New to forum. Need help with Murraya PAniculata BOnsai

[david meyreles]

Hello evryone, I am new to the forum. I have a Murrata Paniculata bonsai, that has had certain branches, die off. First the leaves turn yellow then the branches die and I notice little white things, very tiny, that do not move on the bark, but i dont know if that is related. I started to repot it and noticed that the soil was full of very little very fast moving grayish insects, look like very tiny gray roaches, moving very fast. Does anyone know what this is? Could this be the cause of the sudden death of branches? and what should I do? I dont know if I can apply any kind of insecticide now that the roots have just been cut? What should I do???? I will try to take pictures of the tree and the branches..