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Scale Insect (Lepidosaphes ulmi)


Infestations of Buxus by Scale Insect (Lepidosaphes ulmi), also commonly referred to as 'Oyster scale' or 'Mussel scale', are frequent and will kill Box plants if left uncontrolled.

Buxus semper virens parterre infested by Scale Insect (Lepidosaphes ulmi)   Colony of mature Scale Insects (Magnification X 15)

Identification of Scale Insect

Positive diagnosis is essential, leaf drop and plant death may occur for a variety of reasons such as Box Blight (Cylindrocladium buxicola), physiological damage, root death, or cultural problems and any remedial treatment must be specific to the problem.

Plantsprayers offer a free diagnosis service, for details click here.

Mature Scale Insect (Lepidosaphes ulmi)
Magnification X 30   Magnification X 15

Description of the Scale Insect

The 'cover' of Scale Insect resembles a small oyster shell. The adults are usually clustered together and in severe infestations may cover the bark of infested branches completely. On buxus commonly a small localised area (or a single plant within a row of plants) will exhibit leaf drop and dieback and examination of the stems below will show numerous small (1-2 mm) dark/black marks, examination of these marks through a magnifying glass ( X4 to X10) will show that these are actually individual (or clusters) of mature scale insect which have now embedded themselves into the surface of the plant, become static, and are feeding on the sap.

Description of damage to Buxus

Infestation of Buxus by Scale Insect frequently goes unnoticed until colonies are established and the plants begin to die and show pronounced leaf drop. To the untrained eye the effect on the plants is frequently confused with that caused by Box Blight (Cylindrocladium buxicola)

Damage caused by Box Blight (Cylindrocladium buxicola)   Damage caused by Scale Insect

The bark may become cracked and scaly, trees loose vigor, foliage is dwarfed and spotted with yellow. All phases of the insects feed on Sap and in effect starve the host of nutrients, frequently the stems above where the scale is embedded appear to be dead with severe leaf drop etc.

Populations tend to congregate on one or a few branches or on an individual plant, seldom a few scales scattered over the host. If scale is widespread then it has probably been present for a long period (years). Therefore it is prudent to examine newly purchased/acquired plants before introducing them to your garden.

Life cycle of Scale Insect (Lepidosaphes ulmi)

Eggs are laid in late autumn, 40-150 per female and as these are physically deposited below the insect,in the internal structure of the plant, the overwintering eggs are difficult to treat by conventional contact insecticides. Hatching occurs in late spring. Crawlers (larva stage) move around 1-2 hours to 1-2 days before settling and usually do not travel very far (possibly up to 1 meter ) before embedding themselves into the bark and metamorphosing into adult scale insects.

Nb. There is not a phase in the life cycle when the insects are able to fly, consequently Scale will not spread very quickly and usually form clusters in a localised area. The Scales are light in colour at first but become brown with maturity about the middle of July.

Treatment/Control of Scale Insect


The adult Scale insect is difficult to control, however it can be effectively treated by use of powerful systemic insecticides, these are not commonly available to the general public. This treatment will also control the 'Crawlers' (larvae) and should be applied at time of crawler hatch , repeat applications will usually be required. This treatment is effective throughout the growing season whilst the insects are feeding on the sap.

Plantsprayers offer spray treatments for control of adult and larva Scale Insect.

Overwintering eggs are difficult to eradicate. Specialised spray treatments are possible during the 'dormant' period but these should be considered with caution and applied by an expert. During the 'Crawler' (larva) phase the insects are at their most vulnerable and can be controlled by non persistent contact insecticides.

Careful observation is needed to determine when the crawlers are active ( depending on climate/temperature this will usually be in late spring) it may be useful to wrap some black electrical tape around a stem, coat it with vaseline/grease, and the light coloured crawlers can be seen stuck to it. Repeat sprays (approx 2 weekly) will be required.


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