Scale insects vary greatly in size but are usually about 1-5mm long. They are mainly pests of indoor plants, hedges and of orchards. Like aphids and mealybugs, they are sapsuckers, producing honeydew and sooty mould. Severe infestations of scale insects can result in defoliation and retardation of the plant’s growth, and even in the death of the plant.
Scale insects mostly have a waxy or armoured cover which they use to protect themselves. There are two categories of scales — ‘soft’ and ‘armoured’ — and they can take various shapes. Most soft scales belong to the Coccidae while the armoured scales belong to the Diaspididae. In Australia, citrus red scale is probably the most important of the scale insects considered to be pests. Damage by this scale insect can result in severe downgrading of fruit quality. ‘Damage’ is due to marking of the fruit by the scales themselves and/or marking of the fruit by sooty mould growing on scale insect honeydew. 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.
There are hundreds of species of scale insects in Australia. They attack a wide range of host plants including ferns, conifers and flowering plants and some of the soft scales infest hedges, where the dense foliage makes them hard to control.
In soft scales, some scale insects hatch from eggs, while others are born live. Juvenile scales are known as crawlers. They disperse to favourable sites on the leaf, settle down and start feeding. The juveniles then become sedentary and start building their protective 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. Females are generally headless, legless and wingless, and when mature, they produce eggs, then die. 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. Fertilisation is not necessary in some species of soft scale insects. For some of these species, no males have been recorded.
For a domestic situation, removal and disposal of infected plant material may be effective. For minor infestations scale insects can be rubbed off using a damp cloth. Sprays with horticultural soap will desicate and suffocate the insects and horticultural oil will smother the insects but these low toxicity products should not be used when the weather is above 32°C as plant leaves can burn. Alternative products are the low toxicity, residual chemicals imidacloprid and acetamiprid.
The parasitic wasps Aphytis melinus, Aphytis lingnanensis, and Comperiella spp. are used in Australia in many integrated pest management programs to control armoured scale insects. The scale-eating ladybird (Rhyzobius lophanthae) is also used to control scales, especially citrus red scale.;">
Waxy scale insects are much harder to control with either biological or chemical agents than armoured scale insects.