Miscellaneous Pests.
In this chapter will be grouped for convenience mites, woodlice,
millepedes, slugs, snails, and eelworms.
Mites.
Mites, together with spiders and ticks, belong to a group of animals
distinct from the insects, from which, they differ in many respects;
for example, they possess four, and not three, pairs of legs in the
adult state, no head separated from the body as a movable, distinct
region, while in many cases, especially in mites and ticks, the abdomen
and thorax are continuous; in no case are wings developed.
Mites are of small size, some being microscopic, while others are
just discernable by the unaided eye. All species have the mouth-parts
developed for the purpose of feeding upon liquid food--_e.g._, blood
(in the case of those species that attack animals), decaying vegetable
matter, or the saps of plants. It is the last--that is, those parasitic
upon plants--with which we are here concerned.
The life-history of mites presents some variability, and, though
there are fundamentally four stages of development, additional stages
have been developed by some species which tend to complicate the
cycle. The principal stages in development are as follows (Fig. 13,
1-5):--In practically all cases eggs are deposited, but few species
being viviparous. The larva, on hatching, possesses but six legs, and
resembles an insect in this respect; the larva then becomes quiescent,
and after moulting the eight-legged nymph appears. While in the
nymphal state the mite may undergo one or more moults, giving rise to
additional nymphal forms, that may complicate the life-history. From
the final moult of the nymph the adult mite emerges.
Perhaps the best-known mite in New Zealand is the European red mite
of apple trees (_Paratetranychus pilosus_), though it attacks a wide
range of plants apart from deciduous fruit trees, which it favours; it
has been found on grape vine, raspberry, rose, hawthorn, citrus, etc.
This mite (Fig. 13, 6) occurs in Europe, Russia, British Isles, North
America, Australia, and New Zealand, and it causes considerable injury
to foliage, which assumes a brown appearance, owing to the tissues
drying up where they have been punctured by the mouth-parts of the mite.
In the case of heavily-infested trees, the red eggs of this mite form
conspicuous patches on the bark during winter; these winter eggs are
laid from January onward till leaf-fall, and from them the young mites
hatch in the spring, when the foliage is again attacked. The red mite
develops rapidly, and reaches the adult stage in about two weeks;
several generations are thus produced from spring to autumn, when the
eggs are laid upon the foliage.
The eggs (Fig. 13, 7) are very small, globular, and ribbed on the
surface; from the centre of each projects a hair-like stalk, somewhat
bent at the tip. The colour is bright red, changing to a deep orange.
The red mite lives freely upon the foliage, and does not produce a
web, as do related species; the adult female is bright red to dark
brownish-red, rather globular in shape, with comparatively stout legs
and numerous spine-like hairs on the back. Although the eggs of the
European red mite are exposed on bark and readily accessible to sprays
during the winter, no effective winter wash for their control is yet
known; the most satisfactory method for checking the pest is to spray
the active stages of the mite with summer oil.
Another species of mite, having much the same habits and host plants
as the European red mite, is the brown mite (_Bryobia prætiosa_). The
eggs of this species are of a deep red, with a yellowish tinge in many
cases, but differ from those of the European red mite in the absence of
the polar-stalk and ribbed surface. The brown mites (Fig. 13, 8) are of
a dull red or greenish colour, lack the spine-like hairs on the back,
are decidedly flattened, and have the front pair of legs abnormally
long.
The common red spider (_Tetranychus telarius_) is a species of mite
frequently met with on a wide range of plants too numerous to mention
here; in New Zealand it frequently injures violet, hop, currant,
willow, and many weeds. This mite is to be found in all stages
practically all the year round; during the spring it is mostly found
on weeds and such cultivated plants as strawberry and violet. It is a
web-spinning species, and the minute yellowish-red eggs are to be found
scattered among a fine web attached to the lower surface of leaves as a
rule. The adult mite (Fig. 13, 9) is very active; it is somewhat larger
than the two foregoing species, and of a yellowish-green colour, with
a pair of conspicuous dark spots on the back. Though this mite can be
held in check by the application of lime-sulphur sprays, advantage
should be taken of thorough cultivation during the dormant season,
since the mite hibernates on weeds and among dead leaves and in the
soil.
A mite very often met with by bulb growers is the bulb-mite
(_Rhizoglyphus hyacinthi_), now found in most parts of the world.
Although this mite may possibly be able to attack practically all
tubers or bulbs, it is commonly found infesting narcissus, hyacinth,
tulip, crocus, and Easter lily; it is especially abundant in bulbs with
loose scales, and has been found to be capable of attacking healthy
tissue. The life-history of this species is complicated at times by
the development of additional stages; one of these--the hypopus--is of
particular interest, as it shows more activity than the others, and
attaches itself to the bodies of insects, and is so transported. The
mite develops from egg to adult within a period of nine days under
favourable conditions, or as long as six weeks at other times. All
stages of the bulb-mite occur at the same time in infested bulbs, which
become soft and rotten. The adult mites (Fig. 13, 10) are smooth,
yellowish-white, tinged with pink, and have legs and mouth-parts
reddish. Symptoms of their presence are to be found in stunted growth
and yellowing leaves, failure of flower development, reddish spots on
bulb scales, or a softening of the bulbs. All seriously-infested bulbs
should be destroyed, and the ground where they were grown treated with
calcium cyanide. For the treatment of bulbs, they should be immersed
for ten minutes in a two per cent. solution of formalin heated to 122
deg. Fahr., or simply in water at a temperature of 131 deg. Fahr.
