Why are cassia tree leaves turning yellow?

Last year I had a tree and a bush with yellowing leaves for a few months before I pulled them up, to find borers. Without much thought I planted a small bush in the same area about 3 months ago. Some of the leaves are now yellowing. I have looked at the base and cut off 1 limb without any evidence as yet of the borer. Do borers stay in the soil? For how long? I believe the first 2 plants had the borers when I bought them from a local nursery(Eustis,Fl.).