oystershell scale

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

Updated: August 16, 2022

Key points

  • Learn more about scale insect groups, biology, and management on Introduction to Scale Insects.
  • Inspect ailing plants and learn how to find scale insects using the information on Monitoring for Scale.
  • Numerous scale species can occur in home gardens, though they may not be noticed unless plants show signs of damage. Symptoms of plant damage include leaf yellowing, stunting of growth, and branch dieback.
  • Small populations of scale are rarely a concern, and landscapes that support natural enemies benefit from a lower likelihood of outbreaks. Gardens incorporating a diverse range of plant species attract and retain populations of natural enemies.

Common species in Maryland gardens

  • white euonymus scale covers on leaves

    Euonymus scale

    Photo: Brian Kunkel, University of Delaware, Bugwood.org

  • Large numbers of gloomy scale on a red maple branch.

    Gloomy scale

    Photo: Adam Dale, North Carolina State University

  • heavy infestation of Japanese maple scale

    Japanese maple scale

    Photo: Brian Kunkel, University of Delaware, Bugwood.org

  • San Jose scale on a branch

    San Jose scale

    Photo: Laura Iles, Iowa State University

  • white prunicola scale on a branch

    White prunicola scale

    Photo: Brian Kunkel, University of Delaware

Occasional species in Maryland gardens

Obscure scale (Melanaspis obscura)

This is a native scale species.

Appearance

Close-up of obscure scale on twig.
Photo: Lorraine Graney, Bartlett Tree Experts, Bugwood.org
  • Mature female covers are rounded, brown to gray over black, and up to ⅛” (3mm) in diameter. Individuals may be overlapping.
  • Male covers are oval, similar in color, and smaller.
  • Crawlers are pink.
Illustration of obscure scale covers on twig.
Female (large) and male (smaller) covers on a twig.
Illustration: J.A. Davidson

Common host plants

  • Various oaks, particularly black oak (Quercus velutina), pin oak (Quercus palustris), and willow oak (Quercus phellos)
  • Pecan (Carya illinoinensis)

Where to look

Obscure scale covers blend in well on bark.
Obscure is an apt name, as covers are well-camouflaged.
Photo: William Fountain, University of Kentucky, Bugwood.org
  • All life stages will be found on the bark, especially on three- to four-year-old branches.
  • Gnarled or disfigured branches (or trunks on young trees) can occur from dense and overlapping clusters of live scale damaging the bark.
  • Overfertilized trees are more vulnerable to heavy outbreaks. This includes sites where trees are surrounded by lawn receiving excessive nutrients.

 

Illustration of overlapping obscure scale covers.
Overlapping aggregation of female covers.
Illustration: J.A. Davidson

Plant damage

  • Heavy or prolonged infestations can cause premature leaf drop and branch dieback, plus a rougher, sunken appearance to the bark under dense scale clusters.

Life cycle

  • There is 1 generation per year in Maryland.
  • The crawler emergence period depends on temperature and can vary slightly from year to year. The approximate time to monitor for them is July through mid-September.
  • They overwinter on the bark as juveniles.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

1774 degree days

After the peak flowering of Golden Raintree (Koelreuteria paniculata)

yellow blooms of a golden rain tree
Golden raintree blooms
Photo: Miri Talabac

During the peak flowering of Rose-of-Sharon (Hibiscus syriacus)

rose of Sharon flower
Rose-of-sharon blooms
Photo: Miri Talabac

Natural enemies are active during the beginning of the crawler period. Conserve them by avoiding the use of long-residual pesticides during this time.

Contact insecticides may have limited efficacy since crawlers often shelter under the covers of dead scale from prior generations.

Oystershell scale (Lepidosaphes ulmi)

This is a non-native scale species.

Appearance

Close-up of oystershell scale on a stem.
Photo: D.K.B Cheung
  • Mature female covers are oystershell-shaped, brown to gray, sometimes with yellow bands, and up to ⅛” (3mm) in length.
  • Male covers are similar but smaller.
  • Crawlers are pale yellow.

Common host plants

Over thirty plant families are used by this species.

  • Preferred shrubs include lilac (Syringa), boxwood (Buxus), and cotoneaster (Cotoneaster).
  • Preferred trees include beech (Fagus), birch (Betula), ash (Fraxinus), maple (Acer), poplar (Populus), willow (Salix), elm (Ulmus), flowering cherry (Prunus), dogwood (Cornus), and fruit trees.

