INSECT ENEMIES OF TREES
Whether viewed from a commercial or sylvicultural point of view, the
widespread damage caused to timber from insect attacks can scarcely
be overrated. The depredations in various parts of our own country,
particularly in young plantations, are bad enough, but when compared
with those of Europe and America they appear insignificant. In France
and Germany whole woods have been wiped out by insect pests, while the
Government of Bavaria were mulcted in something like £100,000 by the
destruction of its spruce forests. The United States fares no better,
for we find that over a period of ten years the amount of timber killed
and reduced in value was calculated at fully £10,000,000. The coffee
plantations of Ceylon suffered much from the attacks of a fungus, and
we could go on multiplying cases. In our own country the ravages of the
pine beetle and of the larch disease have caused incalculable damage;
indeed, in the latter case there is hardly a plantation of larch where
the presence of the fell disease cannot be distinctly traced, while the
pine beetle has ruined whole plantations both in England and Scotland.
Though the adult bark and wood-boring beetles do a great amount of
damage, yet that inflicted by the caterpillar or grub from the egg
is greater still, and in the case of fungi we have a typical example
of their destructive properties in the case of the well-known larch
disease.
The =Pine Beetle= (_Myelophilus {Hylurgus_} _piniperda_) is a dreaded
enemy to not a few species of Pinus, but particularly _P. sylvestris_,
_P. laricio_, _P. austriaca_ and _P. Strobus_. The injury done by this
beetle consists in its destruction of the leading shoots of the tree it
attacks. It enters by boring a hole into the side of the shoot until it
reaches the pith, after which its course is directed upwards, and an
exit made at the terminal bud. This tunnelling of the shoot so weakens
it that frequently during stormy weather it is broken across at the
point where the beetle entered. Not only are unhealthy trees attacked
by the pine beetle, but young and robust-growing specimens frequently
fall a prey to its insidious depredations.
June, July and August are the months when it is most commonly found.
The only remedy is to collect and burn the affected shoots—work that
requires to be done with the utmost care to prevent the escape of the
wary insect. Burning all brushwood in plantations is a great preventive.
The =Pine Weevil= (_Curculio_ {_Hylobius_} _Abietis_) is another
destructive insect, which differs from the former in waging its attacks
against the buds of the leaders and branches, as also by eating patches
of the bark here and there on the stems and branches. The various
species of Abies suffer most, but the pines are occasionally attacked
as well. It is always most destructive in young plantations growing on
the margins of old woods, and equally bad amongst trees that have been
planted on the site of a former pine plantation.
The beetle is about half an inch long, and nearly black. One remedy,
probably the best, is to place fresh pieces of pine bark on the ground,
beneath the infested trees. By shaking the trees and examining the
traps the following morning, many may be destroyed.
=Bostrichus typographus= is another pest of our woodlands, and may
frequently be seen, like fine white wool, spreading over the stem
and branches of the silver and other firs. It spreads with terrible
rapidity, first appearing in small patches here and there on the bole,
and particularly on the under sides of the branches. The tree infested
soon becomes unhealthy, and frequently dies off prematurely. Trees
growing in low-lying, heavy ground would seem to fall a first prey to
this insect.
=Bostrichus laricis= is nearly allied to the former, but its
devastations, which are, however, not very deadly, are principally
confined to the larch. It is usually known as the “larch blight.”
The =Pine Shoot Moths= (_Retinia buoliana_ and _R. turionella_) would
seem to be more numerous in this country than is generally supposed.
Quite lately I visited a large plantation of young Scotch fir, the
terminal buds of which were greatly injured by the caterpillars of
this elaborately-coloured moth. The moth lays its eggs at the base
of the buds, and into these the caterpillars enter by hollowing out
the centre, thus destroying their vitality and causing them to take
on a withered appearance and to feel soft and empty to the touch.
