At this time oyster shell scale may be causing more damage than any other insect in the Calgary area. Cotoneaster has been the hardest hit, but other plants are also susceptible -- apples, ash, aspens and others.
Scale feeds by sucking the fluids of tissue underlying the bark. During heavy outbreaks plant death can occur. There is one generation per year. The insects overwinter underneath their protective scales.The eggs hatch at end of May/start of June. Seasonal variations often occur. The emerging young scales are called crawlers; they move over the bark in search of a feeding site. This is the only time they are exposed, so spraying during the crawler stage is essential for control. They are tiny, much smaller than their host scale and it can take some practice to see them. Depending how the season is progressing, crawler activity can begin by late May. Crawlers can be shaken off infected limbs onto a sheet of paper, or caught with scotch tape.
If you find them, spray immediately. Once established at a feeding site they continue to grow, and are full size by approximately mid summer. Firmly entrenched under new scales the eggs are laid in early fall and they overwinter to begin the next year's conquest. Dormant oil application and pruning out of infected branches can be effective.
Rubbing branches with a coarse scouring pad that doesn't cut the bark, works very well. It all depends on the plant and the degree of infestation.