Answer: As with any pest, understanding the scale's life cycle aids in effective control. Here is a link to an Ohio Cooperative Extension Fact Sheet that has photos of adult and juvenile scale (called crawlers), so you'll know what to look for:
The bulletin goes into detail about how to control the pests as well. You'll have to wait until late summer to take action that will be effective, since it will take that long for the crawlers to become most vulnerable to applications of horticultural oil. Until then, you can support your magnolia by keeping it watered if there is a lack of rain. Do not fertilize the tree aside from a topdressing with compost. It may also help to spray your tree with kelp, as the nutrients and compounds in kelp help plants maintain resistance to pests and diseases, and it may help your tree resist secondary infection or pest infestation. You can find powdered kelp for this purpose at Superior Growers Supply (they're located in Michigan): http://www.sgs-hydroponic.com/sg14.htm, e-mail firstname.lastname@example.org.
I hope this helps!
Q&A Library Searching Tips