This insect has become a major issue in Calgary over the past couple of years. Oystershell scale is hardly recognizable as an insect. On close inspection it looks like a tiny oyster shell. It is one species of a group called armoured scales. Oystershell scale can attack fruit trees, lilacs, ash and elm but in Calgary these scales are most likely to be found on cotoneaster hedges. Scales may cover affected stems to the point that the stem appears to merely have rough bark.
Oystershell scales begin life as over wintering eggs under the shells of their now dead mothers. After hatching in spring, the small white crawlers emerge and move a short distance. They insert their sucking mouthparts into the stem to feed on plant juices and secrete a protective shell. Crawlers are easily blown to other stems or plants by the wind. Or transported on tools used by landscapers to prune and shape hedges.
Dead patches within a hedge including dry brown leaves are the usual sign of oystershell scale feeding. By the time this becomes alarming the scales have covered most of the affected branches.