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 21015, 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.