The term “sucking insect” is applied to all insects that have the mouth parts modified as delicate stylets, by means of which the plant tissues are punctured and the nutrient sap sucked up. Not only may such insects weaken the infested plants, but they also cause the destruction of chlorophyll, interfere with the normal functioning of the stomata, and have a toxic effect upon the tissues; further, many serious plant diseases are carried and spread by sucking insects, whilst the punctures made when feeding may allow the entry of disease spores.

Among sap-sucking insects are scale insects, mealy-bugs, aphids, leaf-hoppers, white-flies, thrips, etc. Infestation by most of these insects (especially in the case of scale insects, mealy-bugs, and aphids) is very often detected by the sticky nature and blackened appearance of the plants; this is due to the fact that the insects excrete a sweet, sticky substance known as “honey-dew,” which collects on the foliage and branches, whilst upon it grows a black, sooty mould.

Scale Insects and Mealy-bugs.

Scale insects and mealy-bugs, collectively known as coccids, are of very great economic importance on account, not only of their widespread depredations upon plants, few being free from infestation, but also of the commercial value of some species—?e.g., in the production of lac, cochineal, Chinese wax, etc.; it is with the injurious forms that the New Zealand horticulturist is concerned. The term “scale insects” is derived from the appearance of many of the species that are protected by a scale-like covering, which forms a conspicuous scaly incrustation when a plant is heavily infested.

Of the several kinds of insects injurious to vegetation, the coccids as a family are undoubtedly of major importance, because they infest not one group, or allied group, of plants, as do so many other injurious insects, but an extensive range of widely different plants. Some coccids are much more injurious than others, the San JosÉ Scale, for example, having a very virulent toxic influence, while the Greedy Scale may cause but little damage, even when the plant is completely encrusted by it; further, some plants may be more susceptible to injury than others by the same species of coccid.

Coccids, as a whole, are highly specialised insects, and among themselves exhibit a great variety of forms. Throughout the group the sexes differ to a marked degree. The adult males, which vary but little in all the coccids, are usually minute, and, with few exceptions, two-winged (Fig.5); none has mouth parts, these appendages having become atrophied during metamorphosis, which is complete, while many have one or more hair-like tail appendages. On the other hand, females are never winged; some are comparatively large; all have well-developed mouth parts throughout life, and undergo incomplete metamorphosis, while in many forms the legs and antennÆ are lost before maturity.

In all cases coccids secrete a protective covering, which assumes different forms; this fact, together with the chief methods of female development, is utilised for the purpose of this work to arrange the coccids under three main types as follows:—

1. Less Specialised Forms.—?Examples are the mealy-bugs and cottony-cushion scale, which belong to the more generalised or least specialised representatives. The protective body covering is in the form of a powdery or mealy secretion; the legs and antennÆ are retained throughout life, and the insect remains freely mobile.

A typical-form life-cycle may be studied in that of the cottony-cushion scale (Figs. 5 and 6a). During development the female insect passes through three larval stages; each of these stages is, on the whole, similar, except for size and minor structural changes, and the white powdery secretion that covers the reddish body of the adult.

2. Intermediate Forms.—?An example is the olive scale (Fig.5). In such forms there is a tendency to specialisation, owing to more or less sedentary habits in later life, and protection is afforded by a thickening and toughening of the cuticle on the upper surface of the body. Unlike the cottony-cushion scale, the female olive scale passes through two larval stages; the minute first stage larva is active and very flat; it soon settles upon a leaf and commences to feed, when it becomes much flatter and a little larger; the second stage differs from the first in size and in the development of a dorsal longitudinal ridge, which eventually forms the cross-bar of the two transverse ridges that are characteristic of the third or adult stage, when the insect swells and assumes the shape of the mature form. After settling in the first larval stage, the insect becomes very sluggish, and does not move, except to migrate, as most do, from the leaves to the twigs, there to take up a permanent position. The legs and antennÆ are retained throughout life, but in the adult are functionless, being folded against the body; in some species of intermediate forms the appendages become atrophied during development. In the olive scale, and related forms, the toughened cuticle not only serves as a protection to the insect, but also as a receptacle for the eggs (Fig.5); as these are laid and increase in numbers, the body of the parent diminishes and is crowded against the dome-shaped cuticle.

3. Specialised Forms.—?The apple mussel-scale (Figs. 5 and 7, Nos. 2 and 6) is a representative of this group, the members of which are markedly specialised, the legs and antennÆ of the adult female becoming completely atrophied during development, and the shape of the body profoundly altered; protection is afforded by a scale-like covering not attached to the body. In the mussel-scale development there are two larval stages: the first, like all coccids, has the legs and antennÆ well developed and is active.

