WSU Extension


Annosus root rot 
Phytophthora root rot 
Hemlock scale 
Hemlock woolly adelgid 

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Caption: Hemlock scale
Photo by: R. Van Denburgh
Hemlock : Hemlock scale
(revision date: 2/14/2019)

Hemlock scales feed on the needles of Eastern hemlock (Tsuga canadensis) and spruces (especially Colorado blue). Adult scales are round to oval, dark gray or black in color, and about 2 mm (1/10") in diameter. Immature scales (crawlers) are green to yellow. Scales are typically found as small bumps on the underside of needles. As few as 4-6 scales per needle will cause the needle to drop. Colorado blue spruce in particular loses large numbers of needles. Severe infestations may weaken trees sufficiently to cause death. The hemlock scale is most common on stressed trees.
Management Options

Non-Chemical Management
  • Provide proper culture. Healthy plants are less likely to be infested and more tolerant of damage.
  • Prune and destroy branches with heavy, isolated infestations (when practical).
Select non-chemical management options as your first choice!

Chemical Management

Oils are effective in early spring. Apply all products (excluding oil and dormant sprays) to control the immature crawler stage in early summer. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • All Seasons Horticultural & Dormant Spray Oil Conc [Organic]
    Active ingredient: oil/pet distillate  |  EPA reg no: 4-80
  • Bonide Systemic Insect Control
    Active ingredient: acephate  |  EPA reg no: 239-2461-4
  • R-T-U Year-Round Spray Oil
    Active ingredient: oil/pet distillate  |  EPA reg no: 6218-78
  • Safer Brand Insect Killing Soap Conc II [Organic]
    Active ingredient: potassium laurate  |  EPA reg no: 42697-60
  • This list may not include all products registered for this use.

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Caption: Hemlock scale
Photo by: R. Van Denburgh