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The white louse scale (Unaspis citri) is one of the commonest pests infesting citrus trees in coastal areas. Infestations generally commence on the trunk and main limbs and may spread rapidly up to the tips of the twigs. The scales may also move on to the leaves and fruit. Where infestations are heavy the effects are seen in the reduced size and pitted appearance of the fruit and the yellow markings where scales are feeding upon the leaves. Leaf fall and twig dieback may occur and even large limbs may be killed. With lighter infestations the effects are not so evident but weakening of the limbs may occur and the bark becomes "hide-hound" and later cracks. Such limbs become more subject to attack by tree borers.
The small, white male scale coverings are much more conspicuous than those of the dark females and somewhat resemble fragments of desiccated coconut scattered over the hark. The female scale coverings are inconspicuous and similar in colour to the bark. The eggs arc laid beneath the scale cover and as egg-laying proceeds the body of the female scale contracts leaving a cavity beneath the cover which becomes packed with the white shells of hatched eggs and with the orange coloured remains of dead larvae. Several generations develop each year but two main broods may be recognised and peak larval emergences occur during October-December and March-May. Females over-winter mostly in the adult stage, maturing the eggs which are deposited in the spring. These hatch shortly after
being laid and give rise to active larvae or "crawlers".
Infestations after building up to a peak are usually reduced rapidly by predatory caterpillars of a small moth (Batrachedra sp.), and, to a lesser extent, by chalcid wasp parasites. The moth caterpillars feed extensively on the scales beneath the protection of a mass of fine, white, silken webbing into which remnants of the scales become incorporated. When attack is at its height the white webbing gives the trees an almost whitewashed appearance.