On settling to feed, this first larva commences to produce a covering of white threads that mat together to form the first scale; the second stage larva presents profound changes in the absence of legs and antennÆ, while the body has become pear-shaped, the head, thorax and abdomen seeming as one; a second more waxy scale is now formed. After a second moult, the adult appears, and resembles the second stage larva in form; the adult constructs a third scale, very much larger than the earlier ones, to which it remains attached by its anterior end.

Though many of the specialised coccids form elongate scales, as in the case of the mussel-scale, numerous others construct circular scales, as does the San JosÉ (Fig.5); in the latter, the second and third scales are constructed round the first, so that the first and second appear as pimple-like structures in the centre, or slightly to one side of the completed covering. As with the olive scale, the covering of the specialised forms serves as a receptacle for the eggs (Fig.5).

Some of the more important coccids occurring in New Zealand will now be discussed.

Cottony Cushion Scale (Icerya purchasi).—?This insect (Fig.6a) is a native of Australia, but has now become established in many other countries, including New Zealand. For a time it was a serious pest of citrus, until the introduction and establishment of its natural enemy, the ladybird beetle (Novius cardinalis).

The adult female is more or less oval, and covered with a yellowish powder, partly concealing the reddish-brown ground colour and dark spots along the sides of the body; the legs are black. A characteristic feature is the white corrugated egg-sac attached to the end of the body (Fig.5). As the eggs are laid, this sac increases in size, until it may measure fully 2 ½ times the length of the parent, which becomes tilted up. The eggs are orange-yellow, and as many as 800 may be produced by a single female. The eggs hatch in about a fortnight during summer, and the period of development to the adult ranges from three to five months. The larvÆ most frequently congregate along the mid-ribs of leaves, and as development advances they usually migrate to the twigs and branches. There are two generations each year. A considerable variety of plants is attacked by this insect, chief among which are citrus, acacia, gorse, wattle, and Douglas fir.

Control is effected by the agency of the ladybird, but epidemics sometimes occur with which the beetle cannot immediately cope; in such a case fumigation in the glass-house, or spraying with red oil in the open, should be resorted to.

Mealy Bugs.—?Mealy bugs are characterised in the female by the nature of the waxy protective secretion which forms a powdery meal-like covering over the body, but is developed as a fringe of leg-like processes at the side (Fig.6b); these processes at the posterior end of the insect may be prolonged as longer or shorter tail-like appendages in some species, or they may be no longer than those fringing the body margins in others. Immediately after each moult the larvÆ are devoid of mealy covering and lateral processes, which are secreted anew each time the cuticle is shed. In a mealy bug colony are numerous small, narrow cocoons, in each of which a developing male insect lies.

Most mealy bugs produce eggs, which are laid in a spacious, cottony sac secreted at the posterior end of the female; the female insects, egg sacs, and male cocoons together form characteristic woolly masses on infested plants.

The injury caused by mealy bugs may be considerable, not only through the drainage of plant sap, but also owing to the production of honey-dew and its consequent sooty mould. All parts of plants are subject to mealy bug attack, and the insects are frequently attended by ants.

Mealy bugs are controlled to a great extent by natural enemies, among which are the Tasmanian lace wing (Micromus tasmaniÆ) and the CryptolÆmus ladybird (CryptolÆmus montrouzieri), but the influence of these is insufficient for commercial purposes. Attempts are now being made at the Cawthron Institute, Nelson, to establish other parasites recently imported from California.

Control under glass is effective by means of fumigation, but in the open is a more difficult matter, though red oil and lime-sulphur give some satisfactory results, together with the practice of removing rough bark on trees where the insects hibernate. In New Zealand are several species of mealy bugs, of which the following are of interest to the horticulturist:—

Long-tailed Mealy Bug (Pseudococcus adonidum).—?This species is readily recognised by the long tail-like appendages of the female. It is widely distributed and commonly met with under glass, where it infests almost any plant; in the warmer and moister districts of the Dominion it occurs out of doors. Its list of host plants is a lengthy one, and includes grape vine, passion vine, wistaria, fig, oleander, Phormium, cineraria, begonia, apple, plum, palms, ferns, etc. Considerable injury may be caused by the insect when it occurs in dense masses on the under side of foliage and upon young, succulent growth.

No eggs are produced by this insect, the young being born alive; the production of young lasts for a period of from two to three weeks at the rate of about twelve each day; the time taken to reach maturity varies considerably, according to climatic conditions, the range being from one to three months. There are comparatively few generations each year out of doors, but under glass there may be several.

