Hi ljs, and welcome to ATP.
I'm not sure about plum trees, but I do know that garlic and tansy planted beneath most fruit trees will repel those bugs that you don't want. Tansy repels flying insects, especially Japanese beetles but it also adds potassium to the soil. Garlic takes care of caterpillars and mites.
I'm sure there are others that work well as companion plants, but these two are always my favorites.