Where to look

Close-up of an infested rose stem covered in oystershell scale.
Heavily infested stems can be encrusted with wall-to-wall covers.
Photo: Joe Boggs, OSU Extension
  • All life stages will be found on the bark.

Plant damage

  • High populations can cause leaf yellowing and wilting, followed by branch dieback. Prolonged infestations can kill small plants.

Life cycle

tiny Oyster Shell scale crawlers
Crawlers might be easier to detect by trapping them on double-sided tape.
Photo: Joe Boggs, OSU Extension
  • There are 1 or 2 generations per year in Maryland (depending on population genetics).
  • The crawler emergence periods depend on temperature and can vary slightly from year to year. The approximate times to monitor for them are:
    • First generation – April
    • Second generation – early May through June
  • They overwinter as eggs sheltered under female covers.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

  • First generation – no local data available
  • Second generation – 486 degree days

    During the first flowering of thornless honeylocust (Gleditsia triacanthos f. inermis), Japanese snowbell (Styrax japonicus), and tulip poplar (Liriodendron tulipifera)
Honeylocust blooms.
Honeylocust blooms
Photo: Miri Talabac
Blooms of a Japanese snowbell tree.
Japanese snowbell blooms
Photo: Miri Talabac
Bloom of a tulip poplar tree.
Tulip poplar bloom
Photo: Miri Talabac

White peach scale (Pseudaulacaspis pentagona)

This is a non-native scale species.

Appearance

closeup of white peach scale waxy cover
Photo: John .A. Davidson, Univ. Md, College Pk, Bugwood.org
  • Mature female covers are rounded, white with yellow center, and up to 1/16” (1.5mm) in diameter.
  • Male covers are elongate, white, and 1/16” (1.5mm) in length.
  • Crawlers are white or salmon/coral pink.

 

Illustration of white peach scale comparing male and female covers.
Comparison of female and male covers.
Illustration: J.A. Davidson

Common host plants

Over 250 plant genera in 89 families are used by this species.

  • shrubs red-twig dogwood (Cornus alba) and beautyberry (Callicarpa)
  • preferred trees include peach (Prunus persica) and other stone fruits plus flowering cherry (Prunus), white mulberry (Morus alba), black willow (Salix nigra), and persimmon (Diospyros)

Where to look

Branch covered in white peach scale.
Photo: Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org
  • All life stages will be found on the bark.
  • 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 yellowing and dieback.

Plant damage

Close-up of very well-camouflaged white peach scale on bark.
Infestations on bark can be very difficult to see.
Photo: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org
  • High populations cause leaf yellowing and premature shedding, followed by stunting and branch dieback. Prolonged infestations can kill small plants.

Life cycle

  • There are 3 generations per year in Maryland.
  • The crawler emergence periods depend on temperature and can vary slightly from year to year. The approximate times to monitor for them are:
    • First generation – mid-May through mid-June
    • Second generation – mid-July through mid-August
    • Third generation – September
  • They overwinter as adult females.

Timing details for monitoring and treatment

No local Growing Degree Days and Plant Phenological Indicator data have been compiled for this species.

Pine needle scale (Chionaspis pinifoliae)

This is a native scale species.

Appearance

Close-up of pine needle scale mature females and crawlers.
Mature females and crawlers.
Photo: Joe Boggs, OSU Extension
  • Mature female covers are oystershell-shaped, white with yellow tip, and up to ⅛” (3mm) in length.
  • Male covers are similarly shaped, white, and smaller (2mm).
  • Crawlers are reddish.

Common host plants

  • Prefers pines, such as eastern white pine (Pinus strobus), mugo pine (Pinus mugo), Scotch pine (Pinus sylvestris), and Austrian pine (Pinus nigra).
  • A variety of needled conifers are also used, including spruce (Picea), fir (Abies), hemlock (Tsuga), and cedar (Cedrus).

Where to look

Close-up of a pine needle scale infestation on spruce foliage.
Infestation on Colorado spruce.
Photo: William M. Ciesla, Forest Health Management International, Bugwood.org
  • All life stages will be found on foliage (needles).
  • Trees under stress. Trees growing against buildings and along roads are more prone to damaging infestations of this scale due to the stress caused by pollution, limited root space, and reflected heat.

Plant damage

Mature pine needle scale females on pine foliage.
Photo: Joe Boggs, OSU Extension
  • Light infestations cause little damage.
  • Heavy or prolonged infestations can cause premature needle yellowing, stunted growth, and dieback.

