It was only after long and patient investigation that the various broods of the terrible Phylloxera which, between 1868 and 1888, destroyed half the vineyards of France, became known, their relations to one another determined, and the final cure for the devastation caused by them decided upon and put into practice.

In all ordinary plant-lice or green-fly (aphides) at the end of the summer, the last parthenogenetic brood produces a generation of distinct males and females, which differ a good deal in appearance from the virginal broods of the spring and summer. Each female, after receiving sperm-cells from a male, lays a single egg, which consists of a fertilised egg-cell enclosed in an egg-shell. It is deposited in a safe place in a crack of the bark of a tree, or on the rootlets of some plant, and remains unchanged through the winter. In the spring from every such egg hatches a single female aphis, which feeds and increases in size. In a very short time (a week or so) this solitary female (Fig. 58) proceeds to produce, without male intervention, young which grow from true egg-cells which are not laid but remain inside her. The young are born or pass out of her as small six-legged insects. They feed and grow up, and in turn produce “parthenogenetically” and viviparously broods of young like themselves. The first female thus hatched from the winter egg is called a “foundress,” or “stock-mother,” because she starts a whole colony of young which, by virginal propagation of successive broods, may number many millions in a season. These are known as “virgin-mothers” (Fig. 57), and eventually their later generations always produce males and females, so that we distinguish, in the course of a year, four sets of aphides, starting from the egg, namely (1) the foundresses, (2) the numerous generations of virgin-mothers, (3) the males, and (4) the egg-laying females.

In different kinds of plant-lice any of these “sets” may be either winged or wingless (Figs. 55, 56, 59); many generations of the virgin-mothers are wingless, but not all, in all species. According to the species or kind of aphis and its requirements in regard to the plants on which it feeds, wings are developed so as to enable the aphis to fly from one tree or locality to another, or are not developed if the aphis has to remain where it was born. The whole series of successive broods of some kinds of aphis remain on one plant and about the same part of it, and then there is little need for wings. Others have their summer broods on the twigs or leaves, but the later broods descend in winter to the roots of the same plant. The woolly aphis of the apple trees and pear trees behaves in this way; other species again produce a late-winged brood, which leaves the plant on which its parents were feeding, and travels some distance to the twigs or to the roots of a quite distinct kind of plant to produce an autumn brood, and from these the final males and females are born, and the winter eggs are then deposited. The hop-louse leaves the hop when the hop-vine dies down in autumn. The abundant wingless form (Fig. 58) of which there have usually been ten generations, produces at last a winged “migrant” brood (Fig. 59) which flies away to plum trees and sloe bushes, perhaps a quarter of a mile distant. There the migrants produce wingless females on the plum tree. They are followed to the plum trees by a final migrant brood from the dying hops which are males—the first yet seen (Fig. 56). The males fertilise the wingless females born on the plum tree—and the latter lay each one fertilised egg in the crevices of the bark of the plum tree near the young buds. Winter now sets in: all are dead except the eggs. In the following late spring a foundress hatches out from each egg so deposited. The “foundress” (Fig. 58) in this species, the hop aphis, is wingless. She produces parthenogenetically and viviparously a brood of wingless females. They similarly produce on the plum tree a third generation of virgin females, but these have wings! (Fig. 55). They fly back to the hop-vines, which are now well risen from the ground and offer abundant juice to the wingless virgin brood which escapes from the winged migrants as soon as they have settled on the hop, and feed and grow and produce new wingless broods (Fig. 57) in rapid succession.

The phylloxera of the vine is a plant-louse or aphis, which exhibits an interesting adaptation of winged and wingless broods to the requirements of the insect’s nutrition and multiplication. A “foundress” hatches from an egg on the bark of the vine where it has passed the winter. It proceeds to attack the young leaves and to produce a brood of young. The leaves of the vine when thus attacked swell up and produce galls, in which the young phylloxera are enclosed, and there the phylloxeras continue to multiply, producing more galls and thus destroying the leaves. Some of the young broods now crawl down the vine to its roots; others stay on the leaves and continue their destructive work there. There are several varieties of form and size amongst these broods. Those which go to the roots attack the rootlets and produce knobs and swellings on them, leading to their destruction as feeding organs. Meanwhile the root-phylloxeras multiply exceedingly, and those on the leaves are still feeding and multiplying. From one foundress mother as many as twenty-five millions are produced in six months. At last in the autumn the root-parasites produce a winged generation of virgin-mothers, which come up from the ground and fly away to other vines, upon which they produce males and females. These females each lay a fertilised egg on the bark of the previously healthy vine, and so the infection is spread. The root-infesting forms continue to multiply, and in warm climates there is no cessation of this process even in winter.

This parasite—the Phylloxera vastatrix—was introduced with some American species of grape-vine—brought over as experimental samples from Colorado—about 1864. In its native country it does comparatively little harm, for the roots of the American species of vine are, though attacked by it, not seriously injured. They have the property of throwing out new rootlets when those already existing are punctured and injured by the phylloxera, and so are not killed by the attack, as is the European grape-vine.

The introduction of this deadly parasite to Europe was a mere chance, due to ignorance and stupid want of supervision of importations on the part of the Government, such as is common in this country, though less so in France and Germany—part of the blind mixing-up of the nicely adjusted products of all parts of the earth which civilised man is always bringing about with disastrous and terrifying results. In twenty years France lost 400 million pounds in consequence; three million acres of vineyards were destroyed. Other countries—Germany, Italy, and the Cape—also suffered. All sorts of remedies were suggested and tried, such as the application of poisons to the roots and the sinking of the vineyards under water. Gradually the only effective method of dealing with the case has been established. The old European vine-stocks or standards have been grubbed up in all but the very choicest vineyards, and American vines have been planted in their place. On to these have been grafted cuttings of the local French vines, and they have taken kindly to their new conditions. The produce of the French vineyards is now greater than it has ever been. It had fallen from an annual yield of 1,300,000,000 gallons to 650,000,000—but in 1900 it had risen again to a yield of more than 1,400,000,000 gallons.

This history is a striking instance of the vast importance to civilised communities of a knowledge and control of even such minute living things as the plant-lice, and of the extraordinarily large results which obscure living things may produce. It must tend to convince reasonable men of the importance of accurate knowledge as to living things and of the necessity of expending public money in constantly improving and extending that knowledge.

An ingenious illustration of the enormous fecundity of the plant-lice occurs to me as worth giving in conclusion. The late Professor Huxley—a careful and trustworthy authority—calculated that the produce of a single aphis would, in the course of ten generations, supposing all the individuals to survive, “contain more ponderable substance than five hundred millions of stout men; that is, more than the whole population of China.” And this calculation is held by some authorities to be below rather than above the mark!