All insects do not follow the same method of development from egg to adult, and the adaptations of structure and habit are many and varied as well as simple and complex. Species having a complex development, during which they pass through stages, each differing in form from its predecessor, undergo what is known as a metamorphosis; contrasted with such insects are those developing in a simple manner without pronounced differences in the form of successive stages, the young resembling the adult in most features except size and maturity—​these insects are without a metamorphosis. Intermediate between these two extremes are other insects with a partial metamorphosis.

A consideration of the life cycle of some common insects will serve to illustrate the principles of development discussed above. Firstly, will be taken examples of complex development or complete metamorphosis; secondly, examples of simple development or absence of metamorphosis, followed by a review of species having a partial metamorphosis, thus linking the first two types.

A convenient type of insect undergoing a complete metamorphosis is any common moth (Fig. 4); one of the most suitable, most easily obtained in all stages and commonest in any part of the country from spring to autumn, is the magpie moth (Nyctemera annulata) and its caterpillar, the “woolly bear.” The moth, unlike most of its kind, is a day-flying species, and is very conspicuous owing to its black colour relieved by white wing spots, and orange-yellow bands on the abdomen; the equally conspicuous caterpillar, feeding on groundsel, ragwort and cineraria, is black, with a very hairy body marked with narrow brick-red lines.

The eggs are laid in clusters by the female moth on the under side of the leaves of the caterpillars’ food-plant; at first the eggs are of a pale green colour, but assume a darker yellowish tint within a few hours, and finally a leaden colour some time later. These colour changes are due to the developing embryo, and just before the young insect (the caterpillar in this case) hatches, its outline as it lies curled within the egg is easily seen through the transparent egg-shell; near the top of the egg is a black spot marking the position of the caterpillar’s head, while the numerous delicate black lines below the egg surface are the black hairs with which the caterpillar is clothed. According to temperature and humidity, the incubation period—​that is, the period between egg-laying and the hatching of the young caterpillar—​varies from eight days to three weeks. The process of hatching occupies about two hours, the young insect using its jaws to eat an exit hole through the egg. The caterpillar stage—​indeed, the first stage of all insects—​is known as the larva.

At first the larva of the magpie moth, measuring about one-sixteenth of an inch long, is pale yellow in colour, except for the black head and hairs clothing the body; very soon, however, the body becomes characteristically black, and develops the reddish lines. During growth the larva feeds continuously day and night, undergoing from five to ten moults before becoming fully grown. During a moult the cuticle of the head is cast separately from that of the body.

The body of the larva is worm-like, not only in general form, but also in its segmented appearance; it is, however, a very different animal from a worm. The larva has a distinct head, a pair of eyes, and short antennæ, and a set of mouth parts, similar to those of the weta or grasshopper, well adapted for devouring foliage; the first three segments behind the head correspond to the thorax of the moth, and each bears a pair of short feet; the remaining segments are those of the abdomen, and have no true feet, but six pairs of sucker-like appendages called pro-legs. The number of pro-legs varies from four to six pairs, according to the species of moth, and are found only on the larva.

The time occupied by larval development of the magpie moth varies from forty to eighty days in summer and autumn; but if winter intervenes, causing the larvæ to hibernate before completing their development, the larval period may be as long as two hundred and forty-eight days; normally this insect hibernates in the larval state, completing its development during the following spring. Throughout winter the larvæ hibernate singly or in colonies under loose bark, in leaf axils, or any suitable crevice.

The fully-grown larva measures about one and a-half inches long. Prior to the final moult it ceases to feed, and wanders in search of a suitable place in which to undergo the next transformation, usually among stones, rubbish, or under loose bark, etc. There it spins a white silken cocoon, among the strands of which are entangled the long black body hairs; herein the larva undergoes the final moult, the cast cuticle being easily seen at one end inside the cocoon.

The insect, however, has now assumed a form quite different from that of the larva; this form is the chrysalis or pupa, and as such is incapable of locomotion and feeding. The pupa measures about three-quarters of an inch long, is yellowish at first, but soon becomes black with yellow markings, while the form of the future moth (head, antennæ, thorax, legs, wings and abdomen) can be traced on the pupal cuticle. After from about two to five weeks, the pupa opens by a cross-shaped slit on the back just behind the head, and the moth draws itself out. At first the moth is comparatively helpless after having been confined within the limited space of the pupal cuticle; soon, however, the body hardens, the wings smooth out, and the insect is ready for flight.