Life cycle

  • There are 2 generations per year in Maryland.
  • The 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
    • Second generation – early July through late August
  • They overwinter as eggs sheltered under female covers.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

  • First generation – 307 degree days

    During the flowering of mayapple (Podophyllum peltatum) and flowering dogwood (Cornus florida)
    Before the peak flowering of red buckeye (Aesculus pavia)
     
    Native plant mayapple in bloom.
    Mayapple blooms
    Photo: Miri Talabac
    Blooms of a native flowering dogwood tree.
    Flowering dogwood blooms
    Photo: Miri Talabac
    Blooms of a red buckeye tree.
    Red buckeye blooms
    Photo: Miri Talabac
  • Second generation – 1561 degree days

    Before the peak flowering of golden raintree (Koelreuteria paniculata)
    After the first flowering of swamp milkweed (Asclepias incarnata) and rose-of-sharon (Hibiscus syriacus)
     
    Blooming golden raintree.
    Golden raintree blooms
    Photo: Miri Talabac
    Blooms of swamp milkweed.
    Swamp milkweed blooms
    Photo: Miri Talabac
    rose of Sharon flower
    Rose-of-sharon bloom
    Photo: Miri Talabac

     

Cryptomeria scale (Aspidiotus cryptomeriae)

This is a non-native scale species.

Appearance

Close-up of cryptomeria scale on the underside of fir needles.
Photo: Joe Boggs, OSU Extension
  • Mature female covers are oval or elongate, translucent aging to tan, and 3/16” (4.75mm) in length.
  • Male covers are similar but smaller.
  • Crawlers are yellow.
Illustration of cryptomeria scale compared with elongate hemlock scale.
Comparison of cryptomeria scale with elongate hemlock scale.
Illustration: J.A. Davidson

Common host plants

Utilizes thirteen genera in four families, all conifers.

  • Prefers pine (Pinus), hemlock (Tsuga), and fir (Abies)
  • Yew (Taxus), falsecypress (Chamaecyparis), juniper (Juniperus), and plum yew (Cephalotaxus) may be used. Despite the scale’s name, Cryptomeria (Cryptomeria) is a rarer host in the U.S.

Where to look

  • All life stages will be found on foliage (needles).

Plant damage

Foliage damage on fir needles from cryptomeria scale.
The tops of needles with scale underneath will become discolored.
Photo: Joe Boggs, OSU Extension
  • Light to moderate infestations cause needle discoloration, with hemlock developing yellow blotches and pine and fir developing yellow to brown blotches.
  • Heavy or prolonged infestations can cause dieback.

Life cycle

  • There are 2 generations per year in Maryland.
  • The crawler emergence periods depend on temperature and can vary slightly from year to year. The approximate times to monitor for them are:
    • First generation – mid-June to early July
    • Second generation – late August through September
  • They overwinter as juveniles.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

  • First generation – 937 degree days

    After the first flowering of Japanese stewartia (Stewartia pseudocamellia)
    Before the peak flowering of witherod viburnum (Viburnum nudum)
     
  • Second generation – no local data available
     
    Bloom of a Japanese stewartia tree.
    Japanese stewartia bloom
    Photo: Miri Talabac
    Bloom of a viburnum shrub.
    Viburnum bloom
    Photo: Miri Talabac

Elongate hemlock scale (Fiorinia externa)

This is a non-native scale species.

Appearance

Close-up of elongate hemlock scale on the undersids of hemlock foliage.
Photo: Joe Boggs, OSU Extension
  • Mature female covers are elongate, translucent tan over reddish-brown, and 3/16” (4.75mm) in length.
  • Male covers are elongate, white with waxy threads, and smaller.
  • Crawlers are lemon-yellow.
Illustration comparing male and female elongate hemlock scale covers.
Comparison of female and male covers.
Illustration: J.A. Davidson

See the cryptomeria scale illustration for comparison with this species.

Common host plants

  • Prefers hemlock (Tsuga)
  • Spruce (Picea), fir (Abies), and yew (Taxus) are also used

Where to look

Elongate hemlock scale on the underside of hemlock foliage.
Photo: Joe Boggs, OSU Extension
  • All life stages will be found on foliage (needles).
  • Needle discoloration can resemble spider mite damage. Dense scale populations can give needles a whitewashed appearance.

Plant damage

Elongate hemlock scale on the undersides of discolored needles.
Look for scale on the undersides of discolored needles.
Photo: Joe Boggs, OSU Extension
  • Light to moderate infestations cause yellow spotting on needles.
  • Heavy infestations cause premature needle drop and eventual tree death (even for mature plants).

Life cycle

  • There are 2 overlapping generations per year in Maryland.
  • The 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
    • Second generation – mid-June through early October
  • They overwinter as adult females or as eggs sheltered under female covers.

