All insect pests and diseases have characteristics that can give us the clues we need to create an Integrated Pest Management (IPM) plan to eliminate the cause or to manage the problem.
A proper IPM can remove pest problems with a change to the environment your trees are growing in without using harsh pesticides which kill all of the insects present including the benifitial
insects (Ladybugs, Dragonflies, Parasitic wasps) that may be helping to remove the pests:
Scales can me "washed" off with pressurized water during the spring when they start to crawl
Pressure washing plants in the spring can reduce the presence of other insect pests like Aphids and Lacewing bugs
Birch leaf minor can be reduce to levels that are unnoticeable with the addition of a mulch bed around the base of the tree and turned in the winter. A mulch bed around your birch trees
can aid in the retention of moisture, helping increase the health of the tree and help prevent Bronze birch borer
These are just a few examples of minor changes that can make a big difference. Rather than the "See bug, kill bug dead" theory.