Citrophilus Mealy Bug (Pseudococcus gahani).—?In New Zealand this species is met with on grape vines and begonia in glass-houses, where it becomes epidemic if left uncontrolled; out of doors it infests apple and potato, and no doubt other plants are attacked. It is characterised by the mealy covering being coarse and distributed unevenly over the body, while the marginal fringe is short, the processes being comparatively thick, particularly the tail-like ones, which are much shorter than the body, though conspicuous.

Egg-laying covers a period of about two weeks, from 394 to 679 eggs being deposited by each female; development to the adult is completed in about six weeks, though this will vary according to the conditions. In California four generations in the year have been noted.

Apple Mealy Bugs (Pseudococcus maritimus and P. comstocki).—?Both these species occur upon apple, pear and potato in New Zealand, the former species originating in America, and the latter in Japan; the injury to the host itself is not severe, but the presence of these insects on the fruit is responsible for apples and pears being rejected for export.

Both species are very similar in appearance, and are of the short-tailed type; they differ from the citrophilus mealy bug in having the mealy covering evenly distributed over the body, while the marginal fringe is delicate and thread-like. The eggs hatch in from one to three weeks, and the larvÆ migrate freely, the insects reaching maturity one or two months later, according to climatic conditions. In the open the winter is passed in the egg stage, but under glass or in mild climates activity among the different stages occurs throughout the year.

Apart from apple and pear, these insects have been recorded from many plants: Baker’s mealy bug (maritimus) on lemon, orange, walnut, willow, elder, ivy, iris; and Comstock’s mealy bug on citrus, elder, euonymus, gooseberry, grape, horse chestnut, hydrangea, mulberry, peach, persimmon, plum, poplar, wistaria.

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.

Both leaves and stems are infested by the insect, which resembles the olive scale (Fig.6, e); from the latter it may be distinguished by its light brown colour and smooth surface, there being no ridges; the longest diameter of the adult female is one-seventh of an inch. Between 500 and 1,000 eggs are laid by each female, and the life-cycle is completed in about six months; the young insects settle along the main leaf-veins.

Turtle Scale (Coccus hesperidum).—?This widely-distributed insect, though common in hot-houses and out of doors in the warmer parts of the Dominion, is not especially injurious, except for the copious honey-dew secreted and the consequent sooty mould; it occurs on holly, ivy, camellia, citrus, laurel, myrtle, oleander, and japonica.

The insect infests leaves and stems, and is especially abundant on succulent growth. The adult female is rather reddish-brown in colour, dome-shaped, but with the margins flattened on the host plant; on each side the margin is notched by a shallow depression, and there is a deeper one at one end; over the surface is a reticulation of ridges, resembling the pattern on the back of a turtle; fully-developed individuals measure from one-sixth to one-eighth inch in diameter. This species is viviparous, and development to the adult occupies about nine weeks; there may be three or four generations each year.

Fruit Lecanium Scale (Eulecanium corni).—?This European insect is common throughout the Dominion, where occasionally it becomes epidemic and causes some temporary damage; with it are associated honey-dew and sooty mould. Among the plants infested are apricot, peach, nectarine, plum, pear, grape-vine, wistaria, raspberry, mulberry, blackberry, gooseberry, black currant, ferns.

Leaves and bark are infested, and a narrow twig may be partly encircled by the margins of the scale. The adult female (Fig.6f) is oval and dome-shaped, some individuals measuring one-sixth of an inch in length; the surface is smooth, except toward the margins, parallel to which are some wrinkles. The general colour is dark brown, but just prior to egg-laying there are numerous transverse and longitudinal markings of a lighter colour over the surface. The winter is passed in the egg stage or as partly-grown young.

Another, but larger, species, closely resembling the preceding, and found on grape-vines, wistaria, eleagnus, etc., is Eulecanium berberidis. It is reddish-brown in colour, and measures up to one-third of an inch in length.

Golden Oak Scale (Asterolecanium variolosum).—?This insect is very common upon English oak trees in parts of New Zealand. In many cases so badly are the trees infested, that they become sickly in appearance, and at times the greater part, or even the whole, of the tree is killed through the agency of the pest.

The individual scale (Fig.7, 1) is more or less circular, and about one-sixteenth of an inch in diameter; it is of a greenish-yellow colour, with a narrow paler circumference, though some, with the exception of the rim, are partly or wholly brownish. Each scale forms and lies in a depression of the bark. The insect is viviparous. A minute parasite, Habrolepis dalmanni (note the exit holes made by the parasite during emergence from some of the scales shown in the photograph) has recently been established as a means of control and is proving effective.

Camellia Scale (Pulvinaria camelicola).—?This European scale sometimes heavily infests camellias and euonymus in New Zealand, but is not a very serious pest, though more so in glass-houses than out of doors. After the female has produced her eggs, she drops off the plant, so that, though the latter shows evidence of injury, there may be no sign of the insect.

