What are scale insects?

There many different species of scale insects that can suck the sap from garden and glasshouse plants in Britain. They range in size from less than 1mm to over 1cm in diameter. Many species excrete a sticky, sugary substance, honeydew. Some species also produce white, waxy egg masses on stems or the undersides of leaves, this can be mistaken for mealybug or woolly aphid.

Scale insects can be found on a wide range of ornamental plants, fruit trees and bushes grown out of doors. Several species of scale insects are confined to house-plants, those growing in greenhouses or other sheltered places.

Symptoms

You may see the following symptoms:

• Scales or shell-like bumps on plant stems and the underside of leaves. These are the outer coverings of scale insect
• Large populations may result in poor growth
• Some species of scale insect excrete honeydew, which accumulates on the upper leaf surfaces. This can be colonised by a black non-parasitic fungus known as sooty mould
• Some scale insects deposit their eggs under a covering of white waxy fibres in early summer

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. The best time to treat heavy attacks is when the more vulnerable nymphs are present, for many scales out of doors this is during mid-late summer. Scale insects on indoor plants can breed all year round and so there is no best time of year to treat. 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

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
• Adult scales and egg masses can be removed when seen but this may not reduce large populations
• Encourage predators  in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects
• The nematode Steinernema feltiae can be purchased to as a  biological control of soft scale/ hemispherical and is available from some Biological control suppliers 

 Pesticide control

• For scales on deciduous plants including edible fruits, a plant oil winter wash (considered organic e.g. Growing Success Winter Tree Wash) can be used. This can control the overwintering scale nymphs in December-January when the plants are fully dormant
• With grape vines, peel away the loose outer bark to expose the scales before treatment
• 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

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (downloads pdf)

Biology

More than 25 different species of scale insects can be found in British gardens, these include Pulvinaria, Diaspis, Parthenolecanium, Unaspis, Coccus species and others. They suck sap from the leaves and stems of their host plants. They are mostly 1-6mm (less than ¼in) long, although wisteria scale, Eulecanium excrescens, can be up to 10mm (about ½in), and vary in shape and colour.

All species have a shell-like waxy covering over their bodies when mature. The eggs are often laid under the protection of this shell but with the cushion scales (eg Pulvinaria species) the eggs are deposited outside the scale under a mass of white waxy fibres.

The adults are sedentary but newly-hatched nymphs crawl actively over the plant surface and spread the infestation.

Scale insects in greenhouses can breed continuously throughout the year but those species that infest outdoor plants mostly have one generation a year.