Oystershell Scale

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.

Tree Damage

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.

Control Measures

  • Chemical sprays are effective only on the crawler stage in the early Spring.
  • Horticultural oils may be effective in suffocating eggs if applied before the tree or shrub leafs out, and where the infestation has not progressed too far.
  • Pruning out affected stems or cutting the entire hedge to ground level is the most effective control once dead patches have become evident in a cotoneaster hedge. Cotoneasters with healthy root systems will quickly re-grow. Within a few years, a hedge may be a couple of feet tall again. Monitor re-growth and apply horticultural oil to suspected scales during early spring before leaf-out.