What is mussel scale?

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.

Symptoms

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.

Control

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 starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
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 control

• Where possible tolerate populations of scale insects. Not all scale insects affect the growth of plants and so they do not necessarily require control
• Encourage predators  in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects

Pesticide control

• 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 Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for 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 and fatty acid 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, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic
• More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.
 

Download

Pesticides for gardeners (pdf document)

Biology

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.