Magnolia scale are insects with sucking mouthparts, which they use to remove large quantities of sap from twigs and young branches. Heavily infested branches can be completely covered by scale and may be weakened or killed by it. Magnolia scale also produces a sticky, sweet substance called honeydew, which attracts ants, yellow jackets and other insects that feed on it. The honeydew is largely concentrated plant sap excreted by the insect. A fungus called sooty mold can grow on the honeydew, resulting in blackened leaves and branches, greatly reducing the ornamental value of the tree. 

Controlling the scale depends on the stage of the insect. June to
early July is the time to apply a systemic insecticide which is
absorbed by the roots and will move from the roots into the
branches. The next stage in the insect’s life is the active crawler
stage which occurs in Michigan in August through September when you can spray with a contact insecticide. You should not need to use the second application if you do the first.

MSU’s Enviro-weather site has a tool which states when the crawlers are active.

http://msue.anr.msu.edu/news/magnolia_scale_is_starting_to_appear_on_magnolias

The links below will give you full information about this insect and the best treatment options.

https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now

https://extension.umn.edu/yard-and-garden-insects/scale-insects

If your tree is too large for you to treat or you do not wish to treat
it yourself, the best thing to do at this point for treating it is to consult a certified arborist. An arborist is a certified and 
generally insured expert in trees and their problems. He or she can examine your tree and make recommendations regarding the health of the tree, any needed judicial pruning, and what type of treatment is the best option. Hiring a certified arborist will ensure that you are getting a knowledgeable and qualified tree specialist. You can find a certified arborist in your area here:

http://www.treesaregood.org/


Click on "Find an Arborist" at the top of the page; then click on
United States from the pull-down menu. Then enter your State and City. This should bring up a list of certified arborists nearest to you. Or you may be able to locate one under "Tree Services" in your local Yellow Pages.

Every tree and situation is a little different and in this case, an on-site arborist is the safest and most effective way.