The Gum Scale (Eriococcus coriaceus).—This is one of the most spectacularly destructive scale insects now established in the Dominion. It is a native of Australia, and its normal hosts are the several species of eucalyptus, though it is sometimes found on apricot and willow. A characteristic feature of infected eucalyptus is their blackened appearance, due to sooty mould growing on the copious honey-dew secreted by the scale.
On an infested twig or branch, the insects may be so closely packed as to conceal the bark ([Fig. 6], d); each female lies in a pear-shaped sac of felted secretion, reddish-brown, tawny, or sometimes white in colour, measuring about three-twenty-fifths of an inch long, and having a circular aperture at one end. The enclosed insect is somewhat flattened, oval, and blood-red in colour; when crushed, it leaves a reddish and sticky smear. The developing males are to be found forming white patches of innumerable individuals on the tree trunks under the loose bark ([Fig. 6], d).
The female is viviparous; during spring, mid-summer and autumn immense numbers of young are produced, which escape through the opening at one end of the female sac, and are carried long distances by the wind. These young insects first settle on the eucalypt leaves, whence they migrate, the females to take up their final position on the twigs and smaller branches, and the males to continue their development on the trunk of the tree.
The gum tree scale occurs throughout the districts east of the Southern Alps and in the vicinity of Nelson, in the South Island, and over the southern half of the North Island; it is, however, spreading rapidly northward.
This pest is held in control by means of the black-ladybird beetle (Rhizobius ventralis)—[Fig. 6], d—which was imported for the purpose from Australia; birds such as the tui, wax-eye, fantail, blackbird and thrush congregate on infested trees and eat the insect.
Olive Scale (Saissetia oleæ).—This insect has a world-wide distribution, and is one of the most important pests of citrus in New Zealand, although it occurs on a wide range of plants; in all cases it infests the fruit, bark, and the under side of leaves. The host plants include citrus, apple, pear, apricot, plum, almond, fig, grape-vine, wistaria, pepper tree, oleander, holly, laurel, palms, camellia, rose.
The injury caused by the insect is not so much on account of its weakening influence upon the infested plants as of the fact that it copiously secretes honey-dew, so that black mould develops to a marked degree, necessitating the washing of herbaceous plants and fruit.
The adult female ([Fig. 6], c) is hemispherical, and measures about one-fifth of an inch in diameter, a characteristic distinguishing feature being the three ridges forming the letter H on its upper surface ([Fig. 5]). According to age, the colour varies from brownish or greyish to jet black, the insect being conspicuous against the lighter background of bark or leaf; the small, immature individuals are light brown or yellowish, and almost flat.
In New Zealand the winter is passed in both egg and larval stages, though a few adults may be found at that time; on turning over what appears to be an adult, it will usually be found that the female has died and her place taken by numerous eggs ([Fig. 5]). The average number of eggs produced has been estimated at from 1,500 to 2,000 per female; at first the eggs are white, but prior to hatching they turn a deep orange-red. Development is slow, the adult state being reached about three months after time of hatching; egg laying commences about five weeks after maturity, and continues for a period of about six weeks. There is only one generation each year, and all stages may be met with on the one plant; the greatest activity occurs during the summer months. An important natural enemy of this scale is the steel-blue ladybird beetle (Orcus chalybæus), introduced from Australia.
Hemispherical Scale (Saissetia hemispherica).—This world-wide species is commonly met with in New Zealand, and, though not a serious pest, has a wide range of host plants, both in the open and under glass; some of the commoner hosts are citrus, fig, oleander, palms, japonica, camellia, asparagus, and orchids.