Efficient chemical control depends on targeting the most vulnerable stage of the life cycle - the "crawler" nymphs. These are either born live or hatch from the eggs and disperse to new feeding sites on the plant. Scale insects are sap feeders belonging to Hemiptera (true bugs). It is only when they start to suck sap from phloem vessels that they settle and build up their protective scales. These waxy scales are the retained remains of the exoskeleton shed during moulting.
The large numbers of eggs or live young produced mean infestations can become excessive and damaging if left unchecked. A single female common soft scale can produce up to 250 live young, breeding all year round on protected crops. A single brown scale can lay up to 2,000 eggs that are protected under the parent's scale from May to July outside.
Crawlers may be spread by wind and animal vectors outdoors but in glasshouses the infection source is almost always brought-in stock. As well as direct scale damage, the sooty moulds that grow on their honeydew may reduce photosynthesis and are a cosmetic problem.
How to recognise them
Nymphs and mature females produce white, yellow or brown waxy scales, up to 5mm long. Species include:
Beech scale - Cryptococcus fagisuga: White, powdery waxy colonies of females, eggs and nymphs on trunks and branches. Infestation can render trees susceptible to infection from beech bark disease (Nectria spp.).
Brown scale - Parthenolecanium corni: Similar to Saissetia coffeae. Usually found at lower temperatures and outdoors. Eggs laid May-July and hatch within a month. Nymphs feed on undersides of leaves but migrate to stems and branches to hibernate before resuming feeding from April.
Common soft scale - Coccus hesperidum: Widespread under glass and outside in warmer areas. Flat, oval scales up to 4mm long, with light-brown edge and darker centre, found on underside of leaf near veins. Infests protected and outdoor crops.
Cottony cushion scale - Icerya purchasi: Brown oval scales, 3mm long, produce large white egg sacs up to 10mm long with grooves.
Cushion scale - Pulvinaria floccifera: Resembles common soft scale except in April-May, when females produce 15mm white, waxy egg sacs. Found on camellia, Ilex and rhododendron.
Cycad Aulacaspis scale - Aulacaspis yasumatsui: White scales, 1.2-1.6mm long, with variable shape. Males are white and 0.5-0.6mm long with three longitudinal ridges.
Hemispherical scale - Saissetia coffeae: Common in heated glasshouses. Dark-brown, dome-shaped scales, 4mm long with prominent H-shaped marking.
Horse chestnut scale - Pulvinaria regalis: Dense colonies on trees including acer. Eggs hatch June-July. Nymphs feed on leaves until autumn then migrate to stems to overwinter.
Mussel scale - Lepidosaphes ulmi: Attacks apples, pears, plums, Buxus, ceanothus and other Rosaceae.
Oleander scale - Aspidiotus nerii: 2mm scales resemble tiny fried eggs and colonise glasshouse plants.
White peach scale - Pseudaulacaspis pentagona: Notifiable pest first found in 2005 on protected peaches and an Italian-sourced Catalpa bignoniodes. Other host plants include buddleia, camellia, clematis, Cornus, Fraxinus, hydrangea, Juglans, Malus, Morus, Ribes, Salix, Syringa and Vitis. Adult females are white with a yellow spot, 2-2.5mm long and can be obscured beneath bark flakes. Males are smaller, felted, white and elongated.
Woolly currant scale - Pulvinaria ribesiae or woolly vine scale Pulvinaria vitis: White woolly egg sacs produced May-June on Alnus, Betula, Crataegus, cotoneaster, Salix and vines. Feed mainly on stems.
FERA has carried out pest risk analyses on mango shield scale (Milviscutulus mangiferae), Fletcher scale (Parthenolecanium fletcheri), Hemiptera greedy scale (Hemiberlesia rapax), fig wax scale (Ceroplastes rusci), San Jose scale (Diaspidiotus perniciosus), pink wax scale (Ceroplastes rubens) and wisteria scale (Eulecanium excrescens). See www.fera.defra.gov.uk/plants/publications.
Coccidae are soft scales with a covering that remains part of their body. They usually excrete honeydew from sap feeding. Diaspididae are hard scales with a covering that can be separated from their body. They generally do not produce honeydew.
Females reproduce only once and usually without male fertilisation. In most species, winged adult males are rare or absent. After producing large numbers of eggs or live larvae, the female dies, leaving behind the waxy scale to protect offspring.
Eggs hatch into young nymphs that resemble mealybugs and disperse over the plant to new feeding sites on leaves or stems. In nature, these may also be dispersed by wind or animals. Adult females are generally immobile once attached at a feeding site.
Initial symptoms are yellowing foliage, weakened, stunted or distorted growth and sunken spots on the upper leaf surface that correspond to the position of scales below. Heavy infestations may lead to defoliation. Honeydew deposits and sooty mould on lower leaves may be the first signs of an infestation of soft scale.
Treatment: biological control
Chilocorus nigritus Small predatory ladybird beetles that predate on most hard scale species on protected crops. Use in late spring to early summer or with supplementary light above 18 degsC. Introduce as adults, two per square metre of infested plants, repeated at least twice at 14-day intervals.
Metaphycus helvolus Wasps that parasitise soft scales (Saissetia and Coccus spp.) on protected crops. Use in late spring to early autumn or at other times with supplementary light above 18 degsC. Introduce as adults, two per square metre of infested plants.
Steinernema feltiae Parasitic nematode effective against most scale species. Spray foliage at above 14 degsC.
Generalist predators such as lacewing larvae and the mealybug destroyer Cryptolaemus may also control scale.
Treatment: cultural control
Check brought-in material, establish quarantine and treat if there is any sign of infestation. Monitor crops for early signs including honeydew or sooty mould. Regular cleaning of interiorscape plants helps to prevent infestation and removes honeydew.
Treatment: chemical control
Active ingredient Chlorpyrifos
IRAC code 1B
Action(s) Organophosphorus insecticide. Compatible with some biological controls.
Active ingredient Deltamethrin
IRAC code 3
Formulation Decis, Decis Protech (Bayer)
Action(s) Contact, synthetic pyrethroid insecticide with residual activity. Not compatible with biological controls.
Active ingredient Fatty acids
Formulation Savona (Koppert)
Action(s) Apply when insects first seen. For scale insect, apply several applications weekly after eggs hatch. Harmful to some beneficials. Short persistence on foliage.
Active ingredient Petroleum oil
Formulation Spraying Oil (Certis)
Action(s) Insecticidal oil. Acts by physical means. Blocks insects' respiratory system and makes it difficult for insects to attach to surface. Good contact needed with pest.
Active ingredients Pyrethrins
IRAC code 3
Formulation Spruzit (Certis), Pyrethrum 5EC (Agropharm)
Action(s) Short-term knockdown based on natural pyrethrin extract. Compatible with biological controls after seven days.
Active ingredient Spirodiclofen
IRAC code 23
Formulation Envidor* (Bayer)
Action(s) Label approval for control of mussel scale "crawlers" in late May.
Active ingredient Thiacloprid
IRAC code 4A
Formulation Calypso* (Bayer)
Action(s) Good control at feeding stage. Not compatible with biological controls.
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.