In last few years, the cotoneaster hedges in Calgary are severely infested by Oystershell scale which causes die back of the hedge and makes dead patchy spots. Oystershell scales suck the fluids of cells underlying the bark, often killing the cells at the feeding site.
The eggs undergo overwintering in the shells of dead mother scales and they start hatching in spring. Crawlers emerge and move to find new sites and start sucking plant juices and secrete a protective shell. As summer starts, they gradually increase in size and become full-grown in midsummer. Eggs are laid in late summer and early fall and the mother scale dies at the end of the season. Eggs produced in late summer remain under the protective wax cover of the mother throughout winter.
Oysterscale is one of the “Armored Scales” in which adult has the outer shell that is impenetrable to traditional pesticide sprays. Therefore timing of treatment is very critical for controlling Oystershell scale. The best time to spray insecticides is immediately after babies, called “crawlers,” are hatched and have not developed their protective armoured coating. This is generally from June first week to third week in Calgary, depending on weather.