Yew scale

Yew scale is a sap-sucking insect that can be found on yew (Taxus) light populations of this insect are part of the natural biodiversity that this plant can support. 

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Yew scale Parthenolecanium pomericanum on Yew Taxus baccata

Quick facts

Common name Yew scale
Scientific name Parthenolecanium pomeranicum
Plants affected Yew (Taxus)
Main symptoms Brown, oval convex shell-like objects on the leaves and branches
Most active All year

What is yew scale?

There are many types of scale insects encountered by gardeners. Yew scale is a sap-sucking insect that lives mainly on the leaves and stems of a yew (Taxus). It is similar in appearance to brown scale however, brown scale is found on a wide range of plants but not Taxus.

Yew scale has one generation a year and in early summer the adult insects can be seen on the leaves and stems. The eggs hatch in late June and July and the young scale nymphs then disperse and feed during the summer by sucking sap from the leaves. It is not unusual to find light populations of this scale on healthy yew plants. 

Symptoms

  • Convex, oval, dark brown ‘shells’, 3-6mm long, occur on the woody stems
  • It is not unusual to find this scale on yew, only heavy populations may affect vigour and cause defoliation
  • A black sooty mould often develops on the sugary honeydew, that is excreted by the insects as they feed on sap and deposited on leaves and stems

Control

Light infestations of yew scale are of little consequence, indeed normal for a healthy yew and can be usually tolerated. Note that on large trees there is no treatment available that will reach all parts of the plants.
Check yews frequently so action can be taken if it appears a damaging population is developing. 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 this insect 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.
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.

Non-pesticide control

  • 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
  • Encourage predators, yew scale is a food source for some ladybirds and other predators which often keep it under control.

Pesticide control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

  • The best time for summer spraying is in June and July 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. 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)
  • A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides.
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.

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Pesticides for gardeners (pdf document)

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