That does look like peachtree borer on your sand cherry and your description of "granular" sounds like insect frass (i.e., excrement). It even looks like you may have a pupa of peach tree borer in a photo (circled in white). 

Once the larvae are in the tree, insecticides aren't effective, but you could spray to target eggs and any early larvae that are present and monitor for next year. Next year, you could use a pheromone trap to determine when adults are around and use that to time future spraying. You can read more about this in this CSU Fact Sheet: Peach Tree Borer (Fact Sheet 5.566).

Be sure the plants are receiving adequate (and not too much) water. This can help them recover and make them less susceptible to insects and disease. Trees and shrubs do best with less frequent, but deep watering. 

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