Many gardeners know dormant oil is extremely useful in controlling scale and mites effectively BUT timing is essential.
Wait until the daily temperature is at least 40 degrees F. (4 C.), and will stay that way for at least 24 hours and when in a 24-hour period, no rain or high winds are predicted.
You can use a commercial product or a 2-3% solution of Canola Oil with a 
squirt of Dawn Dishwasher soap. (2-3 tablespoons per gallon). Bottle end sprayers are helpful if you have a lot of bushes to do.
Drench the branches and trunks. Start at top and move around the bush to soak it down.
Yews, magnolia, apples, crabapples, plums, quince and pears all benefit from dormant oil treatment, as do gooseberry and currant bushes. Know blue junipers will lose their blueness afterwards. Do not spray japanese maples.