Is it too late to do drench applications

The species of tree has some bearing on when to drench however Spring and Fall are your best times to apply. Keep in mind that you are applying the chemical to the variables of the ground which includes soil compaction and type. Sand soil will leach faster than clay so dig around the tree and see what the soil is like. Your chemical (usually in this case imidacloprid, can get bound up with soil particles. It can get absorbed by lawn roots or other plant roots. And you need to convince the tree to absorb the material - I include a small amount of plant food (very small) because over the years I've seen how weeds will tak in chemicals better with small amounts of fertilzer. Second is watering enough to leech into the soil but not through the soil.
Obviously this is about as scientific as shooting in the dark which means applying every year is important to getting control.
I have had great results with some species such as Ash keeping the drench right at the root collar but less results with Elm and only okay results with Maple.
One type of application probably will not solve all your insect issues.