Insect of the Month - Scale Insects

  Hort Snacks - February 2019
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 Diaspididae family (armored scales) or Coccidae family (soft scales); there are other scale families

Crops Affected: wide range of plants, including trees, shrubs and perennial plants, both deciduous and coniferous

Life Cycle:

  • Small sucking insects, which may also inject toxins during feeding
  • The majority of stages and types of scales are small, oval to circular in shape, flattened, lack wings and a separate heads or any easily recognizable body part
    • Most scales that are seen are female
  • Adult males are rarely observed; differ in appearance from females – size, shape, wings, antennae
  • Not all species have males
  • Life stages include adults, eggs, and typically at least 2 nymphal instars/growth stages
  • Mature adult females produce 50-200 eggs that may be hidden under their bodies or secreted externally under a protective cottony or waxy cover
  • Eggs hatch (within 1-3 weeks) into the first nymphal stage, referred to as Crawlers, as they are mobile
    • Crawlers are typically yellow to orange in colour
    • Crawlers walk over the plant, are moved on the wind or transported by carriers (people, birds, etc.)
    • Settle down and begin feeding within a couple of days
  • Settled nymphs do not typically move for the rest of their life cycle as they mature
    • Some species will move slightly or will move later in the season
  • Scales tend to be 1/8 to ¼ of an inch in diameter
  • Scales can be found on lower leaf surfaces, stems, branches, bark, etc.
Soft Scales (e.g. brown soft scale, European fruit lecanium, etc.)
  • Considered tropical and would be more of a concern in a greenhouse
  • Secrete honeydew, generally
  • Typically overwinter as second stage nymphs
  • May have tiny legs and antennae (all barely visible) and may move very very slowly after settling
  • Typically ¼ inch in diameter
  • Don’t produce a protective shield that can be separated from the insect body
Armored scales (e.g. San Jose scale, oystershell scale, etc.)
  • Have a hard, waxy shield
    • Shield may have a slight bulge/bump/lump/knob and visible concentric rings
  • Perhaps more of an issue in interior plantings (cooler than greenhouses)
  • Do not generally secrete honeydew
  • Typically overwinter as adult females and first stage nymphs – this is only the case in mild regions or protected areas
  • Most stages lack obvious appendages and spend their entire lives in one location
    • Crawler stages and adult males may have appendages and move somewhat
San Jose scale
Brown soft scale
Photo: United States National Collection of Scale Insects Photographs , USDA Agricultural Research Service,
Photo: Lesley Ingram,

  • In situations with a very abundant population of scales, some plants may appear weakened and slow growing
  • Plants may appear stressed, with yellowed leaves ad premature leaf drop
  • Some plant dieback may occur
  • Dead leaves may stay on dead branches
  • Sticky honeydew can be observed, as well as association visible symptoms, such as black sooty mould and the presence of colonies of ants
  • Check plants for scales or their symptoms (honeydew, sooty mold, ants)
  • The use of tape traps or other methods can be used to determine populations and presence of crawlers in spring
  • Confirm type of scale and whether scales are actually alive prior to considering controls
  • Typically not required for most plants that are healthy – even with heavy populations of scale, plants may not be unduly affected
    • Ensure plants are healthy and have sufficient moisture, fertilizer, etc.
  • Physically remove (and destroy) infestations; wash off crawlers and honeydew
  • Natural predators and controls can keep populations in check; introduced predators and parasites can be effective biological controls
  • Chemical controls must be applied at the correct time (prior to scales settling) and/or must be systemic
    • Horticultural oils applied in the dormant season can be effective
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on January 31, 2019.