Emerald Ash Borer (EAB), is an exotic, invasive wood-boring insect that infests and kills native North American ash trees. Since it's intial detection near Toledo in 2003, the insect has spread throughout the state killing more than 4 billion ash trees.
If left untreated, EAB kills ash trees within three to five years of infestation. Adults are dark metallic green, 1/2 inch in length and 1/8 inch wide, and emerge from the tree and fly only from mid-May to September. Larvae spend the rest of the year developing beneath the bark.
As the adults emerge, they leave small (one-eighth of an inch), distinctly D-shaped exit holes in the trunk and main branches, which is the surest sign of infestation. Adults feed on foliage for one to two weeks prior to mating. Females produce about 50 to 100 eggs, which are laid individually on the bark surface or within bark cracks and crevices. As the larvae hatch, they tunnel into the tree, where they feed through the summer and early fall on the phloem and outer sapwood, excavating S-shaped, serpentine galleries just under the bark. This feeding disrupts the flow of carbohydrates and water between the canopy and roots of the tree, which results in the canopy thinning, branch dieback and finally tree death.
Several insecticide options are available to homeowners to effectively treat landscape ash trees threatened by EAB. It is best to begin using insecticides while ash trees are still relatively healthy. By the time most people notice canopy thinning or dieback, EAB has already caused considerable injury to the vascular system of the tree. An effective insecticide may stop additional damage, but it cannot reverse damage that has already occurred and it takes time for trees to recover. Most insecticides used for EAB control act systemically - the insecticide must be transported within the tree. In other words, a tree must be healthy enough to carry a systemic insecticide up the trunk and into the branches and canopy.
Multi-year studies have shown that if more than 50% of the canopy has been killed by EAB or if the canopy appears to be thin and carrying less than half as much foliage as it should, it is probably too late to save the tree. The ability of trees to recover from low to moderate EAB injury can vary, depending on the extent of the damage and which control options are used. Studies have also shown that if the canopy of a tree is already declining when insecticide treatments are initiated, the condition of the tree may continue to deteriorate during the first year of treatment. If treatment is effective, the tree canopy will usually begin to improve in the second year of treatment. Embedded at the bottom of this post is a YouTube video that exhibits how timely and consistant insecticidal treatments can positively impact infected ash trees.
Systemic insecticide applications should be made in time to allow for uptake and distribution of the insecticide within the tree to ensure adult beetles and very young larvae encounter the toxin. In Fairfield County, that means that insecticides should be applied to ash trees in April each year. Imidacloprid and dinotefuran are systemic insecticides that can be applied as soil drenches or soil injections. Both are sold under numerous brand names for use by professional applicators and homeowners. Homeowners can make applications as a soil drench by mixing the product with water, then pouring the solution directly on the soil around the base of the trunk. The insecticide is taken up by the roots of the tree and then moves (translocates) throughout the tree. Since imidacloprid can bind to surface layers of organic matter such as mulch or leaf litter, it's important to remove, rake or pull away any mulch or dead leaves so the insecticide solution is poured directly on the mineral soil.
Rates of soil applied insecticides needed to provide effective control may vary depending on the size of the tree and the intensity of pest pressure at the site. Higher rates of some imidacloprid products available to professionals and homeowners can be applied to large trees with trunk diameters greater than 15 inches. Lower rates are effective on smaller trees and when EAB populations and pest pressure are relatively low. When treating larger trees with imidacloprid or dinotefuran soil treatments, particularly when EAB density is high, studies have shown that applying the highest labeled rate is most effective. Only some imidacloprid products can be applied at the higher rate and only if trees are greater than 15 inches in diameter, so please review the label closely when selecting a product.
If you'd like more information about EAB, visit the Emerald Ash Borer Information Network.
If you believe your trees to be infested and would like to learn more about treatment, find the complete list of insecticide options in the publication Insecticide Options for Protecting Ash Trees from Emerald Ash Borer.
For an example of how timely and consistant insecticidal treatments can positively impact infected ash trees, see the video below.