Boring and Underground Insects.
CODLIN MOTH(_Cydia pomonella_).--The codlin moth caterpillar burrows
in developing apples and pears, and such “wormy” fruit is known to
everybody.
The moth itself is seldom seen, since it lies concealed until after
nightfall, when it becomes active and lays its eggs. The insect
(Fig. 12a) measures about three-quarters of an inch long, and is
inconspicuously, though beautifully, coloured; the fore wings, which
cover the body when closed, are light grey, crossed by fine bands of
a darker hue, giving the appearance of watered-silk, while at the
extremity of each wing is a large bronze spot; the hind wings, seen
only when expanded, are of a light brown colour. The minute flat eggs
are laid on the foliage of leaves, on the fruit, or even on young bark;
they appear at first as glistening white specks, but, as development
advances, a red ring develops, and finally a black spot just prior to
the caterpillars hatching.
In some places the first larvæ developing in the spring enter the fruit
by way of the calyx, but under New Zealand conditions it is more usual
for entry to be made by boring through the skin of the apple. Having
completed their development in the fruit, the caterpillars bore their
way out and spin their cocoons beneath the loose bark of the tree
trunks; in these cocoons pupation takes place, and from them the next
generation of moths develops.
Although in New Zealand there is usually only one generation produced
each year, three or even four develop in other countries. The winter
is passed by the larvæ in their cocoons, and pupation takes place just
prior to the period when the moths emerge in the spring. As the moths
continue to emerge and lay their eggs for a period extending from
November to February, it is essential that regular applications of
arsenate of lead be made during that time in order to protect the fruit
from the larvæ hatching from the eggs laid by the moths. In localities
where the spring larvæ enter the calyx of the fruit, it is essential to
apply the first spray just after the petals fall, so that the poison
may lodge in the calyx before it closes. The removal of rough bark from
the trunks of both apple and pear trees is a help in controlling the
insect. Another method sometimes used is to band the tree trunks with
strips of scrim; under these bands the larvæ collect, and the former
can be later removed and destroyed with their tenants.
CURRANT CLEAR-WING BORER(_Sesia tipuliformis_).--This destructive
moth has been carried to and established in New Zealand, as well as
many other parts of the world. In currant gardens its larvæ cause
the death of canes by eating out the pith. The moth (Fig. 12b) is a
very conspicuous and beautiful insect; the wings, which expand to
about three-quarters of an inch, are transparent and bordered with
golden-purple, a bar of the same colour crossing the surface of the
fore wings; the body (about half an inch long) is metallic-purple, the
thorax having a yellow stripe on each side, while the abdomen, barred
with golden bands, terminates in a fan-shaped tuft of purplish hairs.
The moths are active each year in the spring, when they lay their
brownish, globular eggs singly on the bark of the currant canes. The
resultant larvæ bore into the stem and destroy the pith, passing the
winter in the damaged canes. In the following spring the larvæ become
active once more and approach the surface, where pupation takes place
shortly before the moths emerge.
There is only one generation each year, and control lies in the removal
and burning of infested canes in late winter.
TOMATO STEM BORER(_Gnorimoschema plæsiosema_).--Tomato growers are
frequently faced with the problem of the destruction of tomato plants
caused by the attacks of the larvæ of the tomato stem borer moth. This
insect caused considerable damage for the first time in Auckland some
fourteen years ago, though it was known in other parts of the country
as well.
The moth itself (Fig. 12c) is a small one, measuring about a quarter
of an inch with the wings closed. In this position the insect is
wedge-shaped and conspicuous. Against the general greyish-brown colour
is a dark brownish area on each side. The eggs are laid on the tomato
leaves, in which the young caterpillars tunnel as they work toward
the leaf petioles, down which they burrow into the main stems. In the
damaged stems, pupation takes place. Under favourable conditions, this
insect may pass through at least three generations during the season.
Control depends upon sanitation and the use of arsenate of lead
sprays. All infested stems, together with plants after the crop has
been removed, should be burned; as the insect is known to attack
potato plants and tubers, care should be taken to destroy all potato
tops after harvesting. Frequent applications of arsenate of lead are
essential to protect the tomato plants, especially during the earlier
part of the season.
When on this subject, mention should be made of the potato-tuber
moth (_Phthorimæa operculella_), which is somewhat similar to the
tomato-stem borer, both in appearance and habits. The larva of this
insect is best known from its habit of boring through potato-tubers;
these burrows become filled by a fungus after the larvæ have vacated
them. The adult potato-tuber moth is a night-flyer, and lays its eggs
upon the leaves of the plants; the larvæ burrow down the stems, and may
even reach the tuber below ground. When seed is not properly buried,
the moth will also lay its eggs in the “eyes,” and so directly infest
the tuber; this danger applies also to potatoes in store or in bags.
In the control of the potato-tuber moth, the following points should
be noted:--Select only sound seed and cover well when planted. On
harvesting the crop, do not leave the bagged potatoes standing in the
field overnight, as they are exposed to infestation; neither cover
the open bags with the potato-tops, as is commonly done, since this
will attract the moths. Destroy all tops immediately after harvesting.
