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.
[Illustration: FIG. 5.--ILLUSTRATIONS OF DIFFERENT TYPES OF
SCALE-INSECT LIFE-HISTORIES.]
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 2-1/2 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.
[Illustration: FIG. 6.
(a) Cottony cushion scale. (b) Mealy bug. (c) The black olive scale.
(d) Gum tree scale: On right, females on twig; upper left, male scales;
lower left, the ladybird beetle; centre, scales destroyed by beetle.
(e) Hemispherical scale. (f) Fruit lecanium scale.
_Photographs by W. C. Davies, Cawthron Institute._]
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.
[Illustration: 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._]
Natural enemies in New Zealand are two species of hymenopterous
parasites, _Aphelinus fuscipennis_ and _A. mytilaspidis_, the latter
also attacking the apple mussel scale. Ladybird beetles also feed upon
the insect.
Control requires close attention, and can be effected by the
application of lime-sulphur in the dormant season, when it is essential
to apply a strong wash to kill off as many scales as possible before
reproduction commences in the spring. At bud movement further
applications are necessary to destroy the young insects.
RED ORANGE SCALE(_Chrysomphalus aurantii_).--The red orange scale
is distributed throughout the world, and is especially abundant in
tropical and sub-tropical regions, the most southern limit being New
Zealand. As a major pest it is peculiar to citrus, but infests to a
minor extent other plants--_e.g._, plum, apple, pear, quince, grape,
fig, euonymus and rose. So far it has been found only on citrus in
New Zealand, it being well established in the Auckland province, and
also in the South Island on Banks Peninsula. It is very often found on
imported oranges and lemons.
This scale is a circular one, with a central pimple-like prominence, as
in the case of the San José, but is flatter, about half as large again,
and is of a characteristic reddish colour. The damage done to citrus
trees by this insect is of a serious nature, as the entire tree or part
of it may be killed in severe infestations. A characteristic feature
of this species is that no honey-dew is secreted, and hence there is a
total absence of sooty mould on infested trees.
Like the San José scale, the red scale is viviparous, and over-winters
as partially mature adults, completing development in early spring,
when the young insects make their appearance. An average of about 55
young is produced by each female, and development to maturity takes
from two or two and a-half months; about one month later young are
produced, and their production continues over a period of one or two
months; climatic conditions, however, have a direct influence on
development.
An important natural enemy is the steel blue ladybird (_Orcus
chalybæus_), imported from Australia; but the most efficient control is
cyanide fumigation, or spraying with red oil or lime-sulphur.
THE BLACK SCALE(_Chrysomphalus rossi_).--Foliage of palms, oleander
and citrus is often infested by this reddish-black to black circular
scale (Fig. 7, 3); it is almost flat, with a central whitish spot, and
measures up to one-tenth of an inch in diameter; when many individuals
are crowded together, their outline becomes irregular. This species is
not especially injurious, though common.
OLEANDER SCALE(_Aspidiotus hederæ_).--This cosmopolitan insect occurs
on orchids, oleander, ivy, camellia, palms, citrus, coprosma, and
karaka, infesting stems, leaves and fruit. In the case of citrus, this
insect delays colouring of the fruit, which becomes blotched with
yellow or green. The insect may be so numerous, that it completely
covers the whole plant, giving to the latter a white appearance; this
is due to the preponderance of white male scales, the female being
slightly yellow, with a purplish tint.
The female scale is almost circular (Fig. 7, 7), having a diameter of
from one-twenty-fifth of an inch to two-twenty-fifths of an inch, and
is rather flat; the male is more oval and of the same size, and in both
cases there is a central orange-yellow spot. The eggs are comparatively
large, and hatch soon after being deposited. The insect reaches
maturity in from four to six weeks.
GREEDY SCALE(_Aspidiotus rapax_).--This European insect is now
widespread, and in New Zealand is common on apple, pear, quince, and
wattle; it has a wide range of hosts. The adult female scale is convex
and of a general grey colour, though sometimes yellowish. The winter is
passed in all stages of development.
ROSE SCALE(_Aulacaspis rosæ_).--This is a very common insect, forming
white incrustations on the bark of roses, briar, raspberry, loganberry,
blackberry, and sometimes pear. The adult female (Fig. 7, 8), which is
from one-twelfth of an inch to one-eighth of an inch in diameter, is
rather thin and flat, circular or oval in outline, but irregular when
crowded; the general colour is white or slightly yellowish. The male
(Fig. 7, 9) differs, in being elongated and narrow. This insect can
withstand severe winters, and is to be controlled by the use of red
oil.