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How do treatments for EAB actually work?
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In later instars, EAB larvae do begin to etch into the xylem and will thus come into contact with some of the insecticide. It should be noted however, that this is not the main area they are feeding in, and that the cambium and phloem will have much lower levels of the product.
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With the arrival of EAB in 2016, many in Nebraska soon became aware that there are several ways to protect ash trees from this voracious insect. While many of these products do in fact work, it is important to remember just how the active ingredient is being moved in the tree, and where it is actually targeting the insect.
By and large, the most common and effective treatments for EAB involve systemic insecticides such as imidacloprid, dinotefuran, and emamectin benzoate. These products are generally applied either in the soil, as a bark spray, or injected. The idea being that the product will move up and out into the tree’s different tissues. The actual life stage which the product is targeting however, is often misunderstood, especially by homeowners.
Neonicotinoid products are transported up into the leaves of the tree. In this area they will target adult EAB as they feed prior to mating and oviposition of eggs. While small amounts of insecticide may be found in the trunk tissues, most of this will be concentrated in the outer three rings of xylem. Since EAB larvae primarily feed in the cambial and phloem tissues they are less likely to come into contact with high levels of the insecticide even though some of the product will leach into these areas. Thus, we can expect to see lower levels of mortality here compared to the adults that feed on the leaves, although the product in the trunk may still lead to high larval mortality.
Emamectin benzoate behaves in a similar way, with large amounts of the product being transported to the leaves to target adults. Unlike neonicotiniods however, this product is believed to be more acutely toxic to EAB. This means that although once again EAB larvae will only come into contact with small amounts, these small doses are more likely to be fatal and may persist longer in the tree tissue.
It is important to remember that when trees are treated with insecticides, most of the product does not stay in its original form. Rather, it is metabolized into other compounds that then persist for a certain amount of time in the tree depending on the species. The metabolites of imidacloprid and dinotefuran have been found to break down fairly quickly in ash trees to the point that the tree needs to be treated yearly (Contrast this with Eastern Hemlock where metabolites remain for 7-8 years). Emamectin benzoate has been shown to persist for longer. However, current research suggests still treating every two years since metabolites present after this point may not be in high enough concentrations to effectively combat high EAB populations. It is also paramount to remember that ash trees must be in a fair condition for treatments to have the desired impact. In most cases at least 50 percent of the crown should be in good health for the treatment to have a chance. Usually upwards of 70 percent is preferred for a greater success rate.
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Compiled by David Olson, Forest Health Specialist
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