This is a native species of scale that is usually kept under control by native parasites and predators. You can spray with a summer rate oil spray when the crawlers are active (July-August). Read the label VERY CAREFULLY for air temperature limits when you spray.
If the scale is low on the tree so that you can reach it, you can scrape the infested areas with a coarse-bristle brush (not wire) before you spray to increase the spray's effectiveness.
Even when the scale is dead, the scale covers ("shells") will remain on the trees for years. To see if they are live, remove the cover and look for living organisms.
Keep your trees as healthy as you can by not stressing them.