Trees infested by this insect resemble greatly in their stunted shoots
and exudation of resin such as have become a prey to the Pine beetle
(_Myelophilus piniperda_), only in the latter case it is the fresh
young shoot and not the bud that is attacked. The _Retinia_ would
seem, from all my notes and observations, to be most abundant in what
might be termed neglected fir plantations, that is, where the trees
have suffered from overcrowding, or from unfavourable conditions as to
soil, etc., and particularly when the wood is composed entirely of one
species. There is no method of dealing with large infested areas, for
the attacked trees have repeatedly been cut over and removed without
any seeming diminution in the numbers of the insect. One experiment
with a small infested corner has been rewarded with good results, viz.
the lighting of a fire to windward, and causing the smoke of coal tar
to pass over the infected area. This might be worth trying in the case
of fruit trees infested by particular insects.
The =Larch Miner= (_Coleophora laricella_).—Few, other than those
specially interested in tree diseases, have the remotest idea that the
yellow, withered appearance of many of our English larch plantations
is due to the larvæ of the above tiny moth. It usually attacks young
trees, say, from five to twenty years old, and although it may not kill
them out, yet the repeated onslaughts year after year tend to keep the
trees in an unhealthy condition, and so render them liable to other and
more deadly diseases.
Unfortunately the attacks of the larch miner are by no means confined,
as is usually supposed, to trees growing under unfavourable conditions,
for I have this season noticed in an unusually healthy, fast-growing
plantation in Sussex that almost every tree was more or less affected.
Certainly in another large extent of larch in Gloucestershire which I
examined lately, where nine-tenths of the trees were being ruined by
the _Peziza_, the larch miner was very abundant; but, I think, that
young trees, whatever may be their state of health, suffer alike,
although where hard-wooded trees form a portion of the crop the larch
certainly suffers less than when grown in pure woods. The moth lays its
eggs at the end of June on the needles of the larch; the caterpillar
mining into and feeding upon the interior of the needle causes it to
turn faded and yellow. It lives in the tube thus formed during the
winter, changing to a pupa, and ultimately to a moth. It is a most
difficult matter in the case of this insect, as, indeed, of all others
that are fairly abundant, to suggest a remedy, and I have looked
over and examined larch plantations that are differently situated in
many respects to find out under what condition the attacks are most
persistent, but with little or no success—healthy and unhealthy, native
or Tyrolese, faring alike when grown as a pure crop.
Where the larches are intermixed with hard-wooded trees—sycamore, oak
and beech—the attacks are certainly less frequent, as I have noticed
in a number of cases. Trees growing at high altitudes do not seem to
suffer less than those only a few feet above sea-level, and this point
I have paid particular attention to.
Whether the wounds caused by this insect will serve as a nidus for the
spores of _Peziza Willkommii_ has yet to be determined, but special
importance should be attached to all larch-feeding insects, and their
depredations minimized to as great an extent as possible.
The =Pine Sawfly= (_Lophyrus Pini_).—Fortunately, this insect is not
abundant in the British Isles, though on the Continent the damage it
does in the pine forests is by no means inconsiderable. The insect
may readily be recognized by its wide, flattish body, and usually
dark appearance. Having attained to full size in the trees, they form
cocoons among the foliage or on the stems, and remain in this condition
until the following spring, when, in April or May, the perfect insects
make their appearance. The male is considerably smaller than the
female, while the full-grown caterpillar, which is of a greenish-yellow
colour, with a row of black spots on either side, is about an inch
long. The remedial measures are not at all easy, especially when
a large number of trees are attacked, but single specimens may be
entirely cleared by shaking the caterpillars into a sheet placed
beneath the tree.
[Illustration: (_a_) WILLOW BEETLE (_Phyllodecta vulgatissima_), AND
(_b_) ITS LARVA.]
The =Larch Aphis= (_Adelgis laricis_) and =Giant Sirex= (_Sirex
gigas_) are both, more or less, harmful to the larch. The latter is a
formidable and splendid insect, which is, however, not very abundant in
this country.
