Darrel, Magnolia scale can be tough to control and it's not often a one and done control process.  I would not recommend a second Imidaclopit drench now.  A drench treatment can take several weeks for the insecticide to work its way through the tree's roots to the parts of the tree where the scale is located.   With fall approaching, the scale insects may be slowing or have stopped their feeding when the insecticide reaches them.  And Imidaclopit hasn't been shown to always be effective against magnolia scale.  Plus Imidaclopit is a neonicotinoid pesticide which can harm beneficial insects.  Dinotefurin, also a neonicotinoid, can be used but its effectiveness against magnolia scale varies.  

  Now, or soon, the crawler stage for the magnolia scale will begin.  Control of magnolia is most effective at this stage using horticultural oil or insecticidal soap.  The magnolia scale overwinters as a crawler and then resumes crawling in the spring, providing a second opportunity for control.  More info on the crawlers is in the link.  Also, if needed, a drench application can be used in the spring using insecticides recommended in the link below.

  If your tree is large or the scale areas on the tree are hard to reach you can consider contacting a certified arborist for an evaluation. 

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Magnolia Scale