Hello Thomas

A large tree can be sprayed at the correct time in the insect’s life cycle using professional equipment. You can hire a certified arborist to do that. Find them by zip code search here - http://www.treesaregood.org

If you are willing to use a soil drench insecticide, you can do it yourself. But, you must apply enough insecticide based on the tree size. The active ingredient to control scale via a soil drench is imidacloprid or dinotefuran. There are several brands on the market available at garden centers. Read the label and measure your tree’s trunk to determine how much you would have to purchase. ( most labels are available online to “read ahead”)
Here are bulletins with more details-
https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
https://extension.psu.edu/magnolia-scale

As you can see from these bulletins, the time to treat the scale will be later this season. In the meantime, keep the tree well watered during droughty times this summer.

Thanks for your quick response

You are welcome!

And I forgot to mention, you may need to treat the tree for a couple years if the infestation is extensive.