Generally felled trees, or such as are somewhat sickly, are chosen by
the female in which to lay her eggs. These are deposited beneath the
bark by means of the powerful ovipositor, and in course of time the
whitish cylindrical maggots make an appearance, and with their strong
jaws form large borings in the affected tree.
Cutting down and burning infested trees is the only practical remedy.
The =Willow Beetle= (_Phyllodecta vulgatissima_) causes considerable
damage to osier plantations, and would appear to be greatly on the
increase of late years. Much damage has been done to osier holts in
various parts of the country, and in northern Ireland the ravages of
this beetle were particularly noticeable during the past five years.
The insect, which is metallic green or blue in colour, passes the
winter in the adult state, at which time it may be found amongst refuse
of the osier beds, such as the heaps of bark, and also at the base
of old stools and beneath stones or other shelter. The larvæ have a
tough yellowish cuticle with conspicuous brown bristles, the head and
prothorax being black and hard. The eggs are laid on the undersides of
the leaves in spring, and when the larvæ are hatched they feed on the
leaves, eating holes quite through to the upper surface. Burning all
rubbish in the osier beds is to be recommended, and spraying with Paris
green or lead arsenate has been found useful.
The =Larch Sawfly= (_Nematus Erichsonii_).—This is a species of
sawfly the larvæ of which bear considerable resemblance to those of
the caterpillar of the pine sawfly, and also to that of the better
known gooseberry caterpillar. The larvæ are about three-quarters of
an inch long, and possess twenty feet. From July to August they feed
on the leaves of the larch, and a plantation that has been attacked
presents a partially leafless condition with quantities of the brown
cylindrical cocoon cases lying amongst the grass beneath the trees.
There have been several notable instances in which larch plantations
have suffered severely from the attacks of this insect, and in northern
England, particularly Cumberland, whole areas of plantation have been
attacked. Burning all brushwood and grass beneath the trees in infected
plantations is probably the best means of lessening the numbers of this
dread insect in our larch plantations.
The =Spruce Gall Aphis= (_Chermes abietis_).—This is a common insect,
and one that renders many fine young spruce trees very unsightly by
reason of the cone-like excrescences that are formed by the action of
the insect on the shoots of the infested specimen. The formation of
this excrescence is brought about by the female aphis piercing with
her beak, or sucker, one of the buds, and drawing off the sap, the
consequence being an unusual growth at that part.
When the young larvæ appear, they also, by piercing the gall, extract
the juices, and the gall enlarging soon causes the larvæ to become
embedded at the bases of the leaves, which, by this time, have become
curiously malformed. The insects are scarcely one-tenth of an inch long.
[Illustration: ELM TREE DESTROYER]
A. Entrance of Parent Tube.
B. Insect in Tube.
C. An imperfect Tube arrested from want of nourishment.
D. Scolytus Destructor. Natural size.
E. Lateral Tubes with Larvæ, some changing to the Pupa state.
The only remedy is to collect the cone-like excrescences and have them
destroyed, except in the case of badly infested trees, which should be
cut down and burned.
The =Elm Tree Destroyer= (_Scolytus destructor_) is about one-fifth
of an inch long, stout and cylindrical, and usually confines its
depredations to the elm.
In the beginning of June this beetle bores into the inner bark, where
it forms galleries, along the margins of which are laid the eggs.
Cutting down and burning badly-attacked trees is the best remedy,
but promoting exuberant health of the infested specimens by means of
enriching the soil has been attended with promising results.
The =Goat Moth= (_Cossus ligniperda_) is most frequently found on the
willow, oak, lime and other trees. Being not only one of our largest
native moths, but also one of the most destructive, its ravages are
much dreaded, the holes or tunnels made by the moth being of large
size—large enough to admit the little finger. Filling up the tunnels
with a mixture of soot, lime and cow manure is an excellent remedy.
