The insect in the photo appears to be a type of scale possibly, Lecanium scale. 
Scales tend to thrive on stressed plants. Following a recommended fertility program and watering regime will promote plant health. However, over-fertilization favors scale buildup. If practical, improve plant sites to reduce stress and promote growth. You can prune out small infestations to try and eliminate the issue or prune out sections of a large-scale problem in order to make chemical control more feasible. 
There are two options when considering management of scales, doing a systemic treatment or treating the crawlers directly. Whichever you choose, management may take repeated applications over a couple of growing seasons.

Lecanium scale numbers building on chestnuts - MSU Extension

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Pam V.
MSUE Master Gardener