Thank you for the question.  The injury from magnolia scale is from the nymphs that are under the protection of the mother’s covering or waxy coating. Since these nymphs are under this brown covering which limits the effectiveness of pesticides, timing is of the essence. The object is to kill the nymphs (crawlers) as they crawl out from under this protective shell usually in early to late fall (Aug-Sept). The key is to monitor them with a hand lens and when you first see the crawlers you will know it is time to act. The usual treatment is the use of horticultural oil which is available at most nurseries or box stores.  Neem oil would be fine.

Another option would be to use a chemical insecticide. Imidacloprid is a systemic insecticide applied as a soil drench. This insecticide is taken up by the roots and transported throughout the plant where it is ingested by the scale insect. Application can be done in late April through early May or August through September. 

Chemical insecticides available are Sevin (carbaryl), Bayer Advanced Power Force Multi-Insect Killer (cyfluthrin), horticultural oil, Bayer Advanced Garden Tree & Shrub Insect Control (imidacloprid), insecticidal soap, BioNeem (neem), and malathion. All are registered for control of magnolia scale crawlers. Depending on the insecticide, you may need to make repeat applications at the intervals recommended on the product label. Always read the label of any insecticide and follow their instructions.  
Thank you for contacting Extension.