Galls, Diseases, and Pests - Scale Insects : European Fruit Scale
Parthenolecanium corni
Despite the name, this tiny insect is native to North America. It feeds off of a large number of species. At Rockville, I've found these scales on Toyon, Chamise, and Chaparral Pea.
When the insects are small, they can still move around on the plant. During this stage, they are called "crawlers."
An adult Scale Insect on Toyon.
And on the back of the leaf, a whole lot more of them!
Scales produce large amounts of waste called "honeydew"--a name that reflects its high sugar content. The honeydew not only attracts ants, it can also cause the growth of sooty mold fungus which blackens the stem.
Younger scales begin growing on the chamise leaves, but as they age they head for the plant's branches. In this photo you can see two of the younger scales on the left, one still located on a leaf.