Unlike most insects, adult oystershell scales do not have legs, eyes, or antennae. Their appearance resembles a brown or grey oyster shell that is about 0.125 mm (1/8 inch) long.
So far, no male oystershell scales have been observed in Colorado; regional oystershell populations are believed to reproduce asexually with one generation produced per year.
The first instars are commonly referred to as crawlers and are highly susceptible to certain controls because they are small and do not have a waxy protective cover like adults. At this stage they are pale yellow and mobile, which allows them to search for sites in the bark for feeding. When first instar nymphs molt, they lose their legs, become immobile, and gradually increase in size over the following months. By midsummer they are fully mature adults.
• The oystershell scale is a scale insect that can be managed effectively with cultural and chemical controls.
• Of the scale insects in Colorado, the oystershell scale is the most damaging. It can attack a variety of trees and shrubs such as aspen, ash, cotoneaster, poplar, willow and lilac.
• Heavy scale infestations can increase plant susceptibility to pathogens. For example, cytospora fungi are frequently observed developing in areas previously infested with oystershell scales.
Oystershell scale on quaking aspen
Whitney Cranshaw, Colorado State University, Bugwood.org
The oystershell scale is the most destructive scale insect in Colorado. Oystershell scales belong to a family of insects known commonly as armored scales and do not secrete honeydew. Oystershell scale infestations weaken plant defenses and make them more susceptible to pathogens. Fortunately, there are many management options available for controlling oystershell scale outbreaks.
Oystershell scale flipped to expose the eggs.
Oystershell scale infestation. Infestations are generally confined to smaller portions of the tree.
Bark cracking on aspen due to a previous oystershell scale infestation.
Cytospora fruiting bodies growing in areas previously damaged by ostershell scales.
Removing oystershell scale from tree trunk using a scrub brush.
Life History and Habits
All scale insects undergo incomplete metamorphosis and have three life stages: egg, nymph (also known as crawlers), adult.
Females lay eggs in late summer and early fall and die at the end of the season, leaving behind the waxy covering as a protective overwintering site for eggs. Oystershell scale eggs overwinter under the mother scale’s old cover (see image) and hatch over a period of 2-3 weeks in late May or early June.
After hatching, the crawlers are mobile and can also disperse by wind or tree dwelling animals like birds and squirrels as they move from one tree to another. Crawlers that do not successfully find a feeding site within a few days after hatching will die.
All scale insects have piercing sucking mouthparts that extract fluids from the plant. It is common to observe dieback of twigs or branches on trees infested with oystershell scales. Reduced leaf size and chlorosis (yellowing) of leaves is another commonly observed symptom. Without effective management, severe infestations can kill trees or shrubs.
Crawlers are the most vulnerable life stage of oystershell scale because they are small and do not have a waxy protective cover. There are many insecticides that can be used to control this pest when applied during the egg hatch/crawler migration period. However, timing is important because these insects become much less susceptible to most insecticides after the waxy cover begins to develop.
Monitoring for crawler activity can be done in two ways. The first involves wrapping double sided sticky tape around infested twigs in mid-May. Use a magnifying lens to check the tape every other day for crawlers.
The other monitoring strategy involves gently shaking or tapping branches over a piece of white paper or a white tray. The fallen debris can then be examined for crawlers with a magnifying lens.
It is important to note that old scales can remain in place several years even after death. Clear old scales from the branch to allow for detection of new infestations. Old dead scales will be dry and flaky, while scales containing eggs will produce some moisture when crushed.
There are few natural enemies of oystershell scale in Colorado. Predatory mites have been observed feeding on overwintering oystershell scale eggs. While there are parasitic wasps in Colorado, they are not typically associated with oystershell scales.
Oystershell scale infestations can be prevented with proper plant care. Eggs and old scale coverings can be removed and destroyed on smaller trees by using a soft plastic brush to scrub the bark. However, branches that are heavily infested might need to be pruned.
Horticultural oils work by smothering insects or mites on plants and can be effective tools in oysterscale management. Those applied during the plant’s dormant period are called dormant oils and target overwintering oystershell scale eggs. It is important to note that other controls may also be necessary to treat heavy outbreaks. More information on horticultural oils is available here.
Using the monitoring methods described above, plants should be examined to determine the crawler period before applying insecticides since seasonal timing varies due to spring weather conditions. While crawlers can be killed with most insecticides, the most effective insecticides are those with persistent activity since eggs hatch over time. For more information on chemical control options for oystershell scale, consult the full factsheet.
Held, D. 2019. Controlling Scale Insects and Mealybugs. Alabama A & M and Auburn Universities: Cooperative Extension. Available https://www.aces.edu/blog/topics/lawn-garden/controlling-scale-insects-and-mealybugs/
Hoover, G. 2003. Oystershell Scale. Penn State: Extension. Available https://extension.psu.edu/oystershell-scale#:~:text=Damage,infestation%20of%20this%20insect%20occurs.
Utah State University. (n.d.). Oystershell Scale. Utah State University: Extension. Available https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/oystershellscale