Late March/Early April is a good time to apply dormant oil spray to fruit trees, 
cane fruits (raspberries, blackberries), and ornamental shrubs and trees such as arborvitae, magnolia, yews, hemlock, spruce and junipers. Insects such as scale, mites and aphids are easily controlled this way.
The key is to wait for outside temperature to be above 40 degrees F. for a 24 hour period.
Use a commercially available Dormant or Volck Oil spray but you can also use household canola oil, too. I recommend a 2% spray solution. For homeowners, use a dial and spray bottle sprayer, with the gauge moved to 2 tablespoons per gallon. A tank sprayer can be used also.
Secondly, drench the leaves until spray ‘runs off’ leaves and branches.