Sucking Insects—(Concluded).
Plant Lice, or Aphides.
The small, soft-bodied plant-lice, or aphides, usually found forming dense colonies on all sorts of plants, are pests well known to every gardener; they attack plants by inserting into the tissues their delicate piercing mouth-parts, and drain the nutrient sap ([Fig. 8], 1g). All parts of a plant may be infested, and the insects, owing to their ability to reproduce abundantly and rapidly, may destroy the plant, or at least injure it by stunting its growth, curling the leaves, or deforming the flowers and fruit. In many cases aphides copiously secrete honey-dew, upon which sooty mould grows, rendering the plant unsightly; on this honey-dew ants feed, and are frequently seen associated with aphides. Apart from their direct injurious effects, aphides are of outstanding importance, in that they transmit some of the most serious plant diseases. Of all the species occurring in New Zealand, only one species is supposed to be a native.
Most aphides live exposed upon the host plant (e.g., Rose Aphis), but some (e.g., Woolly Aphis) secrete a protective covering, while others cause a malformation of the plant tissues which form a partial protection as a semi-gall (e.g., Elm-leaf Aphis), or a complete protection as a true gall (e.g., Leaf-petiole Gall-aphis of Poplar).
Aphides present certain variations in structure, and, generally speaking, the one species presents four or five types ([Fig. 8], 1): the asexual (parthenogenetic) wingless and winged females that give birth to living young (viviparous) in the absence of males, and the sexual forms, both males and females, the latter producing eggs (oviparous).
The best character by which the New Zealand aphides are to be recognised is to be found in the pair of longer or shorter horn-like processes, or “cornicles,” projecting from the upper surface of the abdomen; in some species, however, the “cornicles” are reduced and inconspicuous (e.g., Woolly Aphis), or altogether absent (e.g., Grape Phylloxera). The “cornicles” are frequently called “honey-tubes,” since for many years it was thought that they secreted the honey-dew; it has been shown, however, that the honey-dew is secreted from the rectum, and that the function of the “cornicles” is to secrete a waxy protective substance, which may take the form of a powder or woolly threads. The wings, when present, are membranous, the front pair being much larger than the hind ones, and when not in use usually close roof-like over the body.
FIG. 8.
(1) Life History of an Aphis: A, egg; B, C, and F, wingless females; D, winged female; E, male; G, section of head and plant tissue to show method of attack. (2) Life History of a Leaf-Hopper: H, eggs under bark of twig; I, first stage hopper; J, later stage hopper with developing wings; K, adult from above; L, adult from side. (3) Life History of a White Fly: M, egg; N, first stage larva; O, pupal stage under scale covering; P, adult. (4) An adult Thrips.
In their life-histories and habits aphides present many variations, sometimes of considerable complexity, but fundamentally the processes are as follows:—Eggs are laid on the host plant during the autumn, and give rise to wingless females in the spring; these females (being asexual or parthenogenetic, since they reproduce without being fertilised) are viviparous, producing living forms similar to themselves. Some of these forms remain wingless, while others may develop wings, upon which a wider dispersal of the species depends, but in both cases such females are asexual and viviparous. Several such generations may develop until the autumn, when males and females appear, the latter being oviparous, producing the over-wintering eggs when fertilised by the males. Very often, however, the life-cycle is considerably complicated by the winged forms flying to other host plants and establishing there colonies differing in many respects from the parent stock; from these secondary hosts there is a return migration to the original species of plant. Again, the migrations may be restricted to different parts of the same plant, from the leaves or branches to the roots, for example. Most aphides are readily controlled by means of insecticides, such as nicotine-sulphate, or kerosene-emulsion. They are also very often held in check by natural enemies, such as aphis-lions, hover-flies, ladybirds, and numerous forms of hymenoptera. The following species are some of the commoner aphides met with in New Zealand:—
Black Peach—aphis (Aphis persicæ-niger).—From early spring, even before the foliage develops, this aphis may be found heavily infesting the young, succulent shoots of peach; it also occurs on cherry, plum and nectarine. The adult insects are black and the immature stages pale reddish-brown, dull brown, or lemon-yellow. During the winter the insect lies underground about the roots of the host plant, and thence migrates to the young growth in spring. At first only wingless forms are seen, but as the season advances the winged migratory aphides develop; at that time the foliage is so severely attacked that it becomes crumpled and functionless ([Fig. 9], 1), while the developing fruit is distorted and rendered useless. The heat of the late summer destroys the aphides still on the foliage, but sufficient numbers descend underground for protection, where they live over winter.
