What is mussel scale?

Mussel scale is a sap-sucking insect that lives mainly on the woody stems of its host plants. The small 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 infested plants will lack vigour and stems may dieback, especially on box. 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 be wiped off.

Control

Non-pesticide control

Light infestations can be tolerated without the need for control.

Pesticides control

• The best time to take control measures against scale insects is when the more vulnerable newly hatched crawler nymphs are present. For mussel scale, this is in late May-June
• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep scale insect nymphs in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. Organic insecticides can be used on fruit trees
• More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer). On apple the instructions on the labels must be followed for this crop. Cypermethrin (e.g. Py Bug Killer) can also be used on ornamentals only
• 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
• Dead scales often remain attached to the plant but new growth should stay clean if the treatment has been successful
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

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.