Below is a response to one of our customers from MSU Extension when he inquired about frequency of ash borer injections: This is similar to suggestions offered last year to one of our customers.
Please read the extension's thoughts and then our comments. The next few paragraphs (in red) were from Mary Wilson. Following that is our thoughts and discussion.
The latest research is showing some insecticides are effective for 3 years. So, it depends on what is being used, dosage, and how it is applied. Emamectin benzoate injections, ( one brand is Tree-age) can protect for three years. Soil drenches or basal sprays protect for one or two years. This document updated in 2019 explains the treatment options available and how long they are effective:
Two years after all untreated trees in your area are dead, your treated trees may be able to go to a 4-5 year cycle because the borer insect population will be very low. This response to another client in a location where all untreated trees were dead explains:
“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. From your description you should be able to go to a 3 year cycle when using Emamectin benzoate injections at the correct dosage. Once all untreated trees are dead, a longer treatment cycle can be used. Thank you. —-Mary Wilson, MSU Extension, Horticulture Educator”
We have to respectfully disagree with the suggestion that most or all of the untreated ash trees in Midland are dead and therefore not harboring ash borers.
Our observation is this: On three separate occasions in summer of 2020 we drove around sections of Midland to evaluate numbers of declining ash trees, presumably untreated, but alive and infested with ash borers. In each instance, within 30 minutes we were able to identify many such trees. We also have begun to see ash trees that sprouted after the ash borers arrived in Midland (trees 2 to 5 inches diameter now) that have been infested. So we have a whole new generation of ash trees, small but yet large enough to be attractive to the ash borers.
BOTTOM LINE FOR CUSTOMERS
1. A regimen of injecting vulnerable ash trees every three years will most likely give adequate control of ash borers. The City of Midland has been on a three year cycle since 2011. We have been doing those injections for the City for the past two cycles. We do observe some re-infestation and some small branch involvement in the third year, we have not seen high levels of re-infestation with large branch attrition. So the trees do stay alive, sometimes with small amounts of damage.
2. For OPTIMAL control of valuable landscape trees a two year treatment interval is proven effective. For ADEQUATE control, a three year interval can be used.
3. For longer intervals between treatment, you could THEORETICALLY use high dose emamectin and observe for signs of new damage with the caveats discussed on our ASH INJECTION FREQUENCY page.
Please read the ASH INJECTION FREQUENCY page for more discussion.