Dusting potatoes in store with slaked lime will tend to act as a
protection against the moth.
[Illustration: FIGURE 12.
A--1, Codlin moth; 2, codlin larva in apple. B--1, Currant clear-wing
moth; 2, clear-wing moth larva in stem. C--1, Tomato stem-borer moth;
2, larva of moth; 3, damaged tomato stem. D--1, A long-horn beetle; 2,
larva of long-horn beetle. E--1, A leaf-mining fly; 2, leaf attacked by
leaf-miner. F--1, Subterranean grass-caterpillar moth; 2, subterranean
grass-caterpillar. G--1, A click beetle; 2, a wire-worm. H--1,
Larger narcissus fly; 2, smaller narcissus fly. K--1, A subterranean
spring-tail; 2, a leaf-eating spring-tail.]
ROUND-HEADED BORERS.--Apple, almond, and citrus trees, together
with gooseberry and such ornamental and shelter trees as poplars,
tree-lucerne, and goat-willow, are sometimes damaged by round-headed
borers, which tunnel in the stems and branches. These borers (Fig. 12d)
are white in colour, narrow-bodied, and cylindrical, the segments
being usually well defined, and belong to a group of beetles known
as long-horned beetles, a group of insects to which the common hu-hu
beetle belongs. These beetles are narrow-bodied, and their antennæ are
comparatively long and conspicuous.
To control these pests, the only thing to do is to cut out and burn
the badly-infested parts. Where a borer is located (and this can be
frequently done by the presence of the powdered wood ejected from the
burrows), the culprit may be killed by injecting into the tunnel some
carbon bisulphide and plugging up the openings with some clay or other
similar substance.
LEAF-MINING FLIES.--Very often the leaves of cineraria and
chrysanthemum are disfigured by the tortuous tunnellings of the maggots
of minute flies (Fig. 12e). The adult insects are two-winged, and in
structure resemble in many respects miniature houseflies. The eggs
are laid in the leaf tissues, in which the whole development of the
maggots and pupæ takes place. The white maggots are small, legless and
headless. Spraying with black-leaf 40 would act as a deterrent to the
flies, while infested leaves should be removed and destroyed before
infestation becomes general.
GRASS GRUB(_Odontria zealandica_).--As explained in the preceding
chapter, the grass grub is the larva of a native cockchafer beetle
(Fig. 11, 1). This grub, by feeding upon roots, causes extensive damage
to pastures and lawns, as well as to many garden plants, including
strawberries. In the case of pasture and lawns, the presence of even a
considerable number of grass grubs is not detrimental unless they occur
concentrated in definite areas, when the damage is pronounced. With
garden plants, however, which are isolated when compared with the dense
root masses of grasses, the attacks of one or two grubs upon the roots
of a single plant may cause serious injury.
Grass grub damage to grasses is not merely due to attack upon
the roots. While feeding, the grubs swallow soil with the roots,
rendering the former spongy, and so disturb the normal circulation
of moisture about the grass roots. In the case of infested lawns,
it is advantageous to roll infested areas in order to pack the soil
pulverised by the grubs, and re-establish normal circulation of soil
moisture. Another important feature in grub control is to stimulate
root development by means of fertilisers. A recently-developed method
of “grub-proofing” lawns is to broadcast over every thousand square
feet of turf to be treated one bushel of screened sand or clean soil,
in which 5lb. of lead arsenate powder have been intimately mixed. This
is said to remain effective for a period of three years; but such
fertilisers as nitrate of soda, superphosphate, sulphate of potash,
and potassium chloride should not be used on “grub-proofed” turf, as
they react with the lead arsenate, and reduce its effectiveness, though
rotted manure or ammonia sulphate may be used.
The control of grass grubs damaging the roots of strawberry and other
plants is a difficult matter, though some benefit is to be derived
by making holes about four inches deep with a stick in the soil near
to the infested plants and pouring in a little carbon bisulphide; the
holes should be closed immediately. To protect strawberry beds, if
they are not too extensive, the most satisfactory method is to cover
the plants with scrim, stretched on frames, at dusk during November
and early December, when the beetles are flying; this will prevent the
insects from infesting the ground with their eggs. The use of sulphur
smudges, already referred to, is of great importance in this respect.
SUBTERRANEAN GRASS CATERPILLARS.--These caterpillars are the larvæ of
native moths (Fig. 12, f1) belonging to the genus _Porina_, and when
they become epidemic they cause much more extensive damage to pasture
and lawns than do the grass grubs. When full grown, the greyish-black
caterpillars (Fig. 12, f2) reach a length of about three inches; they
are soft-bodied and rather flaccid, and live in underground burrows
of varying depth. After dark, these caterpillars come to the surface
and devour the grass, eating it close to the ground, much soil being
swallowed by the larvæ during the feeding. This soil is evacuated,
and resembles earthworm castings, but is mixed with silk spun by the
caterpillars; the emergence holes of the caterpillars, about the
diameter of a lead pencil, are conspicuous on the surface denuded of
its covering of grass. Pupation takes place underground, and when the
moths emerge the pupæ first move to and project beyond the surface
of the ground; these pupæ are large and easily recognised by the
wing-cases, which are very short compared with the length of the body.