The =Lackey Moth= (_Clisiocampa neustria_), so called from the gay
colours of the caterpillar, is another destructive woodland pest,
eating wholesale the leaves of the oak, elm, beech, poplar and most
fruit trees.
In April and May the caterpillars are hatched, when the leaves are
just unfolding. They form a nest or web of silken hairs, generally
amongst the smaller branches, in which they live during the day,
sallying forth in the evening to feed on the tender foliage. Being very
plentiful, they are usually difficult to deal with, but hand-picking
and destroying the cocoons are the only practicable methods of meeting
the evil.
The =Winter Moth= (_Cheimatobia brumata_) and the =Lime Looper Moth=
(_Hybernia defolaria_).—The caterpillars of both these moths are very
destructive to the leaves of elms, limes and willows, but particularly
to the buds of the apple tree.
When full grown they descend to the ground, where they cover themselves
and become chrysalides, from which the moths appear from October to
December.
Being almost wingless, it is by no means difficult to prevent their
ascending the trees by painting a band of any sticky substance around
the stems of the trees that it is expected they might attempt to crawl
up.
[Illustration: A. Larva of the Cossus Ligniperda, three years old,
ready to change into the Chrysalis state.]
[Illustration:
A. Cossus Moth.
B. Chrysalis from which the perfect Insect has escaped.
C. Cluster of Eggs.
D. Magnified Ovum.
THE GOAT MOTH]
The =Red Spider= (_Tetranychus_).—In hot and dry summers trees suffer
much from this member of the mite family. Limes and poplars, as also
many other trees, are greatly injured, the foliage turning to a russety
brown colour, and falling off long before the usual time. There are
several remedies, such as fumigating and spraying with a solution of
soft soap, but none of these are applicable to a plantation of trees,
or even a single specimen of large size.
The =Thorn Fly= (_Aphis Cratægi_) attacks whole hedges or brakes of
Quick, especially those in the nursery border; as a rule, the younger
and more healthy plants first fall a prey to its depredations.
Sponging with tobacco water, or almost any of the prescribed solutions
will rapidly exterminate the fly; but such work is laborious when a
large brake or long hedge of the thorn has to be gone over.
The =Cockchafer= (_Melolontha vulgaris_) is usually pretty abundant,
and does most damage by eating the leaves of the sycamore, beech, oak,
cherry and many other trees. It will also eat the roots of most young
trees, but those of pine in particular.
The insect is about 1¼ in. long, and of a chestnut-brown colour on the
upper part of the body, while the head and some other parts of the body
are of a bronzy green, and thickly covered with yellowish-white hairs.
In April and May the eggs are laid in a hole in the ground about 5 in.
deep, and the grubs are hatched in July. They are of a dirty-white
colour and much wrinkled. In this state, however, they do but little
harm; but, after having changed their skins and remained in a torpid
state during winter, come to the surface in spring and eat the roots
of almost any plant that comes in their way. They again burrow deeper
at the approach of winter, coming to the surface again in spring, and,
when full grown, are about 1½ in. long, and almost ½ in. in diameter.
The perfect insects do not live more than about twelve days, and are
easily known by their heavy, awkward flight towards the evening.
The =Laburnum Moth= (_Cemiostoma laburnella_) is fairly abundant—in
England, at least; and, in some instances, every leaf of a tree has
been eaten almost wholesale by the caterpillars of this pretty moth.
The insect is about one-eighth of an inch in length, and three-tenths
of an inch across the fully expanded wing. It is of a silvery-white
colour.
The greenish-grey caterpillars are about ¼ in. long.
By burning the attacked leaves great numbers of the caterpillars may
be destroyed, while, by shaking the trees in May and August, the moths
will fly out, and may be caught in a butterfly net.