Metamorphosis is carried to a much higher state of perfection in the case of such insects as blowflies and houseflies (Fig. 4). The larva, or maggot, is without any external sign of head and legs, though these, together with the wings of the future fly, develop from rudiments within the body of the maggot. At the final moult the larval cuticle is not discarded, as in the case of the moth, but hardens to form a case—​the puparium—​within which the pupa lies.

The life-cycle of the magpie moth is illustrative of the principles of metamorphosis characterising the development of a great many insects, such as all moths and butterflies, beetles, flies, bees and wasps, etc.; but, although the general characters of the larva, pupa, and adult moth are common, with but slight variation, to corresponding stages of moths and butterflies as a whole, these stages in other insects, though readily recognised, have their own characteristics.

Outstanding features in a life-cycle involving metamorphosis are that growth takes place only in the larval state, and that the insect parades through life in different guises—​egg, larva, pupa, and adult—​each with its own peculiarity of habit and form, although the adult and pupa resemble one another much more than do the adult and larva; but no matter how dissimilar the larva, pupa, and adult may outwardly seem, structures common to them all may be traced throughout. Make, for example, a comparative study of the larva, pupa, and moth of the magpie moth; the head, thorax, and abdomen can be seen in each stage, while counterparts of the larval antennæ, eyes, mouth-parts and feet persist in the moth, though more or less profoundly modified during pupal transformation. Although there are no external signs of wings in the larva, these appendages are developing, nevertheless, in concealed “pockets” within the larval thorax, and, at the time of pupal formation, become extruded and lie ensheathed with the legs and antennæ in the pupal cuticle along the sides of the pupal body. Apart from these changes, the larval mouth parts undergo a most profound metamorphosis; apparently, though there is no similarity between the long “tongue” or proboscis of the moth and the jaws and accessory jaws of the caterpillar, the proboscis, adapted for sipping the nectar of flowers, is nothing but the accessory jaws of the leaf-chewing larva greatly elongated; with the exception of the palps of the accessory jaws, the other larval mouth parts are either absent in the moth or reduced to vestiges.

In the case of insects that develop without a metamorphosis, the life-cycle is one of comparative simplicity. An example of such an insect is the so-called “silverfish” (Lepisma saccharina), common in dwellings, especially in damp places, dark and dusty corners, flour and sugar bins, while not uncommonly it causes some considerable damage by devouring the paste and glaze from wallpapers and the binding and leaves of books.

The silverfish (Fig. 4), wingless throughout life, measures about one-quarter of an inch long when full grown; it is silver-white in colour, due to a clothing of glistening scales that rub off as a silky powder when the insect is handled. It glides rapidly about, especially after dark, and is one of the most primitive insects, there being minute leg-like processes attached in pairs to the under side of the abdomen; the normal thoracic legs are well developed. The body is wedge-shaped, tapering to the posterior end, from which three tail-like appendages project, while anteriorly a pair of long, delicate antennæ arises from the head.

All stages of the silverfish, from the minute, freshly-hatched individuals to fully-grown ones, may be found in the one place, the smaller ones being immature developing stages. In the case of another species allied to the common silverfish, the female lays from six to ten eggs at one time in sheltered crevices, and the young hatch forty-five to sixty days later, when the temperature ranges from 65 degrees to 68 degrees Fahrenheit.

Unlike the moth larva, that of the silverfish throughout its growth resembles the adult both in habit and form, the only marked differences being that of size and the absence of the abdominal leg-like appendages. During growth several moults take place, and at the final one the adult appears with all its characteristics. Some species take two years to reach maturity. In this type of insect there is, therefore, no pupal or resting stage, and the larval habits and food are the same as those of the adult insect, while there is but little difference in structure throughout all the stages.

There are many winged insects (e.g., cockroaches, crickets and earwigs) that show a slight advance toward a metamorphosis. Though their larvæ differ from the adults principally in the absence of wings, there are stages between the younger larvæ and the adults in which the wing rudiments appear. These rudiments first appear after one of the moults as small bud-like structures on each side of the thorax (earwig, Fig. 4), becoming larger after each succeeding moult, when the developing wings may be seen enclosed in a sheath of the cuticle; at the final moult the wings, no longer enclosed in their coverings, straighten out and become functional. A very pronounced difference is here noted between the wing development of such insects and that of a moth, in that the wing rudiments of the former develop externally and those of the latter internally.

