Scale Insect Control in Your Garden

Image above: Cottony Cushion Scale (Icerya purchasi) on citrus

There are hundreds of species of insects across several families of the superfamily Coccoidea that are known as Scale Insects. They are all sap-sucking insects. The most common types that occur in home gardens are several species of Hard Scales in the family Diaspididae, and several species of Soft Scales of the family Coccidae. Cottony Cushion Scale (Icerya purchasi) of the family Monophlebidae, and Gumtree Scale (Eriococcus coriaceus) of the family Eriococcidae are also quite common.

Hard Scales produce a separate waxy cover under which the insect feeds, but do not produce honeydew. Soft Scales and Cottony Cushion Scales don’t have a separate covering like hard scales, but the upper surface of their bodies is toughened with protective mealy secretions. Soft Scales and Cottony Cushion Scales also produce vast amounts of honeydew. Gumtree Scales are a bit different in that females encase themselves in a globular sac, and they too produce honeydew.

Best Treatment for Scale Insects

It is important to understand the link between scale insects, honeydew, ants and sooty mould. Soft Scale Insects, Cottony Cushion Scale and Gumtree Scale all produce honeydew, which is a liquid of concentrated plant sugars they excrete. Sooty mould grows on the honeydew, making plants unsightly and inhibits photosynthesis, which may cause leaf drop.

Honeydew also attracts ants that protect the Scale Insects from being attacked by predators and parasites, in return for a feed of the honeydew. The simplest way to manage Scale Insects is to prevent ants climbing into trees, that will allow the natural enemies of the Scale Insects a chance to do what they do. A band of horticultural glue is all you need to prevent ants climbing into trees.

Hard Scale Insects don’t produce honeydew, so you won’t have the added problems of ants and sooty mould to deal with if you find them on your plants. There are several things you can do to prevent populations of Hard Scales Insects building up to damaging levels (see How To Prevent Scale Insects Appearing).

Apart from biological control (see Natural Enemies), the mainstay of Scale Insect control is the application of horticultural oils. Oil sprays suffocate small insects such as Scale Insects. After the insects have died, you can hose them off as well as any sooty mould they may have promoted.

To control scale on citrus and ornamental plants, including roses, spray with Yates Scale Gun. Avoid use on delicate plants such as African violets, ferns, seedlings, mosses or on flowers. Certain plants may be sensitive to oil-based sprays, so do a test spray on a small inconspicuous area and observe over the next couple of days to see if there is a reaction. If there is no reaction, then you may continue with a more widespread spray. If there is a reaction, discontinue use.

If you need to control scale on hedges or larger ornamental plants, stone fruit, citrus, or grapes, then use Yates PestOil Concentrate. Mix the concentrate in a sprayer and apply thoroughly over the leaves or affected plant parts. Read the label prior to use for more details of how and when to apply.

Image above: White Wax Scale (Ceroplastes destructor) on Native Finger Lime
(Image courtesy of Elise Dando)

What are Scale Insects &
How to Get Rid of Them

Common Hard Scale Insects include:

• Red Scale (Aonidiella aurantii), also known as California Red Scale, is an introduced pest that is found in most areas of Australia.
• White Louse Scale (Unaspis citri), also known as Citrus Snow Scale, is an introduced pest of citrus in coastal eastern Australia.

Common Soft Scale Insects include:

• Soft Brown Scale (Coccus hesperidum) is an introduced pest that occurs in all states and territories of Australia.
• Several species of Wax Scales (Ceroplastes spp.) have been introduced into Australia. These include White Wax Scale (C. destructor) found in New South Wales, Queensland and southern Western Australia; and Pink Wax Scale (C. rubens) found in all states and territories except Tasmania.
• Cottony Cushion Scale (Icerya purchasi) is a native insect that is found in most parts of Australia.
• Gumtree Scale (Eriococcus coriaceus) is a common native insect of eucalypts, which is widely distributed in Australia.

Image above: Scale on Grass Tree (Xanthorrhoea sp.) leaves
(Image courtesy of Elise Dando)

Description

Scale Insects don’t look like insects, because the insect’s body is mostly hidden under a hard or soft cover, which looks like a ‘scale’. The Scale Insects you are likely to find on plants are mature females, and juvenile (nymphs) males and females. Adult males are small winged insects that are rarely seen. The first nymph stage (‘instar’) of the life cycle is known as a ‘crawler’ that is a mobile six-legged creature about 0.5 mm long.

Adult female Red Scales are reddish-brown, circular and about 2 mm in diameter. Female nymphs are also circular but smaller. Male nymphs are oval and paler.

White Louse Scale male nymphs are white, about 1 mm in length with three longitudinal ridges. The name ‘White Louse Scale’ is derived from infestations of male nymphs. Adult female White Louse Scales are about 2 mm long, dark brown and mussel-shaped. Female nymphs are smaller versions of adult females.

Mature female Soft Brown Scales are flat, oval, about 4 mm in length, and a greenish-brown colour mottled with brown spots. As females age, they become a uniform brown colour. Nymphs are similar in shape and colour to adult females but are smaller.

Adult female Wax Scales are generally globular in shape and covered in waxy secretions. White Wax Scales are about 6 mm in diameter, white, soft and moist. Pink Wax Scales are about 4 mm in diameter, pink or red, and hard, with two lobes on each side and a depression in the middle. Wax Scale nymphs resemble smaller versions of adult females.

Female Cottony Cushion Scales are about 5 mm long, reddish-brown with black legs and usually covered by a white, mealy secretion. Females usually have egg sacs attached to them that are large, soft, cottony masses with a white fluted surface about 10 mm to 15 mm long. Nymphs are orange–brown with long hairs and covered with cottony secretions.

Gumtree Scale females are rounded and sac-like, about 3 – 4 mm long and reddish-brown in colour.  Female nymphs are reddish-brown and about 1 – 2 mm long, and males nymphs are white and about 1 – 2 mm long.

Image above: Fig Wax Scale (Ceroplastes rusci) on magnolia leaf
(image courtesy of Elise Dando)

Image above: Heavy infestation of White Louse Scale (Unaspis citri) on a citrus tree trunk
(Image courtesy of Elise Dando)

Image above: Red Scale (Aonidiella aurantii) on lemon fruit
(Image courtesy of Elise Dando)

Symptoms of Scale Insects

• Honeydew on upper leaf surfaces.
• Sooty mould on upper leaf surfaces.
• Leaf drop, twig dieback.
• Yellow patches on leaves around feeding scales.
• Ants climbing plants to feed on honeydew.

Image above: Soft Brown Scale (Coccus hesperidum)
Scale (central in image) with exit hole made by a parasitic wasp
(Image courtesy of Elise Dando)

How to Prevent Scale Insects Appearing

Monitor

• Watch for the appearance of sooty mould – a sure indication of sap-sucking pests.
• Watch for ants climbing into plants.
• Look for Scale Insects on leaf surfaces, twigs and fruit.

Prevent

• Control weeds to reduce breeding sites.
• Band tree trunks with horticultural glue to prevent ants climbing up.
• Scrape off minor scale infestations with a plastic scraper.
• Prune off heavy infestations.
• Release commercially available parasites and predators.

Natural enemies

• Parasitic wasps (see beneficial wasps).
• Predators such as ladybirds and lacewing larvae.