Lebmung, may or may not kill 100% of the cottony cushion scale (CCS) but at this point giorgosgr's tree is heavily infested with the insect. A hard direct in line spray with his hose will certainly greatly reduce the highly damaging attach that his tree now has.
Also I found this on the University of California's web site about imidacloprid.
Do not apply imidacloprid (Merit or Bayer Advanced Citrus Fruit and Vegetables) for cottony cushion scale control. Although imidacloprid has scale insects listed on the label, it doesn’t kill cottony cushion scale. To make matters worse, imidacloprid is very toxic to vedalia beetles. The beetles are poisoned when they feed on cottony cushion scale that have ingested imidacloprid. Cottony cushion scale outbreaks have been observed following use of this insecticide because the vedalia beetles were removed and the insecticide didn’t control the pest