The moths are on the wing during spring and summer, the rest of the
year being spent in the larval stage. The moths are night-flyers, and
are amongst the largest species in New Zealand, their wings having
an expanse of from one to over two inches; they are heavy-bodied
insects, and vary considerably in colour. One of the commonest, species
is brownish-yellow, or sometimes a smoky-grey, with a white streak
bordered with black on the fore wings; the hind wings may be pinkish.
The most satisfactory method of controlling the insect is to roll
infested lawns after dark, in order to crush the caterpillars whilst
feeding on the surface. Flooding an infested lawn with water will bring
most of the caterpillars to the surface, when they can be collected and
destroyed. Spraying grass in spring and early summer with arsenate of
lead will tend to poison the immature caterpillars. There are at least
three species of insect parasites that attack these larvæ, and there
is also a fungus which invades and destroys the whole body, taking the
shape of the insect; such fungus-infested caterpillars are commonly
called “vegetable caterpillars.”
WIREWORMS.--The roots of garden plants and germinating seeds are
often damaged by hard, wiry beetle grubs, reddish-brown or whitish in
colour, called “wireworms,” so named from their resemblance to short
pieces of wire; they have three pairs of legs behind the head and a
sucker-like appendage on the last body-segment (Fig. 12, g2). These
grubs transform to narrow-bodied, brownish or blackish beetles, known
as “click-beetles” (Fig. 12, g1) from their habit, when overturned, of
righting themselves by a springing action, during which a distinct and
sharp clicking sound is made; the spring apparatus consists of a spine,
the tip of which fits into a notch on the under side of the thorax.
Practically nothing is known as yet in regard to the biology of the New
Zealand click-beetles. They are extremely difficult to control, and the
larval stage covers a period of two or more years.
NARCISSUS FLIES.--There are two species of narcissus flies--the larger
(_Merodon equestris_) and the smaller (_Eumerus strigatus_) both occur
in New Zealand. The larvæ of these flies attack bulbs of various
kinds, the hosts of the larger fly being narcissus, hyacinth, tulip,
amaryllis, habranthus, vallota, galtonia, scylla, and leucojum; and of
the smaller fly, narcissus, hyacinth, onion and shallot. These flies
are two-winged insects, the hind wings being wanting as such, and
belong to a group called the syrphid, or hover flies.
The larger narcissus fly (Fig. 12, h1) resembles somewhat a humble-bee
(which, however, has four wings); its stout and very hairy body
measures about half an inch long. There is considerable variation in
colour, though black or brown predominates, with greyish or yellowish
hairs, and bands of the same colour; the bands, however, are absent in
some individuals.
During spring the insects fly about in the sun, and lay their eggs at
the leaf bases of the host plants, or on the exposed neck of bulbs, or
in the soil close by. The larvæ, which are legless, yellowish grubs,
enter the bulb, and may completely destroy it. Infested bulbs may be
detected by an unnatural softness near the neck when pressed between
the fingers.
The smaller narcissus fly (Fig. 12, h2) is about half the length of
the larger, of a shiny black colour, with metallic reflections, and is
not clothed with hair. The eggs are laid in the ground, or at times
upon the plant itself. Several larvæ of this fly may be found in the
one bulb; the larvæ resemble those of the larger fly, but are smaller,
and have three small processes at the end of the body. The smaller
narcissus fly usually attacks the bulbs already damaged by some other
agent, though it has been known to infect sound bulbs.
Control of both these flies depends upon the destruction of infested
bulbs. Recent researches have shown that the flies themselves can be
poisoned in large numbers by a spray made of 4oz. of sodium arsenate,
1lb. of crude glycerine, 2lb. of white sugar, and four gallons of
water; this spray is applied during bright and warm weather.
SPRINGTAILS.--These are very minute, soft-bodied insects, which
are very active, and have a habit of springing with the agility of
fleas. There are several species, but two are of interest to the
horticulturist.
One of these (Fig. 12, k1) is white in colour, narrow-bodied, and lives
underground, especially in damp places, where it damages germinating
seeds, or the roots of seedlings; even older herbaceous garden plants
are attacked. As a control, it is important to drain the soil in damp
locations and to dig in calcium cyanide about two weeks before planting
or sowing.
The second species is blackish and more or less spherical (Fig.
12, k2); at times it does considerable damage in the spring to the
seed-leaves of young plants as soon as they appear above ground.
Spraying small areas--_e.g._, of cucumbers, turnips, etc.--with
black-leaf 40 would help to protect the plants; as the eggs are laid in
the ground, and as these develop best under moist conditions, thorough
cultivation prior to sowing the crop is an important controlling
factor.