The adult female is oval and about one-third of an inch at its longest length; in shape it resembles a rather flattened turtle scale, but when laying eggs the body shrivels and numerous transverse wrinkles develop, although the margins of the scale remain smooth. There is at least one generation each year, and in warmer parts probably a second, which may reach maturity before winter or not till the following spring. The eggs are laid in an elongate, white, cottony sac secreted at one end of the female; this sac is sometimes as much as four to five times the length of the insect. The eggs continue to hatch over a period of from four to six weeks, and the larvÆ rapidly spread; the latter settle along the leaf mid-rib, margin, or lower surface.

Apple Mussel Scale (Lepidosaphes ulmi).—?The apple mussel scale is now established throughout the temperate regions of the world. It is commonly met with on apple, but has a long list of host plants, among which are pear, hawthorn, willow, poplar, gooseberry, and currant.

The insect (Fig.7, Nos. 2 and 6) forms incrustations on bark and fruit, and is commonly met with at the stalk end of the apple; the individual scale is chocolate-brown in colour, is shaped like the shell of the salt water mussel—?hence the name “mussel scale”—?and when full grown measures one-eighth of an inch long.

A single female is capable of laying up to 700 eggs, in which stage the winter is passed. The eggs hatch in the spring, and the young insects swarm over the host plant in search of a suitable place to settle. A continuous warm spell of weather in the spring will allow all the eggs to hatch almost at one time, but alternating cold spells will retard development, so that emergences take place over a longer period. After emerging from the egg until maturity, when egg-laying again takes place, a period of three months elapses; the insect is a slow breeder, and produces only one brood a year in colder climates, but is two-brooded in warm districts, such as Auckland.

A small hymenopterous parasite (Aphelinus mytilaspidis), less than one-twenty-fifth of an inch long, attacks this scale, but does not serve as an efficient control; individual scales that have been killed by the parasite show a small hole through which the adult parasite has emerged. The most effective control is secured by treating infested trees with red oil or lime-sulphur during winter.

Cabbage Tree Scales (Leucaspis cordylinidis and Leucaspis stricta).—?Cabbage trees and also New Zealand flax often have the leaves encrusted by the white masses of these two native scales. The adult female of one species (L. cordylinidis) measures one-eighth of an inch long, is very narrow and straight as a rule, and white in colour, except for the yellow anterior end (Fig.7, 4). The other species (L. stricta) resembles the former, except that the adult is one-eleventh of an inch long, and has the anterior half blackish. In the case of ornamental cabbage trees and flax, control can be effected by removing all dead and scale-infested leaves, thus allowing access to sunlight.

San JosÉ Scale (Aspidiotus perniciosus).—?Of all scale insects of major importance, the San JosÉ (Fig.7, 5) is outstanding, in that it is one of the insects most destructive to deciduous trees and shrubs, a considerable number of which are liable to attack. It is of Chinese origin, and first came into prominence when it became established at San JosÉ, in California, hence its name. Owing to its small size, it is easily overlooked, except when epidemic, and is readily transported upon plants from one country to another.

The list of plants attacked is a long one, but the following may be mentioned:—?Acacia, hawthorn, quince, privet, poplar, almond, apricot, cherry, plum, peach, pear, apple, gooseberry, currant, roses, willow, ash, elm.

The female San JosÉ scale is circular in outline, having a diameter of about one-twenty-fifth of an inch; in profile it has the form of a flat cone with a crater-like depression at the apex, in the centre of which lies a minute pimple-like prominence; the immature scales are smaller and whitish in colour, while the male scale is elongate-oval in outline, with the crater-like depression toward one end. The individual scales are greyish and are readily overlooked, but when well established upon a tree they form an incrustation giving a characteristic dull silver-grey appearance to the tree; bark, fruit and leaves are infested. A characteristic feature of San JosÉ scale infection is the discolouration of the plant tissues immediately surrounding each insect, which turn a distinct red or purple, giving at once an indication that this scale is present.

The winter is passed by the insect in almost a mature state; on the advent of spring, development to maturity continues, when, after mating, the females give birth to living young over a period of several weeks. The young reach maturity and commence to reproduce five to six weeks from birth, there being several generations in the course of a season. The average number of young produced by each female has been found to be about 400.

FIG. 7.

(1) Golden Oak Scale; (2) Apple Mussel Scale; (3) Black Scale; (4) Cabbage Tree Scale; (5) San JosÉ Scale; (6) Apple Mussel Scale; (7) Oleander Scale; (8) and (9) Rose Scale.

Photographs by W.C. Davies, Cawthron Institute.