Emerald ash borer in Minnesota's ash forests presents several unique management challenges. Black ash is Minnesota's most abundant ash species and is well-adapted to growing in wet areas. Research is underway to find suitable alternative tree species that might replace black ash, but the conversion to non-ash forests will continue to be very challenging due to limited site accessibility and the expense of planting and growing other tree species across extensive wet sites.
Here are some guidelines for landowners managing ash stands or mixed stands with a high percentage of ash:
- Prioritize forests with the following characteristics:
- Close to confirmed, infested areas
- Contain mostly ash
- High-productivity, forested swamps (sites that typically have other tree species besides black ash and are less sensitive to management activities; for example, northern wet ash swamps).
- Low-productivity sites, such as northern very wet ash swamps, could be deemed lower priority for management since they are less likely to successfully encourage other species.
- Avoid planting ash
- Sites with abundant ash usually are wet, so management activities take longer than on upland sites. Prepare these forests well in advance of emerald ash borer infestation.
- For forests with abundant ash and few other tree species, plant a variety of site-appropriate species before or immediately after tree cutting. Larger seedlings planted in fall have done best in some management trials. In results from trials in wet forests, American elm and swamp white oak have done well. Hackberry, balsam poplar, silver maple, black spruce, tamarack, and others have had varying success in management trials.
- If managers want to capture the timber value of mature ash, it is important to harvest before an emerald ash borer infestation. Once an ash tree is infested, it quickly becomes brittle and potentially loses its economic value.
- When harvesting mature ash trees and promoting other tree species in wet forests, avoid clearcutting. Removing groups of ash (0.1 to 0.5 acre in size) is best on wet forests. Strip clearcuts and strip shelterwoods are methods successful at promoting other species if those other species are present (e.g., red maple and balsam fir). Retain some ash, since they will provide habitat for wildlife when they die.
- Cutting smaller black and green ash promotes stump-sprouting. Consider killing ash stump sprouts the year after harvest to remove competition from desired species.
In addition to the guidelines above, private forest managers are encouraged to consult Managing Ash Woodlands for advice on how to manage ash in their forests and woodlands.
Be proactive when managing the ash trees in your yard. Ash trees can be treated with an insecticide to prevent emerald ash borer infestation. To be effective, treatments must be repeated every two to three years for the entire life of the tree, depending on the label instructions. If you are not interested in or cannot afford to treat your ash tree, consider having it removed now by a professional tree care company and replant with a tree that is right for your area using sound planting practices. Waiting to remove your tree once it has become heavily infested or dead will be more expensive because brittle ash trees are dangerous to cut down. Consider contacting an ISA certified arborist to discuss the best course of action for your tree.
Infested trees with more than 50 percent canopy decline are poor candidates for treatment because emerald ash borer has already caused significant damage to the tree. Lightly infested ash that show little sign of canopy decline can recover if treated with an insecticide. Treatment cannot reverse damage that has already been caused.
Begin insecticide treatments when emerald ash borer has a moderate to high risk of damaging your ash tree, when the borer has reached your community or neighborhood. If you value your ash tree, you could begin treatment when emerald ash borer is in your county, although you may be paying for unnecessary treatments because the risk of infestation is still low in your location. Track emerald ash borer using the Minnesota Department of Agriculture's interactive EAB status map.
The most effective insecticides are injected directly into the trunk. Injection by a professional pesticide applicator is the best option for trees greater than 48 inches in circumference (15 inches in diameter). Sprayed and poured insecticides are not as effective and can drift or leach to surrounding areas and negatively impact insects and aquatic invertebrate animals. To determine the best insecticide option for your ash tree, consider contacting a professional who has a Minnesota Pesticide Applicator License, or visit the Homeowner Guide to Insecticide Selection, Use, and Environmental Protection.
What can I do to help?
Emerald ash borer moves quickly when people move infested ash firewood. A quarantine has been placed in a growing number of counties to help prevent its spread. Firewood and other ash products are regulated by the Minnesota Department of Agriculture.