Timing details for monitoring and treatment

No local Growing Degree Days and Plant Phenological Indicator data has been compiled for this species.

Maskell scale (Lepidosaphes pallida)

This is a native scale species.

Appearance

Close-up of maskell scale on cryptomeria needles.
Photo: Lorraine Graney, Bartlett Tree Experts, Bugwood.org
  • Mature female covers are oystershell-shaped, light brown, and up to 1/16” (1.5mm) in length.
  • Male covers are similar but smaller.
  • Crawlers are pale yellow.

Common host plants

Twenty-one genera in fifteen plant families – both conifer and broadleaf – are used by this species.

  • Prefers cryptomeria (Cryptomeria japonica), Japanese umbrella-pine (Sciadopitys verticillata), and Japanese black pine (Pinus thunbergii)

Where to look

Close-up of maskell scale on cryptomeria needles.
Photo: John .A. Davidson, Univ. Md, College Pk, Bugwood.org
  • All life stages will be found on foliage (needles). Due to their very small size and tendency to tuck into the sheaths between needles (on pine) or around needle bases, they will be difficult to see. Only high populations produce more detectable signs of scale on the surface of damaged needles.

Plant damage

  • Light infestations may cause little damage but also tend to go undetected without close inspection.
  • Heavy infestations cause needle browning and branch dieback on cryptomeria. Needles on pine and umbrella-pine yellow before browning. Young foliage may be stunted.

Life cycle

  • There are 2 generations per year in Maryland.
  • The crawler emergence periods depend on temperature and can vary slightly from year to year. The approximate times to monitor for them are:
    • First generation – June
    • Second generation – August
  • They overwinter as mated adult females.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

  • First generation – 470 degree days

    During the first flowering of thornless honeylocust (Gleditsia triacanthos f. inermis) and Japanese snowbell (Styrax japonicus)
    During the flowering of false indigo (Baptisia)
     
    Honeylocust blooms.
    Honeylocust blooms
    Photo: Miri Talabac
    Blooms of a Japanese snowbell tree.
    Japanese Snowbell blooms
    Photo: Miri Talabac
    Blooms of native perennial false indigo.
    False indigo blooms
    Photo: Miri Talabac
  • Second generation – 2035 degree days

    Before the first flowering of Japanese pagodatree (Styphnolobium japonicum) and Natchez crapemyrtle (Lagerstroemia ‘Natchez’)
     
    Japanese pagodatree in bloom
    Japanese pagodatree blooms
    Photo: T. Davis Sydnor, The Ohio State University, Bugwood.org
    Blooms of Natchez crapemyrtle.
    ‘Natchez’ crapemyrtle bloom

Minute cypress scale (Carulaspis minima)

This is a non-native scale species.

Appearance

Close-up of minute cypress scale on needles.
Photo: D.K.B Cheung
  • Mature female covers are circular, white with a yellow patch, and up to ⅛” (3mm) in diameter.
  • Male covers are elongate, white, and smaller.
  • Crawlers are tan.

Common host plants

Fifteen genera in four conifer families are used by this species.

  • Prefers juniper (Juniperus), spruce (Picea), and leyland cypress (Cupressocyparis leylandii)

Where to look

Close-up of minute cypress scale on needles.
Photo: D.K.B Cheung
  • All life stages will be found on foliage (needles).
  • Cultivars that grow densely and narrowly upright (columnar forms) may be more susceptible to outbreaks of this scale.

Plant damage

  • Heavy infestations can cause needle yellowing followed by browning, and later branch dieback.

Life cycle

  • There is 1 generation per year in Maryland.
  • The crawler emergence period depends on temperature and can vary slightly from year to year. The approximate time to monitor for them is May to early June.
  • They overwinter as adult females.

Timing details for monitoring and treatment

Just before the expected emergence period, 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. A refined estimate of egg hatch and the beginning of crawler emergence is:

  • 511 degree days

    During the peak flowering of tulip poplar (Liriodendron tulipifera), green hawthorn (Crataegus viridis ‘Winter King’), and Virginia spiderwort (Tradescantia virginiana)
     
    Bloom of a tulip poplar tree.
    Tulip poplar bloom
    Photo: Miri Talabac
    Blooms of a hawthorn tree.
    Hawthorn blooms
    Blooms of native perennial spiderwort.
    Spiderwort bloom
    Photo: Miri Talabac

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

Compiled by Miri Talabac, Horticulturist & Coordinator, HGIC 2022