You may be seeing scale on your plants, trees and shrubs. Scale has two life stages: a “crawler” stage and an immobile developmental stage.
Crawlers look for good places on the plant to feed, settle and move into the motionless developmental stage. They feed on the sap inside branches and produce honey-dew that accumulates on stems, leaves and even the base of the plant. In humid areas, fungus may grow on the honey-dew which can produce sooty mold.
Parasitic wasps and ladybugs are natural enemies to scale insects. Keep a look out for a large population of these and you may not need to spray.