If it is black scale you need to more reading.....control is multifaceted.

If your tree is a manageable size, you should physically remove the insects by hand to get a head start. Obviously, that's not something worthwhile doing if the tree is too big.

The are probably hundreds of different kinds of horticultural oils on the market, most of them can be used to help control scale insects on our plants. I have no doubt whatsoever that you can find a horticultural oil in any of your local garden shops.

Most importantly, you should do some research on black scale, their life cycle, the timing of reproduction in your location, etc. You need to know where the eggs are, what a parasitized individual looks like, and the identity of any beneficials that may be helping you control the scale.