THE PERIL OF INSECTS
The Phylloxera--French sport--Life history of the
Phylloxera--Cockchafer grubs--Wireworm--The misunderstood
crows--Dangerous sucklings of greenflies--"Sweat of heaven"
and "Saliva of the stars"--A parasite of a parasite of a
parasite--Buds--The apple-blossom weevil--Apple-sucker--The
codlin moth and the ripening apple--The pear midge--A careless
naturalist and his present of rare eggs--Leaf-miners--Birds
without a stain upon their characters--Birds and
man--Moats--Dust and mites--The homes of the mites--Buds, insect
eggs, and parent birds flourishing together.
The difficulty in describing the Romance of Plant Life does not arise
from a want of romance, but the sieges, battles, and alarms are so
difficult to see, and the enemies are so tiny, that the terrific
contests continually going on escape our notice altogether.
When one does look carefully and closely at the life of a plant, one
sometimes wonders how it manages to exist at all in the midst of so
many and great dangers.
There are great swarms of insects which devour or burrow into it, or
suck its life-juices. These are infinitely more dangerous than the
relatively clumsy, heavy-footed, grazing animal.
Every part of a plant has its own special insect foe, and it is
really difficult to understand how it can possibly escape.
Perhaps the "Achilles' heel" is the root, for, underground, plants
get no help from the watchful and ever-present army of birds, who
are, as we shall see, the natural police of the world.
The Phylloxera, for instance, which ruined the old and valuable
vineyards in France, is a terrible little acarid, or mite, which
attacks the roots. Too small to see, and impossible to kill without
killing the plant, it laid waste the fertile hills and valleys of
all South and Central France, causing millions of pounds damage.
One reason for this destruction sprang from the universal sporting
instinct innate in every Frenchman. Everybody goes out with his gun
to destroy any lark, sparrow, or titmouse that is idiotic enough to
remain in the country. Only birds can deal efficiently with insect
pests. Take this horrible little Phylloxera, for instance; a single
female in her life of forty-five days will lay about two hundred
eggs. Each egg becomes a little grub, which after a few moments of
uncertainty and agitation settles itself, and begins to suck steadily
at any unoccupied part of the vine root. After ten to twelve days'
life it will be laying eggs as rapidly as its mother. Thus in an
ordinary summer the number of young ones produced from a single
female becomes quite incalculable.
These pests are natives of America. Imported on American roots about
1868, they had in thirteen years practically ruined the vineyards in
France, Spain, Portugal, Italy, and Germany.
All sorts of remedies were tried--saturation of the ground by
poisons, flooding the vineyards to drown them, artificial cultivation
of their insect and plant enemies, and many others.
The correct and satisfactory method has been at last discovered.
American vines of sorts which are able to resist these Yankee mites
have been imported, and the valuable French vines have been grafted
on to them.
Another very dangerous root-enemy, which is common in this country,
is the Cockchafer grub or Whitegrub. (But it is not nearly so bad as
in France, where in the summer of 1889, a single farmer collected
2000 lb. of Cockchafers.) The grub (each female lays seventy eggs)
burrows into the earth, and for no less than three summers remains
below ground devouring indiscriminately the roots of everything
he can discover. Underground, the mole is almost his only enemy,
but the rooks, starlings, and gulls, which follow the plough, are
watching for him. The Wireworm, Clickbeetle, or Skipjack, is also an
underground demon which lives for three years, and gnaws and worries
at plant roots for the whole of that time. It, however, shows itself
above the surface.
A gentleman who had passed his whole life in the country complained,
in my presence, of the damage done by rooks. He had had six thousand
of them shot that summer, and remarked that he had seen with his own
eyes one of them pulling out a young cabbage plant by the root. Of
course it was quite unnecessary to point out that the poor bird was
merely trying to get at the wireworms and devour them!
For some time I used to look out for great attacks of wireworm in
turnip-fields: when one was recorded, I never failed to find that the
crows had been ruthlessly shot down a season or two before.
