Botanic Gardens Trust, Sydney, Australia

Scale insects - fact sheet

Scale insects are typically small, averaging about 2-3 mm in length. They are mainly pests of indoor plants and of orchards. They feed by sucking sap from plant tissue, thus weakening the host plant by removing its food supply and other valuable nutrients. Scale insects (like mealybugs, to which they are related) produce large quantities of honeydew. The honeydew - besides being a problem in itself because it is a sticky substance - provides a food source for sooty moulds. Sooty moulds produce a black coating that reduces the photosynthetic capacity and the aesthetic appeal of the affected plant. Severe infestations of scale insects can result in defoliation and retardation of the plant’s growth, and even in the death of the plant.

Types of scale

The term ‘scale’ refers to the typical scaly cover that the insect produces to protect itself. There are two categories of scales - ‘soft’ and ‘armoured’ - and they can take various shapes. Oyster Scale (with oyster-like covers) and Citrus Red Scale are examples of armoured scales, and Pink Wax Scale (pink waxy domes) is an example of a soft scale.

Of the scale insects considered to be pests, Citrus Red Scale is probably the most economically important. Damage by this scale insect can result in severe downgrading of fruit quality and hence drastically lower the prices for affected fruit. ‘Damage’ is due to marking of the fruit by the scales themselves and/or to marking of the fruit by sooty mould growing on scale insect honeydew (excreta). Large infestations of Citrus Red Scale can result in severe weakening of citrus trees, with resulting loss in yield. In particularly bad cases, the citrus trees can die.

Scale insects attack an extremely wide range of host plants, throughout the world, including ferns, conifers and flowering plants. Different host plants have different effects on scale insects - so much so that it can be hard to tell when scale insects on different hosts belong to a single species. This has resulted in some scale insects with extreme variation in morphology (body shape) being mistakenly described as more than one species.

Scale Insect Life Cycle

Soft scales: Some scale insects hatch from eggs, while others are born live. They disperse to favourable sites on the leaf, settle down and start feeding. This dispersal stage is known as a ‘crawler’. The juveniles then become sedentary, and start building their protective scale covers.

Both male and female scale insects live under scale covers. The females of most species remain under the covers for their entire life, while the male insects live under the covers until maturity, when they emerge as winged adults. Males mate with the (larger) females through the females’ scale covers. Females are generally headless, legless and wingless, and when mature they produce eggs then die. Fertilisation is not necessary in some species of soft scale insects, the female reproducing parthenogenetically (i.e. not needing to mate to produce young). In some of these species males have not been recorded at all. Most species of scale insects lay their eggs externally beneath the scale, although some species form a cyst with their egg mass within their bodies. A female scale insect can lay more than 150 eggs in its reproductive phase. Scale insects such as the Cotteny Cushion Scale (which is actually mobile when adult) produce a number of large fluffy egg masses.

Males pass through up to six juvenile stages before becoming winged adults. At their fifth juvenile stage both the male scale insects and their scale covers start to elongate along their longitudinal axes. Female scale insects pass through up to seven juvenile stages before becoming sedentary adults.

Armoured scales: Different species of armoured scale insects use different overwintering strategies: some overwinter as eggs, others as adult females, yet others as nymphal stages.

Control

Cultural: For a domestic situation, removal and disposal of infected plant material may be effective, although for some large-scale commercial operations other means of control will be necessary. In some cases the only realistic option may be to destroy readily replaceable plants that are severely infested - at least this will prevent spread of scale insects to neighbouring unaffected plants. For minor infestations of plants that are sufficiently small and have large leaves, scale insects can be rubbed off using a damp cloth, with or without insecticidal soap and/or spray oil.

Biological: The parasitic wasps Aphytis melinus, Aphytis lignanensis, and Comperiella spp. are used in Australia in many Integrated Pest Management (IPM) programs to control armoured scale insects in citrus orchards and many other places. These wasps control scale insects by laying a single egg through the scale cover into the body of a young juvenile. The wasp larva hatches and feeds on the host as it grows. The wasp kills the scale host once it reaches its pupal stage. The Scale Eating Ladybird (Rhyzobius lophanthae) is also used to control scales, especially Citrus Red Scale. Waxy scale insects are much harder to control than armoured scale insects, with either biological or chemical agents.

Chemical: There are a number of pesticides that are registered for use in controlling scale. Speak to your local nursery for more information.


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Plant Disease Diagnostic Unit
Royal Botanic Gardens & Domain Trust
Mrs Macquaries Rd
Sydney NSW 2000
Telephone: (02) 9231 8186
Facsimile: (02) 9241 1135
Email: pddu@rbgsyd.nsw.gov.au