller trees. We have not seen confirmation that satisfactory results would be obtained on larger trees treated at less frequent intervals with the low dose. As noted above, larger trees would require the high dose and attendant higher cost for treatment. Hopefully, ongoing studies will clarify whether we can go to less frequent intervals and whether the product will be labelled for less frequent injection. (And at what dosage)