Answer: The culprit is an insect called pine bark adelgids (Pineus strobi), and one of the recommended controls is to do just as you did - spray them off with a strong stream of water! These pests are an introduced species from Europe, but there are some native insect predators around that help keep their numbers down. However, they can't take care of an infestation like the one you describe. You can keep them at bay just through monitoring your trees and removing the pests periodically, but as far as I know there is no recommended repellent. Generally, aromatic herbs such as mint, garlic and tansy planted as companions around other plants offer some natural repellent action, and may work with your trees, but as they grow taller, will have less effect. I'll go out on a limb (no pun intended) and speculate that home-made sprays made of crushing these herbs in water may offer some repellent effect. If you feel like experimenting, it can't hurt to try. Good luck!
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