[Illustration: FIG. 13.
(1) Five stages in mite development: (1) Egg, (2) larva, (3) nymph, (4)
older nymph, (5) adult mite. (6) European red mite and (7) egg of same.
(8) Brown mite. (9) Common red spider. (10) Bulb mite. (11) Pear-leaf
blister mite. (12) Common woodlouse. (13) Garden millepede. (14) Garden
slug. (15) Garden snail. (16) Bulb eelworm. (17) and (18) Immature
and mature beet eelworm. (19) and (20) Immature and mature root knot
eelworm.]
Another group of mites of importance to the horticulturist is
that of the blister mites; they are so minute--measuring about a
hundred-and-fiftieth of an inch long--as to be invisible to the
unaided eye. Though so minute, however, their damage to foliage is
characteristic and conspicuous, so that their presence is easily
detected. The most important blister mite in New Zealand is the
pear-leaf blister mite (_Eriophyes pyri_); it differs from the other
mites described above in having a long and cylindrical body, with only
two pairs of legs crowded at the head end, the elongate abdomen having
the appearance of being composed of innumerable segments (Fig. 13,
11). This mite lives in colonies in blisters formed on the leaf, and
sometimes on the leaf petioles. In the spring the yellowish-green
blisters will give the upper surface of an infested leaf a spotted
appearance, and as the season advances these blisters become reddish
and finally brown; in the case of severe infestation, the blisters
become so crowded as to merge into masses.
During the winter the mites lie in the shelter of the bud scales; as
soon as the foliage begins to develop in the spring the over-wintering
mites attack the leaves, each mite forming a blister, in which it
produces a colony of young. The offspring then migrate from the parent
blister and form blisters for themselves, and this goes on until
autumn, when the last generation of mites migrates for the winter to
the shelter of the bud scales.
Owing to the mites being protected within the leaf blisters, summer
sprays are not effective as a means of control, which can be effected,
however, by spraying with lime-sulphur in the autumn, when the mites
are taking up their winter quarters, and again at bud movement in the
spring.
Woodlice.
Woodlice are so well known, that but little description is necessary
here. However, the following features are of interest. They belong to
the group of animals known as the _Crustacea_, which also includes
the crabs; these animals breathe by means of gills, and are usually
aquatic, but some forms, such as the woodlice, have become adapted to
a life on land. In outline (Fig. 13, 12) the woodlice are more or less
oval, with the upper surfaces somewhat arched, and the lower flat; the
body is divided into several segments, which may enable the animals
to curl up in the form of a pill. There is a distinct head, bearing a
pair of antennæ and the mouth-parts, followed by seven large thoracic
segments, to each of which a pair of legs is attached; finally,
the remaining six segments are more or less crowded together, and
constitute the abdomen.
Since woodlice are terrestrial gill-breathing animals, moisture is
essential for them, and it is in moist places that they abound. They
depend upon a mixed diet, being carnivorous, as well as herbivorous;
though normally scavengers, their attacks upon seedlings and tender
parts of plants bring them into the ranks of important garden pests.
Woodlice hibernate under any convenient shelter; in the spring, eggs
are produced and carried by the female on the under side of the body
until the young woodlice hatch. During growth the cuticle or shell is
periodically cast, and a freshly-moulted woodlouse is white in colour.
The best method of control is garden sanitation, all rubbish likely
to harbour the woodlice being removed. Since they are nocturnal, the
woodlice can be trapped by means of moss laid on the ground; the moss
in which the woodlice have taken shelter is collected during the day
and burned, or immersed in hot water to kill the animals, when it can
be used again. Some good results have been secured by means of sliced
potatoes dipped in arsenate of lead or Paris green; the potatoes are
placed within reach of the woodlice, which are attracted to and feed
upon the poisoned bait. Horse manure should not be used in seed beds
likely to be infested by woodlice.
Millepedes.
Millepedes are short, worm-like animals, with a fringe of numerous
short legs on each side (Fig. 13, 13), and have a characteristic habit
of curling up when disturbed. Though scavengers for the most part,
feeding upon decaying vegetation and on small organisms, they may
do considerable damage to sprouting seeds, seedlings, and to tender
plants; they are particularly abundant in damp and warm soil, where
there is an abundance of rotting vegetable matter.
