Trees infested with hemlock woolly adelgid sport what looks like tiny white cotton balls. The most common treatment for most tree infestation is spray insecticide. But especially with tall trees, coverage can be difficult. And since the sprays work by contact, any missed hemlock woolly adelgid will live to breed another season. The best way to get rid of hemlock wooly adelgid on mature trees is to introduce the chemical imidacloprid to the tree’s tissue. This powerful insecticide is not sprayed but absorbed through the soil or trunk of the tree, where it is delivered to the hemlock woolly adelgid through the sap that it feeds on.
Introduce imidacloprid by pouring it into the soil. Unlike other methods, this does not require any special tools. First, clear any debris from the base of the tree then water it. Next, mix the appropriate amount of foliar spray with enough water to facilitate even coverage within the tree's drip line (again, according to the manufacturer's instructions), then pour. Then water the tree with at least as much water by volume as insecticide solution to speed its delivery to the tree's roots.
Introduce imidacloprid to the tree through soil injection. To inject this herbicide, you must first purchase or rent a hand-powered Kioritz soil injector. But this method is more reliable than the first as it allows you to inject the insecticide where the tree's roots can readily access it. First, water the tree. Then fill the injector with the manufacturer's recommended amount of the foliar spray and inject it 3 to 5 inches into the soil within the tree's drip line.
Introduce imidacloprid by injecting it into the trunk. This method is the most invasive, but the best for those who are concerned about ground contamination. Drill several holes into the base of the tree and inject imidacloprid formulated for trunk injection. The number and depth of the holes is dictated by the circumference of the tree and are prescribed by the manufacturer.