Green Peach—aphis (Rhophalosiphum persicæ).—This aphid occurs on a wide range of plants, including the peach, and, as a rule, is most abundant during summer and autumn; as the name implies, the general colour is green, though some individuals are reddish or brownish-yellow; the wingless forms have black-tipped “cornicles,” and on the abdomen of the winged insects are dark markings.
Black Cherry—aphis or fly (Myzus cerasi).—This aphid has now a world-wide distribution. In New Zealand it has been found on cherry and plum, though in other countries its hosts include peaches, red and black currants, and cruciferous plants, such as common mustard, shepherd’s purse, etc. This species exudes copious honey-dew, upon which sooty mould develops, thus rendering fruit unfit for use. The principal injury, however, is due to the destruction of shoots and leaves, the latter frequently curling up when the insect clusters in dense colonies upon the infested plant. The complete life-cycle has not been followed under New Zealand conditions, but the shiny black eggs occur on the bark and buds of cherry trees during the winter. In spring the eggs hatch, and the insects, rapidly reproducing, attack the young shoots and leaves. Observers in other countries have noted that there is a summer migration of winged females to cruciferous plants, where colonies are established, and whence there is a return migration during the autumn to the original host. The wingless females are black, with part of the legs yellow, while the young individuals are pale in colour; the winged females have a green abdomen, from which arise the black “honey-tubes.” Since all the over-wintering eggs have hatched by the time the buds open, the insect can be then controlled by applications of nicotine-sulphate.
FIG. 9.
(1) Peach leaves attacked by Black Peach aphis. (2) Colony of Cabbage aphis on leaf. (3) Stem of insignis pine attacked by Chermes. (4) Grape Phylloxera and galls on vine roots. (5) Grape Phylloxera galls on vine leaf. (6) Woolly aphis on apple twig. (7) Galls of Poplar aphis. (Figs. 1, 2 and 6 by W. C. Davies; Fig. 4. after U.S. Dept. Agric.; Fig. 5. after N.Z. Dept. Agric.)
Cabbage Aphis (Brevicoryne brassicæ).—The cabbage aphis, or cabbage green fly, is widely distributed throughout the world, and has become a serious pest in New Zealand, causing considerable damage to cruciferous crops; it infests rape, turnip, cabbage, Brussels sprouts, cauliflower, as well as related weeds, such as wild mustard, shepherd’s purse and watercress. Most damage is done during dry seasons, when the plants succumb more readily to attack; if the insects are numerous, they cause the leaves to curl, and give a greyish appearance to infested plants, which may become flaccid and sticky from the copious honey-dew of the insect. The wingless forms are bluish in colour and coated with a greyish powder, but the winged females have the head and thorax black and the abdomen greenish ([Fig. 9], 2). In New Zealand all stages may be found throughout the year on winter crucifers or on weeds, though reproduction is retarded during the winter; in the spring the winged females fly to young crops. In very cold climates eggs are laid in the autumn, and these survive the winter. The cabbage aphis is attacked by a number of parasites, and usually the brownish empty shells of a large number that have been destroyed by a small parasite are to be found at any time; other important enemies are the hover-flies, the eleven-spotted ladybird beetle, and the Tasmanian aphis-lion. The insect can be controlled by spraying with nicotine-sulphate to which soap has been added.
