oystershell scale

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

Updated: October 21, 2021

Key points

  • Scale insects belong to a large group of sucking insects that attack a wide variety of trees and shrubs.
  • Generally, they are divided into two categories, armored (hard) and soft scale.  
  • Heavy infestations may cause leaf yellowing, stunting, severe plant stress, and dieback.
  • Eggs are laid under female adult coverings.
  • Typically, eggs hatch in spring or summer.
  • The stage after the eggs hatch is the immature stage or commonly called the 'crawler' stage because they walk about on plants to find new feeding sites. This is how a scale infestation spreads. 

Armored scales

  • Are small, about one-eighth inch, flat, disk-like, circular, elongate or oystershell shaped. 
  • The color is variable.
  • They are found on leaves, twigs, and branches and can be gently scraped off.
  • They do not produce honeydew. Honeydew is a sugary plant sap which is excreted by soft scale and other juice-sucking, plant-feeding insects and which makes leaves shiny and sticky and leads to sooty mold which is a fungus that grows on honeydew that blackens leaves and stems. 
  • Many species have a hard, scale-like covering made up of shed skins and wax and are immobile as adults. 
  • Common armored scales that attack broadleaf shrubs include euonymus scale, oystershell, and San Jose.
  • Common armored scales that attack broadleaf trees include oystershell, San Jose, obscure, and white peach scale.
  • Examples of armored scales that attack needled evergreen trees and shrubs are elongate hemlock scale, cryptomeria scale, juniper scale, minute cypress scale, and pine needle scale. 

Armored scales on broadleaf trees and shrubs

These are some common examples of armored scales found in home landscapes but there are many others. 

Euonymus scaleUnaspis euonymi  

Euonymus scale

Obscure scale, Melanaspis obscura 

Obscure scale
Photo: J.A. Davidson U of MD

A native scale, oaks are a primary host. Common on pin and willow oaks planted in the landscape. Scale covers can overlap on branches and twigs. Small holes in the scale covers indicate that tiny parasitoid wasps have been active to help control this pest. 

Oystershell scale, Lepidosaphes ulmi

Oystershell scale
Photo: J.A. Davidson U of MD

Has a large host range. Covers have a distinct oystershell shape.

White prunicola scale, Pseudaulacaspis prunicola 

White prunicola scale

Preferred host plants are in the Prunus (cherry) family. Is often found on cherry laurel (Prunus laurocerasus).  

"What is this white growth on my cherry laurel?This is a common question we receive through Ask Extension. The white prunicola scale described below is the culprit!

White peach scale, Pseudaulacaspis pentagona

White peach scale
Photo: J.A. Davidson U of MD

Host plants include peach, mulberry, persimmon and red-twigged dogwood. Snow-white patches of male scale covers can be seen hanging down on the underside of branches. 

Armored scale on needled evergreen trees and shrubs

Pine needle scale, Chionaspis pinifoliae

Pine Needle Scale
Photo: L.R. Brown, U of CA

White scale covers are found on green needles. The adult female covers are 1/8 inch (3 mm) long white, oystershell-shaped and only found on needles. Male covers resemble female covers but are smaller. Crawlers or immature scales are reddish in color, very tiny and hatch in May and July. During May and July turn over scale covers and look for the presence of eggs or crawlers. Crawlers are active when they first hatch. They disperse from the egg mass in search of a suitable place to feed where they then settle. Holes in the scale covers indicate the presence of parasites or predators. This scale infests most cone-bearing conifers but prefers white, mugo, Scotch, and Austrian pine. The scale overwinters as red eggs under scale covers.  Light infestations do not cause serious damage. Heavy infestations may cause yellowing of needles, stunting and dieback. Trees along roads and against buildings often suffer from severe attacks.

Cryptomeria scale, Aspidiotus cryptomeriae

Cryptomeria Scale
Photo: J.A. Davidson U of MD

This scale is found on the needles of pines, hemlock, and fir. Adult female covers are 1/16 inch (4 mm), elongate and oval. When newly formed they are translucent and eventually turn light brown. Male covers are similar but smaller. There are two generations in Maryland. Yellow crawlers are present in June and September. Cryptomeria scale overwinters as immature scales (second instars). Damage appears as yellowish blotches on needles. Heavy infestations may result in dieback of branches.

Elongate Hemlock Scale, Fiorinia externa

Elongate Hemlock Scale

Elongate hemlock scale is primarily a pest of Eastern hemlock, but may also be found on Carolina hemlock, Japanese hemlock, yews, spruces, and firs. The female covers are elongated, parallel sided, and light to dark brown. The male covers are white. There are two generations in Maryland with crawler peaks occurring in May and September. The overlap of these generations produces needle scales throughout the summer. The scale covers are waxy and may be so numerous that the needles have a whitewashed appearance.

Juniper Scale
Juniper scale
Photo: J.A. Davidson U of MD
 Dieback caused by minute cypress scale
 Dieback caused by minute cypress scale
Photo: J.A. Davidson U of MD

Management  

Scale insects are difficult to control. To manage a scale infestation, prune out branches with severe symptoms. Parasites generally provide control of many scale infestations. To protect parasites, avoid spraying with insecticides during the summer. If treatment is necessary, spray the tree or shrub with a dormant rate of horticultural oil during the dormant season to control overwintering scales. Also keep trees and shrubs watered during droughty periods and do not overfertilize.  

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