Scale on flowering trees
There are many types of scale insects and the scale needs to be identified before a control can be recommended.
For example, There are several types of scale insects that can be pests of magnolia and we do not know which ones you are referring to. Scale insects in general are attracted to stressed plants. Keep your plants healthy by watering during dry periods and Do not over fertilize. If the trees look healthy and there is minimal damage no control is necessary. It is possible that beneficial insects may be keeping the pests under control. See our publication on maintaining a healthy landscape http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG62%20IPM%20A%20Com...
Send digital photos of the scale insect and the trees if possible.