The oystershell scale may also be called apple mussel scale, appletree bark louse, or mussel scale by some. The cover (test) of the insect certainly looks similar to a mussel. Adult females are oyster-shell shaped and straight in form unless crowded and twisting around one another, and dark to light-brown or gray in color. The soft body of the female beneath is white in color. The male cover is similar in color to the female, but shorter and narrower. Eggs and crawlers (the mobile, immature life stage) are white also. Each life stage of this insect is found on the host plant bark. There are 2 generations of this scale each year. Crawlers active in early summer and again in late summer. Researchers have found slight differences in the form and timing of the life cycle of this insect depending upon which host it is located on. There are differences noted for those on lilac vs. apple. Development on apple may occur up to 2 weeks earlier than on lilac. The lilac form of the insect may also be found on a wider variety of shade trees than the apple form. Also, the apple form is apple to reproduce on lilac, but the lilac form is not able to reproduce on apple. Adult females will lay 20-110 eggs from late August to early September. Eggs hatch from early May to mid-June. Second instar crawlers (immatures) are found from mid-June to late July. Adults from early August to late September. A different form may be found on popular that also has slightly different life history patterns and timing. Egg hatch timing also varies greatly depending upon geographic location. The oystershell scale is common to the northern United States, but has been found throughout most of the country with a limited list of states as the exception. It seems that this may be a species complex. It may have arrived in the United States in the 1700's with European settlers, however the true origin of this insect is uncertain.
The oystershell scale may be found on the bark of twigs and branches on lilac, beech, birch, ash, maple, willow, elm, boxwood, apple, pear, pachysandra, Prunus spp., and possibly on over 168 genera of plants (Miller and Davidson, 2005; ScaleNet). However, those mentioned here are some of the most common host plants in the United States. This species can be slow to spread and can go undetected for years on host plant. This species is known as a pest of shade trees and orchards, and is said to be likely to infest any woody, deciduous plant as well as some conifers (Miller and Davidson, 2005). In apple orchards, the scale can also be found on the fruit. This species of scale has historically been very economically important in the United States. Historically, mortality of entire ash stands has occurred in Ohio and others have described it as "probably the most destructive insect pest of lilac in Wyoming". Heavy infestations have been reported to also kill lilac and willow. Ornamentals are also historically reported to be negatively impacted by this insect. However, the oystershell scale is not reported as frequently or as damaging as it once was in the early 1900's. This may be due to a large complex of natural enemies helping to reduce its population below damaging levels. Miller and Davidson (2005) consider this species to be a worldwide pest. The oystershell scale feeds on host plant fluids with piercing sucking mouthparts. Limb dieback may occur, and scale feeding may make plants more susceptible to fungal pathogens.
On ornamental host plants and shade trees, visually scout for oystershell-shaped scale insects on host plant limbs and branches. These insects may be well camouflaged on their hosts, so look closely. Magnification, such as with a hand lens, may help. Infestations that have been present for many years may be detected any time of the year, as layered "tests" or armored coverings may remain on the host plant following scale insect death.
Host plants growing vigorously due to proper planting, the right site placement, and adequate care are less severely attacked by this insect. Practices that promote overall host plant health and vigor will reduce the impact of this pest. Heavily infested branches can be pruned away and destroyed to help reduce a single plant infestation if it can be done without disfiguring the host plant. Soft pads or brushes can be used to scrub off oystershell scales from host plant bark if they can be safely reached. Do not damage the bark during this process.
Important natural enemies of the oystershell scale are present in their populations. The two most important mortality factors of eggs and adults may be Hemisarcoptes malus and Aphelinus mytilaspidus (Miller and Davidson, 2005). However, up to 24 different genera of natural enemies have been recorded for this armored scale. This includes, but is not limited to: Ablerus, Aphytis, Coccophagus, and Encarsia of the Aphelinidae; Chilocorus and Rhyzobius of the Coccinellidae and many others (ScaleNet). However, these natural enemies may vary in their geographic association with the oystershell scale, as well as their impact on the armored scale's population in any given location.
Abamectin (NL)
Acephate (NL)
Acetamiprid (L)
Azadirachtin (NL)
Buprofezin (NL)
Carbaryl (L)
Chlorpyrifos (N)
Clothianidin (NL)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Dinotefuran (NL)
Gamma-cyhalothrin (L)
Horticultural oil (L)
Imidacloprid (L)
Insecticidal soap (NL)
Lambda-cyhalothrin (L)
Malathion (L)
Neem oil (NL)
Pyrethrin + sulfur (NL)
Pyriproxyfen (L)
Spinetoram + sulfoxaflor (N)
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), dinotefuran (soil drench), imidacloprid (soil drench), and neem oil (soil drench).
When used in nursery settings, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
