You should go cheap, easy and safe with insecticidal soap this late in season. Foliage aphids are easy to kill. This late in the season you don't have to panic. As temperatures drop. Hopefully it will get cold enough soon enough for your crepe myrtle to go dormant and defoliate, SHEDDING ITSELF of the aphids. They should be gone on their own soon enough. Are you seeing them active or just seeing alot of sooty mold? How large of crepe is it? Can you spray all areas needed? Most of the sooty mold will go away with defoliation also.
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