We have this information under the Management section of this page on Hemlock Woolly Adelgid: https://extension.umd.edu/resource/hemlock-woolly-adelgid-trees

Any tree larger than about 8' tall should be treated by a professional - a certified pesticide applicator. They will have the equipment (both to spray and to protect themselves from drift/drips) to treat the trees more effectively, since thorough coverage of the canopy is critical. Or, if foregoing sprays, they will be the only people legally able to apply the systemic pesticides needed to manage this pest. Either landscaping companies or tree-care companies (and occasionally independent garden pest consultants) should have certified employees.

Miri