Scale insects that suck sap from the host tree can cause damage, if they remove large amounts, but otherwise, magnolias tolerate small infestations. One thing to do; keep the tree healthy by pruning dead branches, watering, and mulching. Depending on how widespread the scale infestation on your magnolia is, you can manage it without chemicals. Neem oil really can be effective, but the process for killing off insects is very slow. It is a product that messes with insects' brainwaves, causing them to stop eating and mating -- eventually. You might find that it will be very effective in the long run, but it is not a chemical insecticide that works overnight. Scale insects are vulnerable when they are in the "crawler" stage -- insecticidal soaps and horticultural oils typically do work on them --- but not so much as adults. Adults die off after reproducing (you can remove their bodies by hand). I cannot make a particular brand recommendation here, unfortunately, but you can check with your local nursery or garden store for insecticidal soaps and organic products that will help destroy scale in its infancy.