All these, and many other insects, attack the roots, which would be,
one would suppose, quite well protected in the depths of the earth.
Therefore we find roots producing all sorts of poisonous substances,
tannins, and even strong-smelling bodies, which keep off these pests.
It is perhaps the sucking battalions of the insect army which do the
most harm. In themselves they are weak, stupid, and scarcely move
from their birthplace. They live out their life wherever their long,
lancet-like proboscis needles have pierced the plant's skin, but it
is their power of multiplication that makes them really formidable.
Huxley calculated that if all the offspring of one "green-fly" lived,
and if their broods also lived for ten generations, then the tenth
brood of that original green-fly would contain more animal matter
than the entire population of China. Green-fly would, as a matter of
fact, go on increasing at this rate, were it not for the enormous
number of enemies that prey upon them. A mathematical friend of Mr.
Buckton calculated that in 300 days the produce of a single green-fly
might be 210^15, that is 210 multiplied by 210, and then again by 210
up to 15 times!
In summer time one may often notice, especially on sycamores and lime
trees, a peculiar shining, sticky, honey-like substance which covers
the leaves. It is often so abundant as to drip like a rain of honey
from the upper branches.
This "honey-dew" was a puzzle which greatly intrigued learned minds
in the ancient world. Pliny speaks of it as the "sweat of heaven" or
"saliva of the stars."
In reality, however, it is nothing but the excretions of hundreds
of millions of these green-fly or aphides, which will be found
established on the under side of the leaves, where, moored by their
little anchoring talons and with their proboscis inserted in the
fresh green leaf, they are sucking hard and steadily at the sugary
juice. In twenty-four hours it was observed that a single individual
gave forth forty-eight minute drops of honey.
Bees are very often tempted to collect this honey so abundantly
produced, but this turns their own honey black, and may even make it
poisonous.
Plants try to protect themselves against these pests chiefly by means
of sticky or long hairs, by a thick skin, or by unpleasant tasting
or smelling substances. But it is to insects such as lady-birds
and others which devour the green-fly that they owe a deep debt of
gratitude. In particular, there are certain parasitic insects which
lay their eggs in their bodies. Not only so, but it is known that the
eggs of some other insects are laid _in the egg of the green-fly_,
and in one instance it has been found that yet another insect laid
its egg in the egg of the parasite!
Some of the most interesting objects in nature are the buds in
which, all neatly packed and stowed away, the young leaves and
flowers remain awaiting the warm breath of spring. They are most
interesting to examine: one finds series after series of overlapping
scales which cover one another in the most ingenious way. No two are
exactly alike, but each seems to have been moulded exactly to the
proper shape. There is no waste anywhere, no useless expenditure of
material. Very often turpentine or resin or a sticky gum seals up
the joining of the scales. Every possible precaution seems to have
been taken by nature. Neither rain nor snow can enter a winter bud.
Neither can the cold of winter penetrate to the inside where the
baby leaves and flower petals are cosily and tightly coiled up. But
observe in the very earliest warm days of spring an extraordinary
little insect, which has wakened up after its own winter sleep in the
moss or lichen covering the rough and crannied bark of an old apple
tree. This is the Apple-blossom Weevil, a beetle only about quarter
of an inch in length, but with a curious snout or proboscis half the
length of its body. This creature proceeds to the bud, and fixing
its legs firmly, proceeds to bore a hole through the scales into the
middle of the bud. She then places an egg inside, and goes on to
put an egg in each of fourteen to forty-nine other buds. This takes
a fortnight, and then she dies, probably satisfied that her duty is
fully performed. A little footless, cream-white maggot develops in
the apple-bud, which latter becomes rusty-coloured and dies away.
Another pest is the Apple-sucker, which lays her eggs in September on
the fine hairs which cover the shoots. As soon as the weather becomes
mild and warm, little grubs come out of these eggs; they are very
small, and their bodies are almost flat. These tiny flat grubs, as
soon as they are born, hurry off to the nearest buds and slip between
their scales. They remain sucking the rich juices of the apple
blossom until May or June, when they become perfect insects, and fly
away so fat and well-nourished that they can live until September
without feeding.
