Spraying Wooly Adelgid
The most lethal species of adelgid infests the hemlock tree. Hemlock woolly adelgid is an invasive species that can kill a tree in a couple of years. However, there are other species as well including our native pine woolly adelgid. Pine trees seem to resist the adelgid (a relative of the aphid and scale insects) whereas the hemlock will succumb.
I'm not sure which species of adelgid you have, so here are the recommendations from the University of Maryland for hemlock woolly adelgid. The horticultural oil coats and suffocates the insect,
Spray a summer rate 2% of horticultural oil after all of the eggs have hatched and crawlers are present usually by mid-June.
You can spray a dormant rate 4% of horticultural oil from November to early March before eggs are laid. See our publication for more information http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG2%20Hemlock%20Wool...