Sounds like it could be a seasonal problem, the tree could be slightly stressed with the summer.
So ensure tree is getting sufficient water, preferably have mulch around it but not up to the trunk, organic fertilizer is always good beneath the mulch.
Due to the size of the tree you can forget about sprays which only leaves systemmic insecticides to do the work. It would be good to have natural biological controls, ladybugs etc feed on aphids. Ants are also bad news, if they're in the tree that's not a help, lightly band the trunk with adhesive tape so the ants cant travel up and down the tree, change it every few days, like masking tape applied sticky side out.
Now the danger of using chemicals is that the good bugs also get killed and the bugs themselves can build up resisitance to the chemical. So it's important to get the health and vigor of the tree into shape.
You can purchase a range of insecticides from the local nursery etc, I would suggest the application method be via soil treatment and watering in. The roots will take up the chemical. Follow directions carefully.
Some brands/names etc off hand are Talstar, Metasystox-R®. Try not to do tree injection ... why wound the tree. Remember that roots can travel further than the drip line.
There are natural alternatives but only as a foliar spray, once again, due to size you'll have to go with the soil application method.
Good luck