I can't identify the insects from your description but they sound like the crawler stage of scale insects. These pests attach themselves to the stems and branches and feed by sucking out the plant juices. When they feed they excrete a sticky substance called honeydew and as this honeydew ages it becomes black and is called sooty mold. To keep the sooty mold from forming you'll need to control the insects. In the wintertime you can use a horticultural oil (called dormant spray) which will suffocate any overwintering insects. Wait until all the leaves fall from the tree and spray with horticultural oil according to label directions. Hope this clears up the problem!
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