But those are by no means the only dangers. It is not till the apple
blossom, which has escaped all those perils, opens in the spring
time, after its petals have unfolded in the warm air and the young
apple is already half formed, that the Codlin Moth begins to attack
them. This tiny little moth is then extremely busy. She lays about
fifty eggs, but only one on each young apple. It is put in the one
weak spot of the apple, just at the top, in the base of the withered
flower. The grub tunnels down to the core and feeds upon the seeds,
which are entirely destroyed. When it has grown sufficiently, it
drives another tunnel straight outwards to the skin. If the apple is
still on the tree, the caterpillar lets itself down on a long silken
thread and hurries off to hide in any convenient crack or crevice of
the bark, or if the apple is already stored away, it conceals itself
in the walls or in the flooring of the loft. The moths come out at
the end of next May, just when the blossoms are getting ready for
them. These codlin-moth apples cannot fail to have been noticed by
the reader, as the tunnels in the ripe apple are most conspicuous.
The gradual fattening of the caterpillar can also be traced, for its
first tunnel down to the seeds is quite narrow, while the way out
gets wider and wider as the creature became stouter and fatter whilst
eating its way through the flesh.
The Pear Midge attacks at the same place, but the mother insect has
a long egg-laying tube, and puts from fifteen to thirty eggs into
the opening pear blossom. The pears go on growing, but of course
are quite spoilt by the maggots within. These latter have a curious
springing or jumping habit, and when they reach the soil bury
themselves an inch or two below the surface.
So that all the care and neatness with which the young flowers and
buds are packed up goes for nothing, and these insect pests get all
the benefits of the apple and pear!
Besides these, there are hundreds of sorts of caterpillars which
devour the leaves bodily. Cabbage-white butterflies, magpie-moths,
gipsy-moths, diamondback-moths, and others, lay their eggs in
hundreds. Many lay 300 eggs each.
In the United States, somebody had sent an entomologist a present of
some eggs of one of these moths. They were placed on a paper near
a window which happened to be open; the entomologist went out, and
the paper must have blown across the street into a garden on the
other side. At any rate, two or three years afterwards it was found
that some trees were badly attacked by this moth. Nobody thought
much about this, though of course it was interesting to find a new
moth. But the pest became a very serious one. In consequence of
the stimulating air of the United States the moth multiplied with
the most extraordinary rapidity, and it is said that about 300,000
dollars was spent in one year in the attempt to stamp it out.
All this happened because an entomologist forgot to lock up his eggs
when he went away for half an hour!
These caterpillars and the locusts devour the leaves bodily, but
there are others which live inside them. These so-called "leaf-miner"
caterpillars make white irregularly-winding tunnels between the
upper and the lower skin of the leaf. The tunnel increases or
widens because the caterpillar itself grows fatter as it eats its
tunnel. They can be seen on a great many leaves, and can be at once
recognized by this peculiarity.
Plants cannot run away from their enemies like animals, and it would
seem at first sight that their case was very hopeless. But it is not
so, for there is a vast, active, keen-eyed, and eager army of helpers
always ready for eggs and caterpillars.
It is birds that are of the greatest importance. A titmouse will
eat 200,000 insects in a season. A starling has been seen to fetch
food for its young ones from a grass paddock 100 yards away no less
than eighteen times in a quarter of an hour. All the following are
excellent birds, and without a stain upon their characters: the
plover, partridge, robin, wagtail, starling. Crows and wood-pigeons
are under suspicion, for though the latter do good in devouring the
seeds of weeds, and the former in destroying wireworms, both are fond
of corn and take large quantities of it.
Thrushes, mavises, and blackbirds are amongst the most persevering
and useful of our friends, but they are certainly fond of fruit. Yet
the good which they do is very much more than any possible harm
which an injudicious indulgence in the juicy fruits of summer might
bring about.