[Illustration: WOOD LEOPARD MOTH
(_Zeuzera æsculi_)
(Very destructive to trees, particularly around London)]
=Wood Leopard Moth= (_Zeuzera æsculi_).—The caterpillar of this
beautiful moth is very destructive to the beech, ash, birch, elm,
walnut, privet, etc., which it bores into, eating and living on
the wood. Usually young trees or the branches of old specimens are
attacked, and the tunnelling is confined in the former either to the
pithy centre or the soft wood near the bark. The moths appear about
mid-July, and the female, by piercing the bark with her powerful
ovipositors, deposits her eggs, one in each hole. Three years are
required for the caterpillar to arrive at maturity when it is nearly
2 in. long. Both in form and colour the leopard moth is particularly
elegant, the head and thorax being covered with a thick white pile, the
body with a black down, fringed with white at each joint. The wings
are white with yellowish-brown veins, a row of rounded bluish spots
running between every two.
By stuffing a piece of tow in gas tar, or placing cyanide of potassium
in the hole and closing the aperture, the caterpillar may be
overpowered and destroyed.
A bent wire has often been successfully used in dislodging the
caterpillar.
The =Holly Fly= (_Phytomyza aquifolia_).—The foliage of the holly is
frequently very much disfigured by the grubs of the holly fly, which
burrow beneath the upper skin of the leaves, feeding on the internal
substance. This imparts a blistered and discoloured appearance, which,
in the case of ornamental varieties, is anything but desirable.
Probably no great damage to the infested trees is brought about, but
the wholesale destruction of the leaves, as is often the case, cannot
but weaken the plant.
In May and June the flies make their appearance, and lay their eggs
beneath the upper skin of the leaf, from which the grubs, about
one-fifth of an inch long, are hatched. These work their way beneath
the skin of the leaves, forming small tracks of a more or less circular
shape, thus causing the large and unsightly blisters. They quit the
leaves about March, by making small holes in the skin of the leaf, and
afterwards become chrysalides. The fly is small and inconspicuous.
Picking off and destroying affected leaves, or crushing the grub by
pinching the blisters are the only ways of lessening the attacks.
The =Oak Leaf Roller Moth= (_Tortrix viridana_).—The widespread
destruction caused to oak-woods in almost every part of the country
by the caterpillars of this little moth would seem to be on the
increase from year to year. But it is not the oak alone that suffers,
for numbers of the hornbeam and beech are in an equally pitiable
condition. Having closely watched this insect for several years, mainly
with the view of striving to keep it in check or devise some means
of destruction, I have come to the conclusion that a few individual
trees may, at considerable expense, be rid of the pest, but in the
case of whole woodlands artificial treatment is quite out of the
question. Many observers are under the impression that the caterpillar
is most abundant where the trees grow closest together, and when we
consider that both wind and rain destroy numbers of these, the fact of
isolated trees, which, consequently, are most exposed to storms, being
comparatively free from attack is not to be wondered at. So far as we
know at present, the only way to diminish in any appreciable degree
the numbers of this insect is by encouraging as much as possible its
enemies.
The rook, jackdaw, starling, thrush and sparrow help immensely in
destroying the caterpillars, and the occupants of a rookery will
frequently in a few hours clear the pest from the trees over a
considerable area of woodland. The same has been noted with regard to
the starling, and I have frequently seen the trees over an infested
area almost black with this particular bird when in the act of feeding
on the caterpillars.
In support of this recommendation it may be stated that insects are far
less numerous in the forests of St. Germain, Senart and Fontainebleau
than in the Bois de Boulogne, where, of course, small birds are scarcer.
Sometimes with the oak leaf roller moth its excessive number proves
the means of its extinction, the foliage being devoured before the
caterpillars are fully fed; while, as is usual at the season of attack,
parasitic flies and ichneumons destroy them wholesale, and a box of
caterpillars sent to me the other day revealed the fact that each one
had succumbed to the attacks of one of these enemies.
The life history of this moth is full of interest, and the curious
manner in which it rolls up the leaves is well worthy of study.
