euonymus scale

Euonymus scale. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Updated: April 26, 2022

Key points

  • Euonymus scale (Unaspis euonymi) is a non-native type of armored scale insect. They can be difficult to manage as their waxy body covering shields them from predators and certain pesticides. Learn more about scale insect groups, biology, and management on our Introduction to Scale Insects page.
  • This is a common pest of euonymus and can cause plant decline and dieback when populations are high. Groundcover pachysandra, as well as the euonymus relative bittersweet, are also susceptible (both the native and invasive species for each).
  • Target monitoring and control efforts to the vulnerable crawler stage. Learn more about what to look for on the Monitoring for Scale Insects page. 

Euonymus scale details

euonymus scale male and females on stem
Male and female Euonymus Scale covers on a plant stem
Photo: D.K.B Cheung

Appearance

  • Mature female covers are oystershell-shaped, brown-black, and up to ⅛” (3 mm) in length.
  • Male covers are thin and elongate, white, and smaller.
  • Crawlers are yellowish-orange.

Euonymus scale illustration

Common host plants

  • Euonymus (mainly evergreen species, especially Euonymus japonicus, the hybrid cultivar ‘Manhattan’, and the invasive species winter creeper, Euonymus fortunei)
  • Japanese pachysandra (Pachysandra terminalis) and Allegheny spurge (Pachysandra procumbens)
  • Boxwood (Buxus)
  • Bittersweet (Celastrus) – note that Oriental bittersweet is invasive.

Where to look

  • Mature females will mainly be found on the bark of branches of all sizes.
  • Immature males will mainly be found on the foliage.
euonymus scale on branches
Heavy infestation of Euonymus scale on lower branches, giving them a snowy appearance
Photo: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org
white euonymus scale covers on leaves
Euonymus scale covers on the top of foliage
Photo: Brian Kunkel, University of Delaware, Bugwood.org

Plant damage

  • Moderate infestations may result in stagnated growth or stunting and leaves will develop yellow spots.
  • Heavy or prolonged infestations can result in the death of branches or entire plants.

Life cycle

  • There are 2 overlapping generations per year in Maryland.
  • Crawler emergence periods depend on temperature and can vary slightly from year to year. The approximate times to monitor for them are:
    • First-generation – late April through early June
    • Second generation – early August through September
  • They overwinter on the bark as mated females.

Management

Refer to our general scale management recommendations for both chemical-based and pesticide-free options. When pesticides are warranted, a combination of dormant oil applications and the use of systemic or growth-regulating insecticides is the most effective approach. For large populations, scale suppression may require more than one year of intervention, and professional pesticide applicators will be needed to apply certain treatments.

The overlapping generations mean that monitoring and treatment will need to occur several times a year to be effective. If you do not wish to use pesticides and are not obtaining good control from manual removal efforts alone, remove and replace the infested plants. Plant removal is strongly recommended when the infested plant is an invasive species.

When not too weakened, Euonymus regenerates well from harsh pruning. Cut back heavily-infested plants almost to the ground so regrowth will be easier to monitor; this also greatly reduces pesticide use since there will be fewer plant surfaces to spray.

Pesticide sprays should be avoided when red-spotted black lady beetles are present, as they are a scale predator.

Timing details for monitoring and pesticide use

Begin monitoring for crawlers right before the expected emergence period for each generation. Since weather trends can shift date ranges, a more reliable prediction of timing can be made using Growing Degree Days and Plant Phenological Indicators. Below are refined estimates of egg hatch and the beginning of crawler emergence:

  • First-generation – 522 degree days
    • After the peak flowering of tulip poplar (Liriodendron tulipifera) and green hawthorn (Crataegus viridis ‘Winter King’)
    • Before the peak flowering of American smoketree (Cotinus obovatus)
  • tulip poplar flower

    Tulip poplar bloom
    Photo: Miri Talabac

  • hawthorn tree blooms

    Hawthorn blooms

  • smoketree with white blooms

    Smoketree bloom
    Photo: Miri Talabac

  • Second generation – 2235 degree days

During the peak flowering of Japanese Pagodatree (Styphnolobium japonicum; invasive)

Before the first flowering of Natchez Crapemyrtle (Lagerstroemia ‘Natchez’)

  • Japanese pagodatree in bloom

    Pagoda tree in bloom
    Photo: T. Davis Sydnor, The Ohio State University, Bugwood.org

  • Crapemyrtle 'Natchez' with white flowers

    Bloom of a 'Natchez' crapemyrtle

Adapted from: Davidson, J.A. and M.J. Raupp. 2014. Managing Insects and Mites on Woody Plants: an IPM approach. Third Edition, revised. Tree Care Industry Assoc. Londonderry, NH. 175pp. Illus. and the Pest Predictive Calendar, also the Scale Crawler Emergence Period chart compiled by Stanton Gill, Suzanne Klick, and Sarah Kenney.

Complied by Miri Talabac, HGIC, 2021