Yew scale

Yew scale is a sap-sucking insect that can be found on yew (Taxus) at any time of year, partly because dead scales remain attached to the plant.

Yew scale Parthenolecanium pomericanum  on Yew Taxus baccata

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.

Symptoms

  • Convex, oval, dark brown ‘shells’, 3-6mm long, occur on the woody stems
  • Infested plants may lack vigour and in heavy infestations defoliation can occur
  • 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 are of little consequence and can be tolerated. Yew scale is a food source for some ladybirds and other predators which can keep it under control.

Pesticide control

  • Heavy attacks are best dealt with in June and July when the more vulnerable newly-hatched scales are present. Scale insects can be difficult to control, particularly on plants where there is a dense mass of growth, such as you are likely to get on a yew hedge or tree. Although it is possible to spray the outside of a hedge or tree without too much difficulty, it is difficult to get any pesticide to penetrate very far into the hedge
  • 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 infestation
  • 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 insect nymphs. These pesticides have a very short persistence and so may require reapplication to keep scale insects 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
  • 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
  • Do not spray near plants in flower due to the danger to pollinating insects.

Download

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


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