Creating a schedule for when to spray plum trees for insects is helpful if you are as absentminded as I am. You can do this by specific dates or, more importantly, maintain your schedule by the stage of the tree. For instance, is it in a dormant phase, is it actively growing or is it fruiting? Whichever works for you, the important thing is to stick to the annual spray maintenance schedule for when and what to spray on your plum trees.