Mussel scale is a sap-sucking insect that lives mainly on the woody stems of its host plants. The small 3mm long, soft-bodied insects are concealed under shells or scales that resemble the shellfish of the same name. This is one of several species of scale insects encountered by gardeners.

Scale insects are

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

Mussel scales are up to 3mm long and brownish black in colour. Their shape is similar to that of the shellfish known as mussels. Heavily affected plants will lack vigour and stems may dieback, especially box (Buxus). On apple the scales sometimes spread onto fruits. Old dead scales often remain attached, so the scales can be seen throughout the year.

Light infestations of mussel scale are of little consequence but heavy attacks can affect the host plant's growth.  When mussel scale spreads onto apple fruits it makes them less palatable, although the scales can simply be wiped off.

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.
Light infestations are of little consequence and can be tolerated, but heavy attacks can be dealt with in May and June when the more vulnerable newly-hatched scales are present. Note that dead scales can remain firmly attached to the plants. The success of any treatment can be gauged by the extent to which new growth remains free of scale insects. Large populations of this scale insect often develop on plants already lacking vigour.

Non-pesticide 

• Where possible tolerate populations of scale insects. Well-tended healthy plants are able to tolerate light populations of these insects and so they do not necessarily require control
• Encourage predators  in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects

Pesticides

The RHS recommends that  you don't use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.
 
Where you cannot tolerate mussel scale manage them using the information above as your first course of action.
 
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.

Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife.
 
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
 
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.

Homemade products are not recommended as they are unregulated and usually untested. 

Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.

Plants in flower must not be sprayed due to the danger to bees and other pollinating insects.

• The best time for summer spraying is in early May or June when the more vulnerable newly hatched scale nymphs are present 
• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear Fruit and Veg) can give good control of scale insect nymphs. These pesticides have a very short persistence and so may require reapplication to keep scale numbers in check. Plant oil products are less likely to affect larger insects such as ladybird adults 
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action (e.g. Ecofective Bug Control, RHS Bug and Mildew Control and SB Plant Invigorator). These are not considered organic
• Further information about the use of pesticides available for management of scale is available on the pesticides for gardeners leaflet

Download

Pesticides for gardeners (pdf document)

Mussel scale has one generation a year. Female scales deposit their eggs underneath their bodies during late summer-early autumn, so they are concealed by the shell or scale that covers the insect. The eggs overwinter and hatch in late May-June, when the scale nymphs crawl around for a while and can be distributed on wind currents, before selecting somewhere to feed. This is mainly on the bark but scales can develop on the fruits of apple and Cotoneaster. The scales reach the adult stage in late July-August.