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Time To “Weigh In” on Scales

April 4, 2006
Although it may be “early” in the season to discuss scales, it is important to understand, at this point, the major differences between soft, or bark, and hard, or armored, scales and how this relates to management.

The characteristics of soft scale include

  • There is generally one generation per year
  • Produce honeydew
  • Typically overwinter as immature fertilized females
  • Appear convex in shape or resemble a helmet
  • Females typically can lay >1,000 eggs

The characteristics of hard scale include

  • There are generally two or more generations per year
  • Do not produce honeydew
  • Typically overwinter as either second-instar males or females, or as eggs underneath the female's body
  • Appear circular or rounded in shape
  • Females typically lay <100 eggs.

    Common soft and hard scales are listed in Table 1.

    Table 1. Common scales found in landscapes and nurseries

    Soft scales

    Cottony maple scale (Pulvinaria innumerabilis)

    European elm scale (Gossyparia spuria)

    Fletcher's scale (Parthenolecanium fletcheri)

    Magnolia scale (Neolecanium cornuparvum)

    Pine tortoise scale (Toumeyella parvicornus)

    Tuliptree scale (Toumeyella liriodendrii)

    Spruce bud scale (Physokermes picdae)

    Hard scales

    Hemlock scale (Abragallaspis ithacae)

    Obscure scale (Melanaspis obscura)

    Oystershell scale (Lepidosaphes ulmi)

    San Jose scale (Quadraspidiatus perniciousus)

    Winged euonymus scale (Lepidosaphes yanangicola)

    Euonymus scale (Unaspis euonymi)

    Pine needle scale (Chionaspis pinifoliae)

    Scurfy scale (Chionaspis furfura)

    Juniper scale (Carulaspis juniperi)

    Black pineleaf scale (Nuculaspis californica)

    Soft scales produce honeydew because the food canal carries large quantities of plant sap from the phloem tissues. Hard scales do not produce honeydew because the food canal contains various kinds of cells. As a result, they don't transport large quantities of plant sap. In addition, hard scales use their long stylets to explore large areas within plant tissues to obtain nutrients-not just the phloem tissues. This is the primary reason why systemic insecticides are not effective on hard scales.

    When managing scales with insecticides, which are listed in Table 2, it is important to understand the following points:

    • Repeat applications may be needed, as the eggs don't all hatch simultaneously.
    • Crawlers are most susceptible to insecticides because they have not formed a protective covering.
    • Systemic insecticides are generally effective on soft scales but not hard scales.
    • Dormant oils are targeted at overwintering immatures and females.
    • Contact insecticides are typically less effective on adult female hard scales.

    Table 2. Common insecticides for control of soft and hard scales

    Acephate (Orthene)

    Carbaryl (Sevin)

    Dinotefuran (Safari)

    Imidacloprid (Merit): for soft scales only


    Paraffinic oil (horticultural/summer oil)

    Petroleum oil (dormant oil)

    Potassium salts of fatty acids (insecticidal soap)

    Pyriproxyfen (Distance)

Author: Raymond A. Cloyd


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