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.

• The scale does not usually affect the vigour or flowering of it's host plants, it does however produce a sticky honeydew excrement
• A heavy coating of a black, non-parasitic fungus known as sooty mould can grow on honeydew on the upper leaf surface. This develops over the winter months and can persist into the summer
• Yellowish-brown, oval scale insects up to 3mm (1/8in) long can be seen near the veins on the undersides of the leaves
• Rectangular white waxy egg masses, up to 10mm (almost ½in) long and 2-3mm (1/8in) wide, are produced by the adult scales in spring and early summer. The remains of these egg masses can persist on the foliage throughout the year

Apart from the presence of honeydew and sooty mould cushion scale does not appear to adversely affect the vigour of its host plants and so can be tolerated.

Check susceptible plants frequently from late winter onwards for sooty mould so action can be planned later in the season. 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 early to mid-summer 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. Sooty mould will gradually flake off the leaves during the summer.

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
• On small plants sooty mould can be removed by wiping the foliage with a damp cloth
• Adult scales and egg masses can be removed when seen (spring and summer) but this may not reduce large populations
• 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.

• The best time for spraying is in June and early July 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

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.

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Pesticides for gardeners (pdf document)

• There is one generation of cushion scale a year with eggs hatching in late June-July
• The scales suck sap and excrete a sugary honeydew that coats the upper leaf surface, allowing sooty moulds to develop, especially during the winter
• The level of infestation can vary considerably from year to year. In mild wet winters, populations of overwintering nymphs may be reduced by a (insect pathogenic) fungal infection