Systemic insecticides work by entering the tree tissue and spreading through the whole tree. When the insect feeds on the tree tissue, it ingests the insecticide and dies. Hemlocks are commonly treated systemically for HWA with imidacloprid and dinotefuran. A systemic treatment with a product containing imidacloprid can last 5-7 years with one simple application to the roots or the trunk of the tree. The drift risk is minimal, and it is very effective. See our Chemical Product Comparison Chart under Step 5 of our Chemical Treatment for Landowners page for more information on different forms and brands of systemic insecticide for HWA.
Contact insecticides work by covering the insect and smothering it. Horticultural oils and insecticidal soaps are two common contact insecticides used for HWA. Contact insecticides only work to control HWA if you can cover every branch where HWA is present, particularly the underside of the branches, and must be done once or even twice a year to control HWA. HRI doesn’t use contact insecticides in our work, so we don’t have detailed guidance on using those, but some people prefer them as a less toxic alternative to systemics. There are some resources online, particularly on some state cooperative extension websites, about spraying hemlocks with oils or soaps.