A. It would sound as though your star magnolia Magnolia stellata has magnolia scales, one of the largest scales in North America. The mature female is about 12.5 mm long, oval convex, smooth and dark brown and covered with a waxy coating. It overwinters as a nymph on one and two year-old shoots and the crawlers emerge in late August and September. There is only one generation a year.
You did not say at what time of year your tree was treated, but the best time is late August and early September. You might try using insecticidal soap on the crawlers, it would take several applications. Another suggestion would be to use a dormant oil in early spring before bud break, as this would smother the overwintering nymphs.
The yellow jacket wasps are attracted to the tree by the honeydew and sooty mould on the branches, which is created by secretion from the scale. If you are able to rid the tree of scale, the wasps should also no longer be a problem.
I do hope that this information is of help to you.
Master Gardeners of Ontario Inc.