This is just a thought and I have no idea if it will work. But as cicadas are insects and I assume the the nymphs have soft bodies, maybe an Insecticidal Soap drench will do it. Get some Insecticidal Soap concentrate - this is going to take a lot of IS. Mix it according to the directions on the package. Hopefully you have a tub big enough to hold the pot the tree is in - soak the entire pot up to the rim in IS until the soil is completely saturated. If you have nothing that large, mix some up and pour it into the pot, making sure to cover the entire surface area. Mix a couple bucket fulls and keep dumping to hopefully reach saturation in every corner of the sorl. Insecticidal Soap is a contact poison so the nymphs must be coated for it to work. No guarantees but this would be my approach.
https://www.amazon.com/dp/B00192AO90/
You could also use a systemic but that would make the entire plant poisonous to anything that eats any part of the plant for several months, including bees and you.
