Control of magnolia scale can be challenging. Only young, mobile crawlers are easily controlled. Proper timing of the application of chemical products is critical. If you decide to use spray treatments for control, select a product that contains bifenthrin, carbaryl, cyflulthrin, horticultural oil, insecticidal soap, malathion, or permethrin. Apply spray treatments in late August or early September and follow with a second treatment 10 to 14 days later. Systemic insecticides, such as those containing imidacloprid, have also been effective in magnolia scale control, but such products must be drenched into the root zone in early May. Finally, dormant oils can be used for control, but must be applied in March, before buds open. Be sure to read and follow all label instructions of the insecticide that you select to ensure that you use the insecticide in the safest and most effective manner possible. Excerpted from:

https://hort.extension.wisc.edu/articles/magnolia-scale/

Be certain you are watering during this hot dry period and during all other similar conditions this summer. Maintaining healthy trees is an important component of magnolia scale management, particularly for non-native magnolias. The first and best approach to managing this native scale is to reduce tree stress-inducing conditions such as watering trees during a drought. Excerpted from another Fact Sheet with very detailed information about every stage of this pest, as well as offerings for possible non-chemical control. It can be found here:

https://bygl.osu.edu/node/1986

One more with the basics:

https://ohioline.osu.edu/factsheet/ENT-61