I think the problem you're having is coming from the growing medium.
As you said, the plants in the ground are fine. I'd be looking at the watering situation before blasting them with pesticides. Imidacloprid will be useless if the soil dries out since it will be bound to the soil.
My suggestion would be to enhance the soil biology by adding foods such as fish hydrolysate, humic acid, compost and seaweed powder. This is a long term fix. By doing this, you'll enhance the resiliency of the soil and reduce watering needs.
Most pests are symptoms of soil problems.
I spent 1/2 my career in arboriculture treating symptoms with pesticides which does not work (meaning you have to apply pesticides time after time, year after year).
Careful with Japanese maples, you can damage the foliage even with a blast water.