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Insecticide Treatment of Hemlock Trees for Hemlock Woolly Adelgid, Adelges tsugae, in New York State
Mark Whitmore, Forest Entomologist, Department of Natural Resources, Cornell University.
Systemic insecticides are the best way to treat Hemlock Woolly Adelgid (HWA) in New York. Systemic insecticides should be applied in spring and fall when the soils are moist and hemlocks are actively growing. The spring treatment window opens when the soils thaw and the fall treatments can start with the rains and close usually around the end of October or in early November when soils cool down. The two most effective systemic insecticides recommended are imidacloprid and dinotefuran. Treatments with one of the several formulations of imidacloprid (active ingredient) registered in NY have been found to be effective up to 7 years with just one treatment. Imidacloprid can be applied by soil drench, soil injection, time-release soil tablets, trunk injection, or basal bark spray. The only formulation available to homeowners is a soil drench; all others must be applied by Certified Pesticide Applicators. The most widely available formulation for is Bayer Advance Tree and Shrub. Always read the label and follow its directions carefully for any pesticide application. For the soil drench you pull the leaves and other organic material 2 feet away from the base of the tree, then pour the correct amount onto the mineral soil. Homeowners should be aware that a soil drench can move through porous soil into waterways; it’s best not to use a soil drench within 75 feet of any body of water. Soil drench and soil injection work best when the soil is moistened after a rainstorm, not when it is dry.
The professionally applied time-release soil tablets (Core-tect) need to be placed just under the surface of the mineral soil near the trunk of the tree. Bulb planters have been one of the application tools of choice. One advantage of this formulation is that application can be made when soils are dry in summer since the imidacloprid is only released when the tablets are moistened. Another advantage is that because the full dose of imidacloprid is released over a two year period, twice as many trees can be treated per acre than with other application methods.
Techniques that inject imidacloprid into the tree after drilling a small diameter hole into the xylem have been demonstrated to be effective. Injectors that do not use a drilled hole, but use a needle to push the imidacloprid into a pocket created between the inner bark and wood, have not been found to be effective. Although injection is a time consuming application technique it is useful near water because the imidacloprid does not come into contact with soil.
One of the drawbacks to imidacloprid is that it moves slowly through the tree, sometimes taking up to a year to reach the canopy. Older trees that may have compromised vascular systems or crown decline from HWA may not be able to move imidacloprid into the crown fast enough to survive. A recent development in New York is the Special Local Needs (SLN) registration of dinotefuran (active ingredient), under the trade name Safari 20SG, to be used as a basal bark spray. This is significant because Safari moves into the tree canopy much more rapidly than Imidacloprid, usually within 2 to 3 weeks. The basal bark spray technique uses a low pressure sprayer to apply the product to the bark surface on the basal 5 feet of the tree trunk. Application is made so the bark gets wet, just to the point where it is about to drip. No surfactants are necessary for the product to penetrate the bark. It is faster and easier than injections and is non-invasive (no holes need to be drilled). Basal bark spray can also be used near waterways since no product comes into contact with the ground. Safari is a restricted use pesticide in New York and must be applied only by Certified Pesticide Applicators.
Safari is indispensable when treating large, old trees and any tree that has crown thinning symptoms. The one drawback is that its efficacy is limited to a single year. It is therefore prudent to treat with imidacloprid at the same time. In these circumstances Safari will rapidly reduce HWA so the trees can recover and have time to take up imidacloprid for more long-term protection. In university trials imidacloprid has also been shown to be effective as a basal bark spray. To facilitate rapid and cost effective treatment with both pesticides at the same time some imidacloprid products were recently registered (2ee) in New York for use as a basal bark spray by Certified Pesticide Applicators. Now both imidacloprid and Safari can be applied at the same time as a basal bark spray in a tank mix thereby affording fast protection for up to 7 years with one application.
Tank mixing Safari 20SG and imidacloprid as a basal bark spray for Certified Pesticide Applicators:
In 1 gallon of water mix 9.0oz dry weight of Safari 20SG. There are 128oz in a gallon so at the labeled application rate of 2.0oz/diameter inch there will be sufficient fluid to treat 64 diameter inches. The imidacloprid 2F label states that 0.2oz is necessary per inch of trunk diameter. Therefore the amount of imidacloprid necessary to treat 64 diameter inches would be 12.8oz. For 1 gallon of Safari 20SG and imidacloprid 2F tank mix you need 9.0oz Safari 20SG and 12.8oz imidacloprid 2F.
You then need to calibrate the amount of time it takes for your sprayer to deliver 2oz of liquid given a constant pressure. You can then multiply that amount of time by the diameter of the tree to determine the correct amount to apply.