Oystershell scale. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
Updated: October 17, 2024
Key points
Armored scale are insects that have a hard, shell-like covering over their body. They feed on the contents of plant cells under bark or in foliage. Learn more about scale insect groups, biology, and management on Introduction to Scale Insects.
Symptoms of plant damage from scale insects include leaf yellowing, stunting of growth, and branch dieback. Numerous scale species can occur in home gardens, though they may not be noticed unless plants show these symptoms.
Inspect ailing plants and learn how to find scale insects using the information on Monitoring for Scale.
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 of armored scale in Maryland gardens
Euonymus scale
Photo: Brian Kunkel, University of Delaware, Bugwood.org
Gloomy scale
Photo: Adam Dale, North Carolina State University
Japanese maple scale
Photo: Brian Kunkel, University of Delaware, Bugwood.org
San Jose scale
Photo: Laura Iles, Iowa State University
White prunicola scale
Photo: Brian Kunkel, University of Delaware
Occasional species of armored scale in Maryland gardens
Obscure scale (Melanaspis obscura)
This is a native scale species.
Appearance
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.
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
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.
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)
During the peak flowering of Rose-of-Sharon (Hibiscus syriacus)
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
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
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
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)
White peach scale (Pseudaulacaspis pentagona)
This is a non-native scale species.
Appearance
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.
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
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
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:
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
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
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)
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)
Cryptomeria scale (Aspidiotus cryptomeriae)
This is a non-native scale species.
Appearance
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.
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
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
Elongate hemlock scale (Fiorinia externa)
This is a non-native scale species.
Appearance
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.
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
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)
Second generation – 2035 degree days
Before the first flowering of Japanese pagodatree (Styphnolobium japonicum) and Natchez crapemyrtle (Lagerstroemia ‘Natchez’)
Minute cypress scale (Carulaspis minima)
This is a non-native scale species.
Appearance
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
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)
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