Question
I live in Ohio.  Magnolia gets morning full sun most of the day.

I have a 10 yr. old Magnolia bush, which I have never had problems w/until now.  There are whitish/grayish areas on the leaves & the bush is swarming w/flies and bees.  Help!

Answer
Sounds like Magnolia scale, an insect. This small insect lives in a scale and sucks the plant juices from the tree. It deposits a sticky substance called honeydew and a fungus called sooty mold grows on the honeydew. The bees are feeding on the honeydew. To control the sooty mold you will have to control the scale. Here is a web link to information on the scale and its control.

http://ohioline.osu.edu/hyg-fact/2000/2003.html

I would treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied around the tree to the soil and the roots will transport the product to the twigs , branches and leaves. And when the insect sucks the plant juice it will be killed. Check with your local nursery fro this product.
Here is a link to more information on this product.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html