Armored scale insects have a waxy covering that protects them from most pesticides. You'll probably never actually see these things move — they're kinda like currants stuck on the plant. Scales seem to begin clustering in or near leaf or branch junctions as well as on tender new growth.
Scale insects can be serious pests on all types of woody plants and shrubs by sucking sap from host plants. If infestations are heavy, they may kill branches or entire shrubs or trees. Scales are so unusual looking that many people do not at first recognize them as insects. Armored scales and soft scales are the most common types of scales on woody plants. Adult female scales and many immature forms do not move, are hidden under a disk-like or waxy covering, and lack a separate head or other recognizable body parts. Scales have long piercing mouthparts with which they suck juices out of plants.
Most scales have many natural enemies that often effectively control them, but ants protect scale from these beneficials. If scale is a problem in your yard, you may also notice ants climbing up the plant as well. So try to reduce the ant infestation to allow beneficials a chance at controlling scale. While some horticultural oils and pesticides claim to control scale, possibly the best and lowest impact solution is the use of a toothbrush, water and mild detergent, such as Simple Green. Oils and pesticides often cannot penetrate the scale insect's wax-like armor, while the toothbrush will easily knock off the insects; following up with a horticultural oil or soap can finish the job for you. In Davis, this may require only annual or semi-annual maintenance to manage the problem.
UC Integrated Pest Management program provides a wealth of information on pest control, especially in the Davis area, while Bob Villa's site has a great primer on fairly safe pesticides. To learn more about the various flora growing in our town, visit our Town Flora and our Town Wildlife to see what other critters infest our neighborhoods.