“But how can the stem of a rose get so completely covered with those little green lice?” asked Emile.
“That is easily explained,” answered his uncle. “Plant-lice multiply very rapidly, since each one, without exception, from the first to the last, whatever their number, becomes capable in a few days of procreating a family. The newly born settle down beside their mothers, and are themselves soon surrounded [[281]]by their own progeny. These in turn, in a little while, have offspring of their own; and so on, indefinitely, as long as the season lasts. Thus the stem, the branch, the entire plant, become covered with lice so closely packed one against another that in places the real bark is hidden by this bark of vermin.
“Have you ever seen a garden-patch of broad beans overrun by black lice? There, better than anywhere else, may be seen the rapidity of propagation. On that green expanse appears at first a small black stain, announcing the beginning of the invasion. It is a family of lice installed at the top of a beanstalk, the tenderest part of the plant, where the insects’ suckers can work to best advantage. The gardener, as soon as he is aware of what is going on, hastens to cut off this part of the stalk and crush it under his heel. He hopes to exorcise the evil by destroying this nest of vermin.
“His hope is short-lived. A few days later, instead of one plant invaded there are dozens. He lops off again; he turns up the remaining leaves and examines them one by one; he crushes what vermin he finds, taking all pains to make the extermination complete. Will he make an end of it this time? Not at all: the black hordes reappear in greater numbers than ever; the invaded stalks can no longer be counted. A few lice that escaped the slaughter were enough to infest the whole patch of beans. The foliage hangs down, foul and withered; the young pods, riddled with punctures and corrugated with [[282]]scars, shrivel up and can grow no larger. For this ill there is no remedy; the harvest is ruined.
“The gardener pulls it all up and throws it on the dung-hill. His care and vigilance have been unable to arrest the invasion. In vain he crushed legions at a time under his angry heel: in a few days the half-dozen survivors had propagated a larger colony than ever. Man is hardly in a position to contend successfully against this lowly vermin which braves extinction by virtue of its countless numbers.
“As I told you, the plant-louse does not like to change its place. It plants its sucker on the very spot where it has just been born, and thenceforth sticks to that spot, filling its stomach with sap and surrounding itself with a family. This love of repose explains to us very well how the twig of a rosebush or the top of a beanstalk undergoes a progressive colonization; but it does not account for the distant propagation of the species.
“With its home-keeping habits the insect ought to be confined within narrow limits, on a single leaf and not on all leaves, on one rosebush and not on the neighboring rosebushes. But as a matter of fact it is disseminated everywhere. When one patch of beans becomes infested, those in the neighborhood are equally unfortunate; when one rosebush shows a colony of plant-lice, all those around it are similarly visited. No vegetable growth can defend itself from the pest. How, then, is it that this obese animalcule, which totters with fatigue after one step forward, succeeds in passing from rosebush to rosebush, [[283]]from garden to garden? By what means is it able to spread in all directions without limit?
“Let us examine a number of rosebushes, and we shall have a prompt answer to our question. In addition to the wingless plant-lice, big of belly and all grouped on the tender twigs, we shall see others, green like the first ones, but more elegant in form, of greater freedom of movement, and provided with four wings, very beautiful wings too, diaphanous and gleaming with rainbow tints. These creatures are no lazy sap-bibbers forever squatting over the well their sucker has bored. They are seen to come and go, circulating briskly among the stationary herd, inspecting the foliage, passing from branch to branch, and even taking flight for some distant goal. They are the travelers of the family. Their function is to propagate the race in the surrounding district, with the aid of their wings, and even at considerable distances when a puff of wind carries them thus far.
“Two classes, then, dissimilar though related, are to be noted among the green lice of the rosebush and the black ones of the beanstalk, as also among countless others. The members of one class have no wings; they pass their lives where they were born, and multiply in serried legions. Those of the other class, which is relatively small, are equipped with wings. Confined to no one spot, they fare forth as some passing breeze or their own strength of wing may determine, and deposit in favorable localities the germs that are to serve each as the beginning of [[284]]a community of wingless plant-lice. The first kind procreate on the spot with a fecundity almost beyond belief; the second take leave of the stationary family and go out to start new centers of population in various quarters. The first propagate without limit; the second colonize.
“To soil the stem of a rose with a coating of lice is not exactly a capital offense; but to lay waste a field of beans, the hope of the farmer, is a far more serious matter. Yet even that is as nothing when compared with other depredations committed by plant-lice. There is a species of these insects that lives underground, subsisting on the roots of the grape-vine. Oh, the hateful creature! Never has agriculture known anything to equal the ravages it commits; no floods or droughts or inclement seasons have ever wrought such woes. Its terrible sucker has, up to the present time, caused us losses estimated at the fabulous sum of ten milliard francs. What a mouthful for a miserable little louse hardly visible to the naked eye! And to think that the combined efforts of nations cannot succeed in exterminating this pest! Alas, how feeble is mere force when confronted with the exceedingly minute infinitely multiplied!