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Oystershell scale treatment for trees

Image of a tree with a filed and a forest in the background

Oystershell scale, or apple mussel scale, is a potentially fatal tree disease that affects many trees and woody plants in Western Canada. This disease is caused by oystershell scale insects and has been quite destructive in many residential neighbourhoods. The good news is there are ways to protect your trees from this menacing pest. Here’s what to look for and how to treat oystershell scale.

Cause of Oystershell Scale

Oystershell scale is caused by tiny scale insects that attach themselves to a tree’s bark and suck out its sap for nourishment. Moreover, these pests form a tough, waxy exterior, which protects them from predators, weather and most pesticides.

Oystershell scale has been recorded on over 120 species of plants and trees. For instance, it’s been found on aspen, apple and maple trees. However, the insect’s favourite tree is the cotoneaster hedge. Other at-risk trees include:

  • Hawthorn
  • Mountain ash
  • Green ash
  • Lilac
  • Poplar
  • Dogwood
  • Saskatoon
  • Amur cherry
  • Schubert chokecherry
  • Willow

Oystershell Scale Life Cycle

Once a year, female insects lay tiny white eggs that hibernate underneath her waxy cover. The eggs hatch between late May and early June. Once the small crawlers emerge, they start looking for a new location to feed. They do so by permanently attaching themselves to a tree’s bark and quickly forming a protective shell.

In addition, the wind can blow crawlers to other stems and plants. Critters like squirrels and birds can also spread the insects from tree to tree.

Symptoms of Oystershell Scale

The signs of oystershell scale aren’t usually evident until your shrub or tree is heavily infested. The tell-tale sign are tiny grey or brown, oyster-shell-shaped bumps covering the tree’s bark. You may also notice yellow leaves, dead branches or gaps in your hedges.

Oystershell Scale Control and Treatment

It’s best to treat your trees for oystershell scale with a non-toxic pesticide when the crawlers emerge in late spring. However, the treatment window is narrow. You only have about seven to 10 days to act.

Furthermore, infested trees can benefit from a holistic plant management program. For instance, the trained arborists at ArborCare can apply an effective horticultural oil to your trees to help control insect activity. The oil is an all-natural solution that softens the insect’s shells, suffocates the eggs and deters feeding.

You can also protect your trees by pruning dead and infected branches. Cotoneaster hedges may need to be cut down to the ground to eliminate pests. However, the hedges regrow very quickly.

Tree Disease Treatment in Calgary, Edmonton, Kelowna and Winnipeg

At ArborCare, our mineral-based horticultural oil can effectively treat oystershell scale without damaging your plants and trees. Our experienced arborists can apply the product as part of ArborCare’s plant management program. You can count on our team to provide regular care to all your plants to keep them healthy and happy. Contact us in Calgary, Edmonton, Kelowna and Winnipeg for more information or to book a tree assessment and consultation.

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