San Jose scale on a branch

Infestation of San Jose scale on apple.
Photo: Laura Jesse Iles, Iowa State University

Updated: August 16, 2022

Key points

  • San Jose scale (Quadraspidiotus perniciosus) is a non-native type of armored scale insect and can be difficult to manage as their 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 fruit trees and can cause plant decline and dieback when populations are high. This scale species has a very wide host plant range.
  • 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, with concentric rings of gray to brown with a yellow center, and up to 1/16” (1.5 mm) in diameter.
  • Male covers are elongate to oval, similar in color and pattern, and slightly smaller.
  • Crawlers are yellow.
illustration of San Jose scale covers

Illustration of juvenile and adult San Jose scale covers.
Illustration by J.A. Davidson

Common host plants

Plants in the rose family (a large group, with many popular species) are preferred. Overall, species in 81 plant families can be utilized by this scale.

  • Fruit trees, with apple (Malus domestica) and pear (Pyrus) being more severely damaged. Even so, it remains a serious pest on sweet cherry (Prunus avium), peach (Prunus persica), prune-type plums (Prunus domestica), and other tree fruits.
  • Nut trees and several fruiting shrubs (edible berries).
  • An array of large-statured shade trees, smaller-statured flowering trees, and ornamental shrubs, including flowering plum (Prunus cerasifera), crabapple (Malus), rose (Rosa), and cotoneaster (Cotoneaster).

Where to look

  • They will be found year-round on the bark of branches of all sizes.
  • They can also be found on the skin of fruits and occasionally on foliage.
  • heavy infestation of San Jose scale on bark

    Populations on trunks and branches can be well-camouflaged and hard to detect, even when high.

    Photo: P. Jentsch, Cornell CALS

    Click on image to enlarge.

  • San Jose scale on apple

    Scale can look like scarred spots on apple skins

    Photo: Laura Jesse Iles, Iowa State University

    Click on image to enlarge.

Plant damage

  • Scale feeding through the skin of fruits can cause reddish halos around each individual insect. Blotchy skin coloration may be unsightly but fruits are safe to consume.
  • Scale feeding through the bark can cause reddish-purple staining to the underlying sapwood.
  • Light to moderate infestations may cause leaf wilting, branch dieback, stagnated growth, or overall stunting.
  • Heavy infestations result in extensive branch dieback or the death of entire plants.
     
  • fruit skin damage caused by San Jose scale

    Reddish halos on fruit skin around scale insect feeding sites

    Photo: Central Science Laboratory, Harpenden, British Crown, Bugwood.org

    Click on image to enlarge. 

  • reddish staining under the bark from San Jose scale feeding

    Staining of the sapwood from scale feeding

    Photo: P. Jentsch, Cornell CALS

    Click on image to enlarge.

Life cycle

  • There are 3 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 – May into early June
    • Second generation – July
    • Third generation – September through October
  • They overwinter as juveniles on the bark.

Management

Refer to our general scale management recommendations (link below) 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 overlapping and high number of generations per year 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.

Branches exhibiting dieback will not recover, even with insecticide intervention, and can be pruned off.

Introduction to Scale Insects

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.

Complied by Miri Talabac, Horticulturist & Coordinator, HGIC 2022