Imidacloprid (most popularly by Bayer, but also Bonide and in Bio Advanced 3-in-1 spray) its one of the most commonly used insecticides. Both it and Safari are systemic, and work anytime, but most apply them mid to late spring after trees have leafed out. Both can harm bees, so flowering trees when flowering shouldn’t be exposed, but otherwise their use is fine. Perhaps the recommendation to use in fall is so it will be in the tree’s system over winter and early spring, but otherwise, I don’t understand the logic.