The =Felted Beech Coccus= (_Cryptococcus Fagi_).—Judging from the
numerous specimens of the beech coccus that are being forwarded to me
for identification, and the inquiries as to how this insect pest may
be dealt with, its presence in almost every part of the country is
indicated. It was only in 1862 that Dr. Balfour reported the presence
of this coccus in Scotland, and I remember well how the beautiful
beech hedges on the Penicuik Estate, Mid-Lothian, were ravaged by the
insect in 1875. In Germany, however, the beech coccus was noticed as
early as 1849. It is probable that the beech coccus also extends to
the Weymouth pine, as at Keston, Lord Derby’s estate in Kent, several
of these trees were badly affected with an insect that appeared to me
identical with this pest, and other instances of the Weymouth pine
being similarly attacked have been recorded. Since the appearance
of the insect in Scotland it has spread southwards rapidly, but it
is only of late years that its ravages have been felt severely, and
that owners of woodlands have been driven to do everything in their
power to combat its injurious effects. I think I have nowhere seen
the insect so abundant as on the Burnham beeches, which I visited in
company with several members of the Royal Horticultural Society at the
request of the Lord Mayor and the Corporation of the City of London.
Here, speaking broadly, all the trees are affected in a greater or less
degree, independent altogether of the age or health of the trees.
Amongst the beautiful beech woods of Hertfordshire, where the timber
produced commands a higher price than that from any other station
in Britain excepting the Chiltern Hills, the trees are suffering
severely. After a careful inspection of many beech woods, I am at a
loss to account for the spread of this insect. Usually, in the case
of injurious forest insects, the presence of dead and dying wood and
the general health of the plantations have much to do with the attack,
but this would not appear to be the cause in the case of the beech
coccus. Even old and diseased trees do not appear to suffer more than
the young and healthy specimens, and at Burnham some of the youngest
and fastest growing specimens were by far the worst affected, appearing
in parts as if coated with drifted snow. Neither would soil appear to
have anything to do with the spread of the insect, as on chalky, sandy
and loamy soils, as well as on shale rock, the trees are all more or
less affected. I cannot say that unhealthy trees are more liable to
be attacked than vigorous ones, for I have observed diseased trees to
be sometimes quite free from the coccus, whilst healthy specimens in
the immediate neighbourhood were badly infested. This was particularly
noticeable on some park trees on two estates that I visited lately; in
each instance the majestic boles rose to a height of fully 90 ft. and
contained about 100 cubic ft. of wood. At one time I had an idea that
beech trees when grown too thickly were most liable to be attacked,
but more extensive observations have disproved the hypothesis. By some
it has been suggested that the drainage of ground on which the beech
is growing will facilitate the spread of the insect by bringing about
an unhealthy state of the trees. To some extent this has been noticed
at Burnham, and particularly in Gloucestershire, where a lake had been
formed contiguous to a beech plantation. In another instance that came
under my notice the burrowing of rabbits to an unusual extent was
blamed for the appearance of the insect and the gradual death of the
trees, the soil in this case being sand and gravel. It is a strange
fact, too, that a badly infested tree may be standing amongst others
that are perfectly free from the attack. Under a magnifying glass the
insect appears of a yellowish colour, irregularly globular in shape,
and almost transparent. It is thickly covered over by a secretion
from the body, which looks like fine waxen-white threads or cotton
wool, which effectually screens the insect from observation. With its
proboscis it sucks up the juices, which are readily reached through
the thin, smooth bark. When badly attacked, the foliage becomes meagre
and has a burnt appearance, then the tips of the branches, and finally
whole branches die off. Afterwards the bark becomes dry and cracks,
usually longitudinally, falling off in flakes with the death of the
tree. The timber of trees that have been killed by the coccus is
dry, short-grained, and by no means comparable with that of healthy
specimens. From whatever cause, the presence of the insect in our
plantations is much to be deplored, and already many fine old trees
in different parts of the country have succumbed to the attack. It is
a curious fact that certain trees fall victims to the attack of this
insect very rapidly, often in the short space of two years, while
others of equal age and vigour, and, as far as can be seen, similarly
situated, live for many years. Fully grown trees are, in my opinion,
more liable to be killed quickly than younger specimens.
