Q What is cushion scale?
A Cushion scale is a small, mostly immobile, sap-sucking insect (Pulvinaria floccifera) that attacks a range of outdoor plants.
Caption: Cushion scale often attacks camellias and causes sooty mould
Q What plants does it affect?
A Cushion scale favours evergreen shrubs, particularly camellia, holly, euonymus and rhododendron, and evergreen climbers including ivy and star jasmine (trachelospermum).
Q How do I spot an attack?
A The first sign of infestation by cushion scale is usually the appearance of sooty mould on the upper surface of the leaves. This is most noticeable in winter and appears as a black, powder-like coating. This colonises the sugary honeydew excreted by the scale insects that are living on the undersides of the leaves.
Q How do I identify it?
A Adult female scale insects attach themselves, limpet like, to leaves or stems and remain immobile, This scale insect has distinctive looks and a penchant for evergreens so they often go unnoticed. You are more likely to spot them when they produce eggs, which are protected by a mass of white, waxy scales. Cushion scales appear near the leaf veins, are about 3mm long, light brown, oval in shape and slightly domed. Their egg masses are spread out in short bands, like little bits of string, 2-3mm wide and up to 15mm long.
Q Could I mistake it for anything else?
A Many scale insects – and other sap-sucking insects, such as aphids and whitefly – produce honeydew, which, in turn, attracts sooty mould. You need to take a close look under the leaves to identify which pest is responsible. Young cushion scales look similar to soft scale, which is an elongated oval shape up to 5mm long, fairly flat and light brown in colour with a darker centre. However, once the cushion scales start to produce eggs they are quite distinctive.
Q What damage does it do?
A Small numbers of scale insects cause little harm, but large colonies can extract sufficient quantities of sap to weaken plants. Sooty mould does not damage the plant directly, but is unsightly and, if present in sufficiently large amounts, can interfere with the light reaching the leaf surface, and hence the plant’s ability to photosynthesise.
Q How serious is it?
A Cushion scale is not regarded as a serious pest, more of a nuisance – but it does seem to be getting more common in Britain.
Q What is its life cycle?
A Young scales, known as crawlers, hatch in late June to July. They are mobile at first and spread out over the host plant. They can travel longer distances in air currents or rain splash. Scale insect populations are often all female, but if males are produced they develop wings, remain mobile and are relatively short lived. When the females mature they settle in one place, feeding through the winter, laying eggs in the summer and then dying.
Q When do attacks occur?
A Scale insects are likely to be present on host plants all year round, but they become most obvious in late summer when the eggs are present, or in winter as the sooty mould builds up. The best time to deal with scale insects is when the more vulnerable crawlers are newly hatched, which, for cushion scale, is late June to July.
Q What are the early warning signs?
A You are unlikely to see the immature crawlers, and even the adult scales are well camouflaged until they produce eggs. The appearance of sooty mould is likely to be the first sign of an attack.
Q How do I control an attack?
A The most effective control is to use a systemic insecticide, such as Bug Clear Ultra, in late June to July. These are absorbed by the plant then taken up by the scales as they feed. The scales and the egg-protecting wax often remain on the plant after the scale insect has died. Check whether new growth is affected to judge how well your control measures are working.
Q What are the organic options?
A Organic insecticidal sprays are based on fatty acids (Doff Greenfly & Blackfly Killer) or plant oils (Growing Success Fruit & Veg Bug Killer, Vitax Organic 2-in-1 Pest and Disease Control). These work on contact and are only effective against the immature crawlers, not the adults. You will need to make several applications in late June to July. A quick and effective alternative, where the scales are accessible, is to flick them off with a fingernail or wipe them off with a wet cloth in spring before they lay their eggs.
Q How do I get rid of the sooty mould?
A Once sooty mould has developed it can prove surprisingly persistent, though it will eventually wear off or can be wiped away with a damp cloth where practical. Once the scales are under control it should not reappear.
Q How do I prevent it in the future?
A There is no practical preventive action you can take against scale insects. Dealing with attacks as soon as they are spotted should help avoid large numbers building up and prevent significant damage.