Scale insects are sap sucking true bugs belonging to several families in the Hemiptera. Typically the adults are immobile having a flattened or raised appearance, with no visible legs. They often look like a ‘scale’ on a leaf or stem, many species produce a white wax often covering egg masses. There are more than 100 species found in Britain, 26 of which have been introduced. More than 25 species can be found in gardens or on houseplants.

Yew scale is a sap-sucking insect that lives mainly on the leaves and stems of 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. 

• 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

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 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.
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

• 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

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.
 
Where you cannot tolerate yew scale, manage them using the information above as your first course of action.
 
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.

The shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife.
 
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
 
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.

Homemade products are not recommended as they are unregulated and usually untested. 

Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.

Plants in flower must not be sprayed due to the danger to bees and other pollinating insects.

• 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 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

Download

Pesticides for gardeners (pdf document)