white prunicola scale on a branch

Clusters of white prunicola scale on a branch
Photo: Brian Kunkel, University of Delaware

Updated: April 15, 2024

Key points

  • White prunicola scale (Pseudaulacaspis prunicola) is a non-native type of armored (hard) scale insect. It can be difficult to manage because a waxy body covering shields them from predators and certain pesticides. Learn more about scale insect groups and biology on Introduction to Scale Insects.
  • This is a common pest of cherry laurel that causes plant decline and dieback when populations are high. Several other shrubs serve as hosts, but less frequently.
  • Target monitoring and control efforts to the vulnerable crawler stage. Learn more about what to look for on Monitoring for Scale.

Appearance

  • Mature female covers are circular, white with a yellow center, and up to 1/16” (1.5 mm) in diameter.
  • Male covers are elongate, white, and up to 1/16” (1.5 mm) in length.
  • Crawlers are salmon-colored.
closeup of white peach scale waxy cover
Close-up of white peach scale, a species whose covers are identical in appearance to white prunicola scale.
Photo: John A. Davidson, Univ. Md, College Pk, Bugwood.org

Common host plants

Sixteen plant families are used by this scale, but it is regularly found on cherry laurel in our area.

  • Cherry laurel (Prunus laurocerasus) and other Prunus species (such as flowering cherry) are preferred.
  • Populations may occur on lilac (Syringa), holly (Ilex), boxwood (Buxus), and evergreen privet (Ligustrum; invasive).

Where to look

  • They will be found year-round on the bark of twigs and branches.
  • Juvenile males tend to congregate on the underside of branches in snowy-looking clusters.
  • Infestations tend to begin on outer or smaller branches associated with leaf dieback. 
heavily infested lilac
Heavy infestation of White Prunicola Scale on Lilac, giving the lower stems a snowy look.
Photo: Brian Kunkel, University of Delaware
white flocking of white prunicola scale on stems of a shrub
Look for clusters of white flocking on the stems
Photo: Brian Kunkel, University of Delaware, Bugwood.org

Plant damage

  • Light to moderate infestations may result in leaf yellowing and shedding, stagnated growth, or stunting.
  • Heavy infestations can result in the death of branches.

Life cycle

  • There are 3 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 – May into early June
    • Second generation – mid-July through mid-August
    • Third generation – 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. Mature trees should be evaluated by a certified arborist.

The high number of generations per year for prunicola scale means 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.

Timing details for monitoring and pesticide use

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

  • First generation – 513 degree days
    • During the peak flowering of tulip poplar (Liriodendron tulipifera) and green hawthorn (Crataegus viridis ‘Winter King’)
  • yellow tulip poplar flower

    Tulip poplar bloom
    Photo: Miri Talabac

  • hawthorn tree blooms

    Hawthorn bloom

  • Second generation – 1637 degree days
    • After the peak flowering of Golden Raintree (Koelreuteria paniculata; invasive)
    • Before the full bloom of Rose-of-Sharon (Hibiscus syriacus)
  • yellow blooms of a golden rain tree

    Golden raintree bloom
    Photo: Miri Talabac

  • rose of Sharon flower

    Rose-of-Sharon bloom
    Photo: Miri Talabac

  • Third generation – 3270 degree days
    • After the peak flowering of seven-son flower (Heptacodium miconioides)
  • Seven-son pink flower

    Seven-son flower post-bloom
    Photo: Miri Talabac

Related information

What is this white growth on my cherry laurel shrub? | Maryland Grows blog

Introduction to Scale Insects

Monitoring for Scale 

Understanding Degree Days

 

References:
Adapted from, Managing Insects and Mites on Woody Plants, authors: Ph.D. John A. Davidson, Ph.D.and Michael J. Raupp, Ph.D. The Pest Predictive Calendar, and Scale Crawler Emergence Period chart compiled by Stanton Gill, Suzanne Klick, and Sarah Kenney.

Author: Miri Talabac, Horticulturist & Coordinator, HGIC
2022