A decided advance toward a metamorphosis is exhibited by insects known as thrips (Fig. 4). Though readily overlooked on account of their minute size (one-twenty-fourth of an inch and less), they are nevertheless conspicuous on green foliage and white flowers owing to their blackish or yellowish colour. Thrips, when magnified, are easily recognised by their peculiar wings; each is feather-like, being formed of a narrow rib-like membrane clothed along the margins with long and delicate stiff hairs. Thrips’ eggs are laid upon the plant surface or within the tissues, according to the species, and are very minute (about one-twenty-third of an inch long). The larvæ puncture the plant tissues and feed upon the juices just as do the parents, which they resemble in general form, except that there are no wings and the antennæ are very short and the eyes small. There are two or three larval moults, after which the insect is more like the adult, though still resembling the larva. It now differs from the latter, however, in the antennæ being considerably shortened, and in the appearance of a pair of finger-like processes on each side of the body attached to the thorax and lying along the sides of the abdomen; these processes are the sheaths enclosing the wing rudiments of the future adult. The insect again moults, changing to a form resembling the preceding stage in many respects, but differing in the wing sheaths being much longer, and in having the antennæ, enclosed in sheaths of cuticle, turned back over the head. Although during these two stages the insect is capable of moving about, it is nevertheless sluggish and does not feed; from this second semi-quiescent stage the adult emerges. In the thrip’s cycle, therefore, although the habits of the larva and adult are similar, the presence of the two intermediate semi-quiescent stages, during which feeding ceases, shows a decided advance toward a true metamorphosis and represents a pupal stage.

In the case of those insects not involved by a metamorphosis, as discussed above, the structure and habit of both adult and the immature stages differ but little, the development of wings being the principal change, except in the case of the thrips, where there is a definite tendency toward a pupa. However, passing on to a consideration of the common cicada (wrongly called a locust), a change in both structure and habit occurs during the life-cycle, the immature stages being adapted to a subterranean life, while the winged adult frequents the foliage of trees; all stages agree, however, in puncturing plant tissues with their proboscis and sucking up the nutrient juices from the roots by the larva and from the stems and leaves by the adult.

The female cicada (Fig. 4) lays its eggs in colonies beneath the young bark of trees and shrubs; the larvæ, on hatching, drop to the ground, into which they burrow; the antennæ and soft body are comparatively long, while the fore legs are greatly modified for grasping plant roots and as digging tools. After a number of moults, the body shortens, the antennæ come to resemble those of the adult, and the rudiments of the wings appear. Growth and the activities of the developing insect continue until finally the larva constructs an earthen underground chamber, in which it lies torpid until ready to undergo the final moult; in this inactive state, though still resembling the later larval stages, the insect corresponds to the pupa of the moth. For the final moult the pupa leaves the ground, crawls up some support (a tree trunk or post), where the winged adult emerges, leaving the empty pupal husk attached to the support. Besides the change in habit and the possession of functional wings, the adult cicada differs in many structural features from the immature stages. Outstanding differences are the normal fore legs, the development of a “voice-box” in the male, and an ovipositor in the female.

An insect that shows some linkage between those having a true metamorphosis and those having a partial metamorphosis is the aphis-lion (Micromus tasmaniæ), though undergoing a true metamorphosis itself. The larvæ are predaceous and feed upon aphids (Fig. 4). Its larva, pupa, and adult are distinct forms, as in the moth, but the larva is not of the specialised caterpillar or grub type, rather resembling in general appearance the silverfish, or the type of young larva peculiar to such insects as the earwig or thrips before the wing rudiments develop. Furthermore, the pupa, though one in the strict sense, is capable of great freedom of movement, its head, mouth-parts, antennæ, legs and wings, ensheathed by the cuticle, being freely movable, and not rigidly attached to the body.

A review of the early larval stages of the earwig, thrips and cicada, prior to wing development, and of the aphis-lion larva, shows a conformity to a generalised type exemplified by the primitive silverfish. On the other hand, the moth caterpillar exhibits another larval type more highly specialised, though still retaining a modified semblance to the silverfish type, while specialisation is carried to the highest degree in the blowfly maggot, where all outward sign of the primitive larval type is lost. Regarding the pupæ, there are three types; the most simple is the free pupa, like that of the aphis-lion, and some moths, beetles, etc., where the appendages are freely movable. The most complex is the pupa of the blowfly, enclosed in its puparium, while intermediate between these two extremes are many moth pupæ that have the appendages firmly attached to the body, but nevertheless visible.

CHAPTER V.

Sucking Insects.

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.