Oystershell scale: a major problem in cotoneaster hedge in Calgaryby mrgadmin on April 22, 2023

In last few years, the cotoneaster hedges are severely infested by Oystershell scale which causes die back of the hedge, fail to leaf out and make dead patchy spots. Oystershell scale affects several plant species including apple, Hawthorne, lilacs. It is a major problem in cotoneaster hedge in Calgary and surrounding areas. Developing oystershell scales feed by sucking the fluids of cells underlying the bark, often killing the cells at the feeding site.

Over winter eggs under the shells of dead mother scales hatch in spring, and crawlers emerge and move about to find new sites and start sucking plant juices and secrete a protective shell. Within few of months, 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 crucial for controlling Oystershell scale.

The best defense against Oystershell scale is prevention, keep the hedges healthy by thinning the thick ends of branches and stems to allow more light and air penetration to the inner canopy.

If the Oystershell scale only affects a small area, spot spraying with insecticide kills the bugs and stop further spreading. The best time to spray the insecticides is immediately after babies, called “crawlers,” are hatched and have not developed their protective armored coating. This is generally from first week to third week of June in Calgary, depending on weather. Chemical spray is only effective during this time period. Spraying systemic insecticides offers control for several weeks. However, if the infestation is really severe, it is better to mow the hedge back to about 6- 8″ above ground in late winter, before bud break.  In June when crawlers start feeding, spray the sprouting stumps. Monitor the pest population is required as new hedge starts growing.  Another spray application after 10-15 days will guarantee better control of the pest. A new hedge will grow back within a few years as long as the roots are healthy. Slow release granular fertilizers can be applied to enhance the growth of the hedge.