Pine Tree Chermes (Chermes pini).—This is a widely-distributed species, occurring upon both Austrian and insignis pine in New Zealand. The insect lives in colonies upon the cones, twigs and branches, as well as around the bases of the needles; each aphis exudes a woolly covering, which forms conspicuous white masses when the trees are heavily infested ([Fig. 9], 3). Young trees seem to be the more subject to infestation, from which they may recover as they grow, but some damage is caused by the insect by a weakening of the trees, especially where grown in unsuitable localities. It is frequently noticed that individual trees in a plantation are heavily infested, while adjacent trees of the same species are not. The wingless form of the insect, covered by its mat of white threads, is brownish in colour and ornamented with numerous dark spots; there are no “honey-tubes” on the abdomen. The life-cycle of this insect becomes complicated, when it develops on two types of conifers; in the latter case the primary host is a species of spruce upon which the insect forms galls, and the secondary host may be larch, Douglas fir or pine, upon which gall formation is unusual. So far as is known, only the pine-infesting form of the aphis occurs in New Zealand.
Grape Phylloxera (Phylloxera vastatrix).—This destructive aphis, sometimes called the grape louse, is a native of North America, where it normally infests grape vines. It was accidentally introduced into the grape-growing districts of France, where it became very destructive. It later made its appearance in New Zealand. The insect infests both the leaves and roots of grape vines, the root-feeding stages being the most destructive, in consequence of which vines are now grown on resistant root stocks. The leaf-infesting stages of the insect cause pocket-like galls to form, which open on the upper surface of the leaf by a narrow aperture concealed under a tuft of delicate hairs ([Fig. 9], 5). In each gall the aphid matures and deposits several hundreds of eggs, from which wingless females hatch; these wander to other leaves, and each insect forms a new gall for itself. Several generations develop thus, but later many of the offspring migrate underground and join the root-infesting colonies. The irritation set up by the latter causes yellow flabby nodules to develop on the roots ([Fig. 9], 4). These nodules, or galls, later decay. The root-feeding aphides are wingless, and reproduce by means of eggs for several generations. Although they may go on developing thus for many years, it usually happens that, toward autumn, some of the insects transform to winged females, which fly to other vines or are carried thence by the wind. There each female feeds on the lower leaf surface, and deposits two kinds of eggs, some larger and some smaller; from the larger develop wingless females, and from the smaller wingless males, which are unable to feed. After fertilisation, each of these females deposits a single egg upon the older bark of the vine; such eggs do not hatch until the spring, when they give rise to the wingless females that start the galls on the leaves. Control depends on the use of phylloxera-resistant stocks, since it is from the root colonies of the aphis that the foliage is re-infested in the spring. An important feature is to prevent the scion from sending down roots where the union of the scion and root stock is close to the soil; if such scion roots form, they should be cut away and the soil removed from the union.
Rose Aphis (Macrosiphum rosæ).—The rose aphis is perhaps one of the best-known insects of the garden, mainly owing to its prevalence upon the young growth of all kinds of roses; it sometimes occurs on apple, tomato and rhododendrons. In a colony some of the insects are pink, and others bright green, though in the winged forms the head, antennæ, thorax, a row of spots on each side of the abdomen, and the “honey-tubes” are black; in both winged and wingless forms the eyes are red. In the case of severe infestations, plant growth is retarded and the leaves and flowers become distorted. Control can be effected by applications of nicotine-sulphate, kerosene, or soap solution.
Apple Woolly Aphis (Eriosoma lanigerum).—Although frequently called “American Blight,” the apple woolly aphis is probably a native of Europe. It occurs throughout New Zealand, and was a very serious pest until controlled by the Aphelinus parasite. The presence of this insect is made apparent by the characteristic white woolly patches ([Fig. 9], 6) which appear upon the apple trees, due to the woolly material secreted by the aphid. Another feature is that the part of the tree attacked, even after the insects have disappeared, is disfigured by gnarled swellings, due to abnormal thickening of the inner bark. This species also infests apple tree roots, which become similarly malformed. However, root infestation has been overcome by using root stocks, such as Northern Spy, that are immune. The individuals comprising a colony of woolly aphis are variously coloured, yellow, green and red predominating; a considerable amount of honey-dew is secreted. This species has been found to migrate to the foliage of the elm and mountain ash, but in New Zealand the elm-infesting form has not been found to occur. The insect becomes active in spring, and rapidly increases until the autumn. Under favourable climatic conditions, winged females develop and produce males and females, the latter laying eggs. The woolly aphis is preyed upon by the nine-spotted ladybird, but, as this beetle is itself the victim of another insect, its utility is greatly minimised. The most important check to the aphis is the Aphelinus parasite (Aphelinus mali), the influence of which has been spectacular under New Zealand conditions.
