Chemical control of olive scale revolves around pyriproxyfen-based products, which is an insect growth regulator. For this reason, application timing is crucial because insecticides are effective only against the crawlers (the most sensitive stage of the olive scale). Therefore, spraying applications should take place when crawlers emerge.
One can use traps to determine when crawlers are hatching. In spring, take transparent tape that is sticky on both sides and encircle several branches of infested trees. Change the sticky tape on a weekly basis. Use a hand lens to identify the crawlers; they will stick to the tape and look like tiny yellow dots.