Scale Insects usually are not as harmful to plants as they are unsightly and as a cause for secondary pest problems such as the black mold that grows on the honeydew that certain scale species exude.
Found on bark, fruit and leaves, adult females are less than 1/10" in diameter, wingless, of circular or oval shape, have no distinguishable body parts or separate head while adult males have a single pair of wings, are rarely seen, feed for a few hours and die.
These tiny insects feed on the sap from a wide variety of plants from roses, grapes, raspberries, bird-of-paradise and shrubs to fruit, nut, palm and citrus trees. Severe infestations can cause a yellowing of leaves and ultimately, death of the plant.
The "Armored" scale varieties named for their flattened, hard covers, like California red, Oleander, Oystershell and San Jose, do not exude honeydew, produce several generations per year and mostly spend their entire lives feeding on one spot, enlarging their armored cover as they grow.
Conversely, "Soft" scales like cottony cushion, cottony maple, black, brown, and hemispherical are up to 1/4" in diameter, produce only one generation per year and exude honeydew which in turn, grows mold and attracts ants, hindering the predation of certain beetles and wasps.
As always, the number one rule in pest control is to identify the pest, so check with your local garden center, online or your county extension agent for help.
Many infestations of scales require no intervention as beneficial insects and parasites will keep the scale population in check. Yet during dusty, dry conditions or in areas where systemic insecticides have been persistently applied, horticultural oil sprays can be used to treat severe infestations. In addition, infected branches should be pruned and destroyed and soft scales can be scraped off with a dull blade or fingernail. Lastly, pest-prone plants should be replaced with cultivars that are proven pest-free under local growing conditions.