The sparrow cannot be given a character. Indeed, he is objectionable
in every way, for he not only does no good himself, but he devours
corn and drives away starlings and other valuable and interesting
helpers.
But it is very difficult to say what will happen if man interferes
with the regular working of Nature. The starling has been a pest in
Australia, though here it does nothing but good work. We are still
grossly ignorant of many simple but very important facts. Even
when we do know something, as for instance, that the peewit's or
plover's whole life is occupied in clearing the ground of wireworm,
daddy-long-legs grub, insects' eggs, and the like, that does not help
the bird in the least. Plovers' eggs are regularly sold in enormous
quantities. Every farm-labourer collects them, and the farmer never
dreams of interfering.
Man shoots down owls, kestrels, hawks, who prey upon mice, voles,
and sparrows. Then, when some farmers are half ruined, he has Royal
Commissions to find out why the voles have increased so much.
There are one or two peculiar contrivances found in plants which are
intended to keep off insects, and which may be noticed here.
Thus, the importance of a moat (which almost always formed part of
the defence of a medieval castle) had been already found out by one
or two plants.
In a particular kind of Teazle and in a large Sunflower-like
Composite (_Silphium laciniatum_) every pair of two opposite leaves
run together, so that a little cup-like hollow is formed surrounding
the stem, in which water collects. Insects climbing up the stem
and trying to get at the heads of flowers fall in and get drowned
in this water; their bodies may be seen floating about in it, and
probably when these decay, their decay-products are of some use to
the plant.
This curious contrivance is only a development of a very common
arrangement. In most leaves you will find that rainwater is intended
to run in a particular direction. There are little grooves and canals
down which it is supposed to go, and dry, thirsty hairs may be found
so arranged as to intercept part of it. Thus in summer the plants
are not confined entirely to the water from the ground, but are also
refreshed by the rain from above.
But if you look closely along these little channels, and especially
at the base of the leaf where they join the stem, you will find
that dust particles washed down by the rain collect and form little
streaks and patches. The air is full of all sorts of dust particles
which are made up of every conceivable substance. Many of these
minute grains of dust will be dissolved in the water, and help to
supply the plant with food. Nor is that all, for if you take a
hand-lens and examine these dust particles very closely, you will
very probably find small animalcula moving about. They are not
pretty; in fact they are quite horrible to look at. These are tiny
mites which live in these places. Their office is probably to eat up
everything eatable (including eggs of insects and spores of fungi),
and their excreta as well as their own bodies will probably be
dissolved in the water and go to help the plants.
The most certain place to find them is on the leaves of the lime and
other trees in August. On the under side of the leaf little bushes
of hairs can be found just where the veins fork. It is necessary to
take a pin and stir up these hairs to frighten them out, but when
this has been done, the lens will show the disgusting-looking little
creatures running hurriedly away. They are no doubt exceedingly
annoyed at being disturbed in the midst of their sleep, for they
come out and forage for anything eatable at night, retiring for the
day into these hairy grottos. The structure of these grottos is very
complicated. They are often like little caves with a narrow entrance,
and the sleeping chamber is quite within the leaf.
A great many trees have these curious mite homes. The insects are
generally the colour of the hairs, and are not easy to distinguish.
All those insects mentioned here have so arranged their life
histories that they come into existence exactly at the proper season.
The warmth of the sun, which opens the apple buds ever so slightly,
stirs also the egg of the mite, the egg of the beetle, or the
hibernating weevil, so that all these insect populations come into
full active life just when they can do the most damage.
But one must not stop there; the bird population is also ready, and
is building its nests and feeding its young, just so soon as the
insect swarms are at their thickest and most dangerous stage.
Man walks clumsily through this intricate tangle of living plants and
animals: he sets his big foot on a hedgehog (good for the insects),
or on a mole (so much the better for wireworm), collects plovers'
eggs (to the great help of every insect), shoots an owl (to the
delight of voles and mice) or a whole brood of partridges, and in
other ways makes a---- we had better say, shows that he is not so
clever as he supposes himself to be.