Hydrangea scale

Hydrangea scale is a sap sucking insect found on hydrangeas but can also feed on many other woody plants including Acer and Prunus. Infested plants can suffer a lack of vigour and leaf loss caused by the insect sucking sap from the foliage and stems. This is one of  many types of scale insects encountered by gardeners. 

The immature scales (nymphs known as crawlers) are difficult to see but infestations become apparent in early summer when the mature scales deposit their eggs on the stems and leaves. You may see:

• Eggs which are covered in white waxy fibres that form smooth, oval patches 3-4mm long. These white patches will persist on the plant long after the eggs have hatched
• Newly hatched scales are less than 1mm long and are pale yellow
• The mature scales are oval in shape and brown
• Heavily infested plants lack vigour and may shed leaves and this insect can spoil the appearance of plants by the covering of white egg masses
• The crawler stage can be distributed in wind currents 

Non-pesticide control

Heavily infested plants can be removed and replacements planted in the autumn. Some manual removal of egg masses and adult scale can be attempted but may have a limited affect.

Pesticide control

Control is best attempted in July when the more vulnerable scale nymphs are present. Hydrangeas can be prone to damage from sprays to the leaves, to reduce this risk avoid spraying hydrangeas in hot sunny weather or if the soil is dry.

• 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 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
• 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 on or near plants in flower due to the danger to pollinating insects

Download

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

Hydrangea scale is a sap-sucking insect that develops on the underside of leaves on Hydrangea and other host plants.

There is one generation a year with young nymphs hatching from eggs in mid-summer and sucking sap from the undersides of leaves. In late summer the nymphs move to the stems where they overwinter before maturing in late spring. Eggs are deposited under a covering of white waxy fibres in early summer.

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