Hi Linda, Thank you for your inquiry. This is magnolia scale, an insect that seems to be running rampant in West Michigan this year. The white-covered adults are firmly attached to the limbs of your tree; they do not move at this stage of their life cycle.

Magnolia scale is a soft insect that sucks plant sap, and excretes large amounts of drippy, sticky honeydew. Honeydew provides an excellent growth medium for  sooty mold fungi, and these fungi can give affected leaves and branches a black, sooty appearance. In addition, honeydew and sooty mold can stain lawn furniture and other items in the vicinity of an infested tree, and sugars in honeydew can attract wasps and ants. Magnolia scale is seldom a direct killer of established landscape trees. However, a substantial loss of sap from a heavy scale infestation represents a serious loss of energy resources to the trees. The accumulated stress coupled with other stress-producing conditions may cause leaf yellowing and loss; branch dieback and canopy thinning, and even the death of entire trees.

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

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

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

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

If your trees are too large for you to treat or you do not wish to treat them yourself, the best thing to do at this point for treating them 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 trees 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.