Having a keen sense of smell, millepedes are readily attracted to
poisoned bait in the form of sliced potato spread with Paris green:
another method is to place a piece of freshly-cut potato under an
inverted flower pot, to which the millepedes will be attracted, when
they can be collected and destroyed. A satisfactory control measure
is to treat infested soil with black-leaf 40, using one part in one
thousand parts of water.
Slugs and Snails.
Plants are very often greatly damaged by the depredations of slugs and
snails; frequently young plants are devoured as soon as they appear
above ground. These animals attack the plants after nightfall, and
during the day seek cover. Though slugs will shelter in the soil,
they, together with snails, will shelter in any convenient place,
such as under old boards, sacking, bricks and stones upon the ground,
or under large leaves or amongst rank herbage--indeed, in almost any
place that affords cover and moisture. Slugs and snails are especially
active during wet weather, and at such times, owing to the overcast
conditions, they will continue their depredations in the daytime.
Though slugs are active throughout the year, and even during winter
when the temperature is favourable, snails pass the winter, as well
as hot, dry spells in summer, in a dormant state, often being found
together in sheltered positions where the conditions are dry.
Both slugs and snails copiously secrete a slimy substance, that affords
them protection against chemicals used for purposes of control. In the
case of the slug (Fig. 13, 14), the shell is small and inconspicuous,
but the large spiral shell of the snail (Fig. 13, 15) affords the
animal adequate protection, into which it withdraws itself in times of
danger. Both slugs and snails reproduce by means of eggs; these are
white, spherical and opaque, and are deposited in the soil or under
decaying vegetation.
One of the best means of control is to dust the plants with powdered
tobacco. Another method is to treat infested plants with soot or lime,
but this must be done at night, and the material used must come into
actual contact with the pests. An effective poison bait, but one that
requires to be carefully handled, owing to its poisonous nature, is a
mash made of 6lb. of bran mixed with 1lb. of arsenate of lead and an
equal weight of treacle; this is made into a stiff paste, water being
added if necessary. Lumps of this mash are placed about the plants to
be protected. As a barrier to prevent the inroads of slugs and snails,
plants may be surrounded by a belt of calcium cyanide; this would have
to be replaced each night, and the utmost care taken in handling, since
the substance and the gas evolved from it are highly poisonous; out
of doors, however, the gas, being diluted with air, would not be very
injurious as long as one did not stand over the treated ground longer
than was necessary for laying the cyanide.
Apart from the above methods, the key to the control of slugs and
snails is “clean farming”--that is, the removal of all places, such as
rubbish and rank vegetation, where the animals will find shelter; the
compost heap is a favourite breeding place, and this should be turned
over at intervals and dressed with lime.
Eelworms.
Eelworms are minute, unsegmented worms, related to the parasitic
thread-worms of animals, and are abundant in soil and water; it is
usually the surface layers of the richer soils that are inhabited
by them. Of the long list of species, only a few are destructive to
vegetation, but these constitute one of the greatest problems of the
horticulturist. It is thought that the injury caused to plants by
eelworms is toxic rather than mechanical, and some plants apparently
are capable of producing anti-toxins, which neutralise the toxins
of the eelworms; such plants possess an immunity. There are three
important species in New Zealand.
The so-called bulb-eelworm (_Anguillulina dipsaci_) attacks more
than two hundred kinds of plants, but is of especial interest to the
horticulturist on account of its attacks upon hyacinths, daffodil,
narcissus, and gladiolus, causing deformity and rotting of the tissues
(Fig. 13, 16). It has been found that this eelworm develops from egg
to adult within a period of between three and four weeks; the eggs
are capable of lying dormant in the soil for as long as seven years.
Infested bulbs and corms should be treated by immersion for three hours
in water heated to 110 deg. Fahr.
Potatoes are often damaged by the beet eelworm (_Heterodera
schachtii_), which causes what is known as “potato sickness,” when the
growth is retarded, and wilting takes place; the root-system shows an
abnormal development of secondary or “hunger-roots.” The eggs are
retained in the body of the female, which forms a protective sack or
cyst (Fig. 13, 17 and 18), and in this state the eggs pass the winter
in the ground, where they are known to remain dormant for a period of
ten years; under favourable conditions in the spring, the larvæ emerge
from the eggs and attack the rootlets of suitable host plants, entering
them at the extreme tip. Satisfactory methods of control have not yet
been developed under field conditions, but a four-year crop rotation
following potatoes is suggested; seed potatoes from infested ground
should not be used.
The roots of tomatoes are often found to be a mass of galls, due to
attack by the root-knot eelworm (_Heterodera radicicola_), which also
infests tobacco roots as well as other plants (Fig. 13, 19 and 20). All
stages of this species are to be found in the root galls; the female
lays her eggs in a gelatinous egg sack, which remains attached to the
parent. The larvæ, on hatching, either remain within the parent gall or
leave it and enter the soil, where they seek out and attack the roots
of another plant. In tomato gardens steam sterilisation of the soil is
the most effective means of control.