As regards remedies, these can only conveniently be applied to
single specimens and are scarcely practicable on a large scale. That
success has, however, attended the application of certain remedies I
am quite convinced, for several valuable ornamental trees on a lawn
in Buckinghamshire that were badly affected by the pest are now,
twelve months after being treated, quite free from the insect, and do
not appear to have suffered in consequence. In this case the remedy
was simple, and consisted merely in scrubbing the tree stems where
affected with a stiff brush dipped in an emulsion of soft soap, this
being rubbed well into all crevices of the bark. Only one application
was given, but as the trees were nearly 100 ft. high with many large
limbs, which also showed the presence of the coccus, the work took a
considerable time. However, the results well repaid the expense of
labour. Another excellent remedy is to mix together equal portions of
paraffin and soft soap, and when required for use add twenty times
their bulk of hot water, stirring all well together. This may be
applied either with a scrubbing brush or syringed on to the affected
parts. Other measures of a more drastic nature have been found to
be beneficial, but those given are probably the simplest and best.
Three or four handfuls of lime to a bucket of water is an excellent
application, the only drawback being the unpleasant colour imparted to
the trees. This is a simple remedy, and may be applied with a painter’s
or scrubbing brush. It has saved many trees on an estate near London.
=Wireworms.=—These occasionally do a great deal of damage in beds of
seedling trees, particularly conifers, and in some instances they
attack and destroy the seeds before germination. In the case of
young conifers they are gnawed completely through just above or at
the ground level, the beds in many instances being strewn with the
cut-over plants. _Abies nobilis_ and _A. Nordmanniana_ suffer to a
great extent, and I have frequently been at my wit’s end to put a stop
to the repeated depredations. In the case of a newly-formed nursery or
freshly-made-up seed-beds the attack of the wireworm is always most
pronounced. In the case of fresh nursery ground, paring off and burning
a couple of inches of the top soil in the autumn has been attended with
excellent results, as has also dressing the ground with gas lime. When
seedlings are attacked, hand-picking, with the use of sliced carrots,
mangold or potatoes, varied with pieces of oilcake as a bait, are to be
recommended. Dressing the seeds with red lead is very advantageous to
prevent birds and mice from attacking freshly-sown quantities. Injury
from wireworm is not likely to cause any serious consequences after the
first year’s growth of the plants, especially if the ground has been
kept clean and free from weeds during the egg-laying season in June.
[Illustration: WITCH’S BROOM ON THE WILLOW (SUMMER APPEARANCE)]
The above are only a few of the many insects that injure our forest
trees; but those treated of are the most familiar to the forester, and
those whose attacks he has most frequently to wage war against. Willow
trees all over London are suffering severely from attacks of a mite
insect. It is known as the “Witch’s Broom” on the Willow (_Eriophyes
triradiatus_).
Remedial measures are frequently of little avail, more particularly
when a whole wood or plantation is attacked; but with single specimens
the numbers may readily be lessened by the methods suggested.
[Illustration: WITCH’S BROOM ON THE WILLOW (WINTER ASPECT)]
Keeping the woodlands free of dead and dying trees and branches is a
preventive of insect attacks that should never be neglected.
As showing the injury that can be inflicted on our forest trees by some
of these insect pests, the following cases may be mentioned:—
During fifteen years (between 1853-68) the spruce in East Prussia,
Poland and Russia was killed over an area of 7,000 square German miles;
while in the Bavarian forests, the loss to the Revenue in one year was
£40,000.
In both cases the destructive insect _Liparis monacha_ was the cause of
injury.