Pestipedia

Yew Scale

**Yew Scale** (*Parthenolecanium fletcheri*) is a type of soft scale insect specific to Yew (*Taxus* spp.) and arborvitae. The conflict is **plant stress and aesthetic nuisance**: the scale insects suck phloem sap from the twigs, weakening the plant and causing needles to yellow and drop. They excrete massive amounts of sticky **honeydew**, which coats the foliage and surrounding surfaces, promoting the growth of black **sooty mold**, leading to aesthetic damage.

Taxonomy and Classification

Yew Scale belongs to the Order Hemiptera (True Bugs) and the Family Coccidae (Soft Scales). They undergo simple metamorphosis. The scale is a single-brood species, reproducing once per year.

Physical Description

Adult females are small, 3 mm to 6 mm diameter.

  • **Female (Key ID):** Hemispherical or turtle-shaped, shiny, **brownish-yellow soft shell** (which is part of the insect); found clustered on twigs.
  • **Damage ID (Key):** Needles yellowing and dropping prematurely; excessive clear, sticky **honeydew** on branches; heavy coating of black **sooty mold** over the foliage.
  • **Contrast:** Soft scales produce honeydew; armored scales (like Yellow Scale) do not.
  • **Conflict:** Ornamental.

Distribution and Habitat

Found throughout temperate regions where Yew and related evergreens are grown as ornamentals. Habitat is the woody stems of the host plant.

Behavior and Conflict

The conflict is the honeydew production and cryptic life stage.

  • **Honeydew:** The continuous dripping of honeydew is the most common nuisance complaint, often attracting ants and flies.
  • **Cryptic:** Young scales (crawlers) are tiny and easily overlooked, allowing populations to build unnoticed.

Management and Prevention

Management is **Biological Control and Horticultural Oil**.

  • **Biological Control (Key):**
    • Protection of naturally occurring predators, such as lady beetles and lacewings.
  • **Chemical Control:**
    • Application of **dormant horticultural oil** in the late winter/early spring to smother overwintering nymphs.
    • Targeted treatment of the susceptible crawler stage in summer with insecticidal soap or oil.

    • Conservation and Research

    Research focuses on optimizing the timing of insecticidal oil sprays to maximize control while minimizing phytotoxicity to the evergreen foliage.