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Scale insects and mealybugs on ornamentals | AG0183 |
David Williams, Knoxfield
June, 2000 |  |
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A number of different scale insects and mealybugs attack ornamentals in Victoria. These insects feed by sucking the sap; heavy infestations can seriously reduce the vigour of plants. Severe infestations may kill the plants. Mealybugs and some scale insects produce large amounts of honeydew which spreads over the plants and encourages the growth of sooty mould. The honeydew also attracts ants, which help to spread the insects and protect them from predators and parasites. The growth of sooty mould gives the plants a blackened, unsightly appearance and restricts the photosynthetic activity of the leaves.
Scale insects
Scales are broadly divided into two types: the soft scales and the hard or armoured scales. The soft scales have outer coverings that appear to form an integral part of the insect. The covering of hard scales can be lifted off to expose the insect underneath.
Mature female scale insects are immobile under their scale covering. The eggs are laid under the scale. The young scale insects or "crawlers" are tiny, oval, six-legged yellow or orange insects that can move about. This is the dispersal stage of the insect. Once the crawlers find a suitable feeding site they settle and start to produce a waxy protective covering. The adult male scale insect is a very small, short-lived, two-winged insect.
Soft brown scale Coccus hesperidum
Adults are brown, oval and flattened insects 2-5 mm long. The active crawlers appear in early summer and settle down in their permanent position on the midribs and main veins of the leaves. They have an extremely wide host range including citrus, fig, daphne, oleander, camellia and palms. This scale insect secretes copious amounts of honeydew, which often drips from the trees.
Soft brown scale prefers warm dry climates and mainly affects trees up to 10 years old. Mature trees are usually only infested on part of the tree. Considerable ant activity is associated with this scale. Control of ants can assist in preventing spread of the scale infestation.
Cottony cushion scale Icerya purchasi
This is a native insect found commonly on wattles. It attacks most garden plants but is most often seen on roses, pittosporum and grevilleas. The mature female is about 5 mm long, reddish-brown with black legs and is usually covered by a white, mealy secretion. The oval red eggs are laid into an egg sac beneath the body. The sac develops as a large, soft, cottony mass with a white fluted surface. Crawlers hatch during spring and another generation occurs in autumn. Young nymphs settle along the mid-veins on the undersurface of leaves but the older nymphs migrate to twigs or older branches. The immature stages and the adult can move until the ovisac develops. The abundant honeydew produced is attractive to ants, which often transport young scales to fresh host material.
Rose scale Aulacaspis rosae
The rose scale is a hard scale. Adult females are white or grey-white, roughly circular and 2.0-2.5 mm in diameter. The adult male scale is elongate, 0.8 0.3 mm, flat, white and distinctly ribbed. Eggs are laid under the female scales and crawlers emerge in spring to infest new growth. Heavy populations of rose scale can debilitate the plant. Bloom production is usually reduced, vigorous new shoot growth does not occur and response to bloom picking is poor. Neglected bushes may die.
Mealybugs
Mealybugs are closely related to scale insects but are free-living and completely mobile, although their movement may be sluggish. Adult female mealybugs are six-legged wingless insects, usually less than 6 mm long, with an oval body like that of a slater, and covered with white waxy powder or threads.
Mature females usually produce a mass of mealy filamentous material into which the eggs are laid. The young crawlers look like young scale insects and the male is, like the male scale, a small two-winged insect.
Mealybugs rarely attack annuals but are important pests on perennials. They attack roots, stems and leaves. Woody trees, shrubs, grasses and ferns are the most important plants infested. In Australia there are no reports of mealybugs transmitting viruses but some have been recorded as vectors overseas.
Mealybugs prefer plants with a high nitrogen content. Trees grown in the shade and those that received nitrogenous fertiliser have been observed to have significantly heavier populations of mealybugs. Drought conditions can bring about an increase in amino acid concentrations favourable to mealybugs but so can overwatering.
Nutritional differences may explain why some plants are infected with mealybugs while other plants remain free from attack. Even differences in the chemical composition between parts of any one plant could affect the choice of feeding site.
Mealybugs of the genus Pseudococcus are probably the most important and widespread pest mealybugs in Victoria.
Long-tailed mealybugs Pseudococcus longispinus.
This mealybug has anal filaments that are usually at least as long as the body, and produces an almost colourless yellow exudate. Eggs are laid beneath the body and hatch almost immediately. It is found on a wide range of host plants in greenhouses and in the open.
Citrophilous mealybug Pseudococcus calceolariae
The adult female has anal filaments that are about one-third as long as the body. The dark body colour shows through the white waxy secretion in four dark longitudinal dorsal lines. This mealybug produces a claret-coloured glandular exudate. Eggs are produced in an irregular cottony sac.
Biological control
Before attempting chemical control, growers should check to see if biological control agents are present. The most important predators of mealybugs are lacewing larvae and ladybirds. Cryptolaemus montrouzieri, the mealybug ladybird, has been used to control mealybugs in glasshouses in Canberra. Advice on its availability can be obtained from the Association of Beneficial Arthropod Producers. Both scales and mealybugs are parasitised by tiny wasps that lay eggs in the body of the host insect.
Chemical control
Oil sprays have been used traditionally to control scale insects and mealybugs by smothering the insect. This treatment relies on thorough coverage of the plant. It will not kill eggs that are under the adult scale insects.
Various contact and systemic chemicals are available to kill scale insects and mealybugs. Contact chemicals are only effective on scale crawlers and mealybugs that are actively moving over the plant. Adult scales and mealybugs that have developed their waxy coverings are difficult to kill with contact pesticides.
Systemic chemicals should give good control of adult scales and mealybugs that are feeding. Once the pest has stopped feeding it is too late to control it.
Current registrations of pesticides can be checked at local offices of DPI. It is the users responsibility to read the label and ensure that the chemical is used only for its registered purpose.
For effective pest and disease control, correct diagnosis is essential. A commercial diagnostic service is available at the Institute for Horticultural Development. For further information, contact the Diagnostic Service. ph: (03) 9210-9222 or fax (03) 9800 3521.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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