Plum Aphis (Rhophalosiphum nymphææ).—This insect is sometimes very common during spring upon the shoots and leaves of plum in New Zealand; in other countries it has been found to migrate to and infest the flowers and leaves of water lilies. The insects assume various shades of green, the winged females having the head, thorax, and legs blackish; the “honey-tubes” vary in colour, and may be reddish, blackish or yellowish.
Poplar Gall Aphis (Pemphigus pupuli-transversus).—Upon the leaf stems of poplar trees in many parts of New Zealand sac-like growths ([Fig. 9], 7), measuring anything from half an inch to one inch in length, may be found. These are the galls formed by the North American poplar gall aphis. In each gall are colonies of the aphis surrounded by a mass of flocculent secretion. The walls of the gall are thick and tough, with the outer surface wrinkled, while at the end, toward one side, is a slit-like, or sometimes circular, opening surrounded by a thickened rim, presenting much the same appearance as the mouth of a sack gathered together and tied. For the most part, these insects are wingless females only, but during the summer, and particularly toward the end of autumn, winged females develop and migrate to cruciferous plants, such as cabbage, rape, mustard and turnips, or weeds allied to these cultivated forms, upon the roots of which they establish colonies surrounded by a woolly secretion. In spring a return migration to the poplar takes place, and galls are again established.
Leaf-hoppers.
Leaf-hoppers form a group of small, narrow-bodied, sap-sucking insects; as the name implies, they infest the foliage of a variety of plants, and when disturbed have the habit of suddenly leaping or hopping to safety; the species present in New Zealand are usually of a greenish or yellowish colour. The adult insect is winged ([Fig. 8], K, L), and the female lays her eggs in the plant tissues (H); from these eggs the young wingless hoppers (I) hatch and attack the plant; as they grow, wings develop (J), but until then the insect depends for locomotion upon its hopping powers.
The most outstanding species in New Zealand is the apple leaf-hopper (Typhlocyba australis). This insect causes considerable damage to apple trees unless controlled, which can be effected by spraying with nicotine-sulphate against the young insects in the spring.
White-flies.
White-flies, or mealy-wings, are minute sap-sucking insects, having the body and wings covered with mealy wax. The female ([Fig. 8], P) lays her eggs, frequently in circular batches, upon foliage, and the young insects (N) are active, but settle down and commence feeding soon after hatching. Later the insects change to another form (O), without legs and antennæ, and so resemble scale insects to a certain extent; from the latter, however, they may be distinguished by the waxy covering, bearing spine-like processes, and by being surrounded by a distinct marginal area. An important species in New Zealand is the greenhouse white-fly (Trialeurodes vaporariorum), against which fumigation with calcium cyanide is the best as a check.
Thrips.
The foliage of many plants is sometimes infested by very minute black insects, known as thrips. A species commonly met with is that found upon ripe peaches. Thrips are readily identified by the structure of the wings ([Fig. 8], 4), which are but narrow strips fringed with long, rigid hairs. These insects, by puncturing the plant tissues and sucking up the nutrient sap, very often are responsible for infecting healthy plants with disease, such as mosaic.
According to the species of thrips, the female lays her eggs either in the plant tissues or upon the surface. The young insects are wingless, but attack the plant in the same manner as does the adult; as development proceeds, the insect transforms to a pupa, from which the adult ultimately emerges. A characteristic symptom of thrips infestation is a silvering of the foliage, while the leaves are further rendered unsightly by the minute specks of hardened excreta ejected by the insects. Many thrips pass their whole development upon the host plants, while others pass part of their lives underground. One of the commonest species met with under glass and out of doors is the greenhouse thrips (Heliothrips hæmorrhoidalis). Thrips are readily controlled by means of nicotine-sulphate.