Controlling Magnolia Scale on a Mature Tree

Identifying the Infestation

A 20+ year old magnolia in Toronto (zone 5) shows white scales covering interior branches and heavy honeydew dripping onto hostas below, attracting ants and sooty mold. The dense canopy hides the infestation from outside view.

Why Sprays Fail on Large Trees

Foliar sprays cannot penetrate the thick canopy to reach scales on inner branches. Systemic soil treatments are far more effective.

Recommended Treatment

Apply imidacloprid (Bayer Advanced Tree & Shrub Insect Control Concentrate) as a soil drench around the root zone. Roots absorb and distribute it throughout the tree; scales ingest it while feeding and die.

Monitoring Success

• Dead scales become dry and don't "bleed" when crushed; live ones exude liquid.
• New foliage should appear healthier, with normal color and turgor.
• Sooty mold and sticky leaves will gradually disappear.

Timing

Apply in early spring or fall when the tree is actively transpiring. Follow label rates based on trunk diameter.