Scale insects

Adult scale insects are usually covered in waxy shell-like cover. They feed by sucking sap from a wide range of plants, including house-plants, greenhouse plants and many fruit and ornamental plants grown outdoors. There are more than 25 species of scale insect in the UK. Some scale insects can weaken host plants and many excrete a sticky substance (honeydew) on foliage, which allows the growth of black, sooty moulds.

Scale insects on bay. Image: RHS, Horticultural Science

Scale insects on bay. Image: RHS, Horticultural Science

Quick facts

Common name: Scale insects
Scientific name: Various species
Plants affected: Many greenhouse and garden fruits and ornamental plants
Main symptom: Scales on stems and leaves, sooty moulds on foliage
Most active: Year round

What are scale insects?

There are several different species of scale insects that can suck the sap from garden and glasshouse plants. 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 and 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
  • Heavy infestations 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

Non-pesticide control

  • Biological controls can be attempted during the summer in greenhouses
  • Parsitoid wasps Metaphycus helvolus, Encyrtus spp. and Encarsia citrina which can be used against soft scale (Coccus hesperidum) and hemispherical scale (Saisettia coffeae) are sometimes available
  • The small black ladybird Chilocorus nigrtus is sometimes available for diaspid scale control
  • These controls are available from mail order suppliers of biological controls
  • Adult scales and egg masses can be removed when seen but this may not reduce heavy infestations
  • Not all scale insect affect the growth of plants and so they do not necessarily require control

Pesticide control

  • The shell or scale gives some protection to adult scale insects from insecticides, so spraying is more effective against the newly hatched nymphs. With scales on outdoor plants there is usually one generation a year and in most species the eggs hatch in late June to July
  • Scales in greenhouses or on house-plants usually breed throughout the year so all stages in the life cycle may be present at the same time. Scale insects can remain attached to the plant long after they are dead but new growth should be free of scales once they have been brought under control
  • Deciduous fruit trees and roses can be treated with plant oil winter tree wash (e.g. Vitax Winter Tree Wash) on a mild dry day during December to control overwintering scale nymphs
  • 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 scale insects. 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 
  • 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)
  • 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
  • Do not spray plants in flower 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

Downloads

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

Biological control suppliers (Adobe Acrobat pdf document)

Biology

More than 25 different species of scale insects are pests in the UK and 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.


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