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.
[Illustration: 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.
[Illustration: 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.