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Photo of the Friendly Neighborhood City Forester and superhero, The Smart Ash, Discussing Emerald Ash Borer Treatment Option, Soil Drench

Emerald Ash Borer Treatment Option: Soil Drench

Smart Ash’s Note: If it’s detailed information about emerald ash borer trunk injection treatments you want, you’ve come to the right place. If you want higher-level, more actionable information about your options as an ash tree owner, visit our emerald ash borer treatment page.

Officially dubbed either imidacloprid and dinotefuran, soil drenches 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.

Soil applications can be applied as a drench by mixing the product with water, then pouring the solution directly on the soil around the base of the trunk, or injected a few inches below ground at multiple locations near the base of the ash tree. The insecticide is taken up by the roots of the tree and then moves throughout the tree.

Soil drenches offer the advantage of requiring no special equipment for application other than a bucket or watering can. However, imidacloprid can bind to surface layers of organic matter, such as mulch or leaf litter, which can reduce uptake by the tree. Before applying soil drenches, it is 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 ash 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.

Products designed for homeowners have some restrictions that do not apply to professional formulations. Homeowner products can be applied as a soil drench or as granules that are watered into the soil, but not as a soil injection. Homeowners are also restricted to making only one application per year. Professionals can apply these products as a soil injection as well as a soil drench.

Soil injections require specialized equipment, but offer the advantage of placing the insecticide below mulch or turf and directly into the root zone of the ash tree. This also can help to prevent runoff on slopes. Studies have shown uptake is higher and the treatment more effective when the product is applied at the base of the trunk where the density of fine roots is highest. As you move away from the tree, large radial roots diverge like spokes on a wheel and fine root density decreases.

Efficacy of soil-applied systemic insecticides for controlling EAB has been inconsistent. In some Ohio State University (OSU) and Michigan State University trials, EAB control was excellent, while others yielded poor results. The biggest reason? Even among researchers, application protocols vary considerably and are difficult to replicate, making it equally difficult to reach firm conclusions about efficacy.

This inconsistency may reflect the fact that application rates for soil-applied systemic insecticides are based on amount of product per inch of trunk diameter or circumference. As the trunk diameter of a tree increases, the amount of vascular tissue, leaf area and biomass that must be protected by the insecticide increases exponentially. Hence, application rates based on diameter at breast height (DBH) may effectively protect relatively small ash trees but can be too low to effectively protect large trees. Some systemic insecticide products address this issue by increasing the application rate for large trees.

For example, in an OSU study in Toledo, Ohio, which began in 2006, imidacloprid soil drenches have effectively protected ash trees ranging from 15-22 inches in diameter when applied at the 1X rate in spring, or at the 2X rate when applied in spring or fall. These treatments were effective even during years of peak pest pressure when all of the untreated trees died. Trees treated in fall with the 1X rate, however, declined and were removed. In another OSU multi-year trial with trees up to 22 inches DBH, dinotefuran soil applications were effective when applied at the highest labeled rate. However, lower rates were less effective.

Insecticide placement may also affect efficacy. Recent studies have shown that soil drenches and injections made at the base of the trunk result in more effective uptake than applications made on grid or circular patterns under the canopy away from the trunk.

Treatment programs must comply with the limits specified on the label regarding the maximum amount of insecticide that can be applied per acre during a given year. This restricts the number of trees that can be treated in an area.

Soil applications should be made when the soil is moist but not saturated. Insecticide uptake will also be limited when soil is excessively dry. You may need to irrigate the soil surrounding the base of the tree before and possibly after the insecticide application if soils are dry. However, water-logged soil can result in poor uptake if the insecticide becomes excessively diluted and can also result in puddles of insecticide that could wash away, potentially entering surface water or storm sewers.

To further protect surface and ground water, soil applications should not be made to excessively sandy soils with low levels of organic matter that are prone to leaching, especially where the water table is shallow, or where there is risk of contaminating gutters, lakes, ponds, or other bodies of water. No soil applications should be made where there are roots of flowering plants that are visited by bees and other pollinators. This situation is most likely to occur where flowering plants are established around the base of an ash tree. In these situations the flowering plants should either be destroyed or insecticide should be applied via trunk injection to ensure the toxins will not be taken up by the flowering plants.

This information was adapted from a multi-state study on emerald ash borer insecticides performed by Colorado State, Ohio State, Michigan State and Purdue universities. So which EAB treatment method is the best for ash trees in Metro Denver, an area that includes 1.45 million ash trees? Unfortunately, there isn’t one right answer.

The best treatment for your Denver ash depends on a variety of factors, and is ultimately dependent on property owner preference. While some experts might not recommend treating your ash tree before EAB is found in Denver, it is worth noting that it may take three years for your tree to show obvious signs of infestation. And by then, it may be too late to save. Before making any decisions, you should consult multiple sources. This list of Certified Smart Ashes, all of whom are licensed by the Colorado Department of Agriculture to apply pesticides, is a great place to start.