On small trees, (with a canopy smaller than 15' tall and 15' wide) I have seen noticeable improvement by introducing...ladybugs! Aim for a dose of about 300 bugs/weekly (most will fly away) for as long as the client can tolerate ($).
On bigger trees, you'll need a systemic with something like imidacloprid (did I spell that right?). I tried a pesticide spray and it didn't do squat.
love
nick