It can demolish even the largest ash trees in just a few years, and it’s spreading like wildfire. Emerald ash borer has become a serious pest since it was first found in Michigan in 2002, spreading west into Colorado, south to Georgia and north into Ontario and Quebec.
Dan Herms, professor and chairperson at The Ohio State University department of entomology, shared some new study findings and best tips for handling the pest at a recent Emerald Ash Borer University presentation.
The green beetle moves quickly, generally killing 30 percent of trees in the first 10 years and 60 percent of trees in just four years after that. “If you do nothing, in an urban environment, you can anticipate that you’re going to get … an exponential increase in mortality,” Herms says. “It’s going to start out slow; it appears that you can keep up with it but it’s going to reach an inflection point when you’re not going to be able to keep up with it without expending huge resources.”
While the problem is best managed with preventative treatments, you can still save infested trees.
“Conservation of ash is a viable strategy now,” Herms says. “We know a lot about how to consistently and effectively protect even the largest ash trees. There’s still this meme out there that is repeated in the press that insecticides are not effective. We know that that’s not true.”
According to multi-year studies on ash trees in the Midwest, dinotefuran and emamectin benzoate are the best options for an adult infestation.
Dousing with dinotefuran.
“If you find a tree after adults have emerged and larvae have started feeding in June or July – if that’s the first time a tree comes to your attention – you would want to use a treatment that’s taken up very rapidly and dinotefuran or a trunk injection would be the thing to do,” Herms says, adding that optimal timing is before adults begin to emerge, around early and mid-April. The next best option would be to apply the highest rate allowed in the fall.
In looking at dinotefuran applications, there are two options: a basal trunk spray and a soil treatment. And both are equally effective.
To apply a basal spray, technicians simply spray the product at low pressure right onto the lower 6 feet of the tree. The insecticide is absorbed through the bark and spread throughout the ash. “You apply it like you’re spray painting the tree with the appropriate amount of insecticide, which is based on the diameter of the tree,” Herms says.
For a soil treatment, technicians must make sure that the soil isn’t frozen or saturated. Then they can either inject the insecticide a few inches under the soil or use a drench. Spring treatments are the most effective, but fall drenches will also the treat the problem if used at higher rates.
Technicians should keep in mind that lower rates may not show results, especially on larger trees. “So on these larger trees, we definitely recommend that you don’t try to save money by using lower rates, that you use the higher rate that’s labeled, keeping in mind to that you have per acre restrictions,” Herms says.
Injecting emamectin benzoate.
Emamectin benzoate gives two to three years of control, which Herms says offers more flexibility to landscape operators. “If you’re treating every other year, you treat June and July. And two years later you treat June and July, it’ll still carry so you do have time to treat throughout the growing season because of that multi-year efficacy.”
Applied via trunk injection, emamectin benzoate has been shown to be stop emerald ash borer for two years, even under high pest pressure. In one study, researchers applied a low, medium and medium-high rate to trees in June of 2006 and found that even the low rate of 2.5 ml per inch was warding off canopy decline in late summer of 2008.
It’s also a good option for larger ash trees. Studying trees 32 to 47 inches in diameter, Herms found that canopy decline at first stabilized and then began to drop off, even at the medium rate allowed by the label. “In the west side of Cleveland area, there’s intense pressure. Ash trees are mostly dead in this area, but these trees are doing quite well,” Herms says.