What is hydrangea scale?

Hydrangea scale is a sap sucking insect found on hydrangeas but can also feed on other woody plants including Acer and Prunus. Affected 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. 

Symptoms

The immature scales (nymphs known as crawlers) are difficult to see and the presence of these insects becomes apparent in early summer when the adult scales deposit 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 (crawlers) are less than 1mm long and are pale yellow
• The crawler stage can be distributed in wind currents
• The mature scales are around 3mm in diameter, oval in shape and brown
• Heavily affected plants lack vigour and may shed leaves 

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. 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 the damage 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.

Non-pesticide control

• Where possible tolerate populations of scale insects. Not all scale insects affect the growth of plants and so they do not necessarily require control
• Heavily infested plants can be removed and replacements planted in the autumn.
• Adult scales and egg masses can be removed when seen but this may not reduce large populations
• Encourage predators  in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects

Pesticide control

• The best time for summer spraying is in 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. Solabiol Bug Free, 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)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Download

Pesticides for gardeners (pdf document)

Biology

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