Page:Popular Science Monthly Volume 5.djvu/17

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7
THE GRAPE PHYLLOXERA.

number of eggs found in a single gall averages about 200; yet it will sometimes reach as many as 500, and, if Dr. Shimer's observations can be relied on, it may even reach 5,000.[1] I have never found any such number myself; but, even supposing there are but five generations during the year, and taking the lowest of the above figures, the immense prolificacy of the species becomes manifest. Small as the animal is, the product of a single year, even at this low estimate, would encircle the earth over thirty times if placed in a continuous line, each individual touching the end of another. Well it is for us that they are not permitted to multiply in this geometrical ratio! Nevertheless, as summer advances, they do frequently become prodigiously multiplied, completely covering the leaves with their galls, and settling on the tendrils, leaf-stalks, and tender branches, where they also form knots and rounded excrescences (Fig. 3, e), much resembling those made on the roots. In such a case, the vine loses its leaves prematurely. Usually, however, the natural enemies of the louse seriously reduce its numbers by the time the vine ceases its growth in the fall, and the few remaining lice, finding no more succulent and suitable leaves, seek the roots. Thus, by the end of September, the galls are mostly deserted, and those which are left are almost always infested with mildew [Botrytis viticola, Berkely), and eventually turn brown and decay. On the roots, the young lice attach themselves singly or in little groups, and thus hibernate. The male gall-louse has never been seen, and there is every reason to believe that he has no existence. Nor does the female ever acquire wings. Indeed, I cannot lay too much stress on the fact that gallæcola occurs only as an agamic and apterous female form. It is but a transient summer state, not at all essential to the perpetuation of the species. I have found it occasionally on all species of the Grape-vine {vinifera, riparia æstivalis, and Labrusca) cultivated in the Eastern and Middle States, and on the wild Cordifolia; but it flourishes only on the River-bank grape (riparia), and more especially on the Clinton and Taylor, with their close allies. Thus, while legions of the root-inhabiting type {radicicola) are overrunning and devastating the vineyards of France, this gallæcola is almost unknown there, except on such American varieties as it infests with us. A few of its galls have been found at Sorgues, on a variety called Tinto; and others have been noticed on vinifera vines interlocking infested American vines, or have been produced by purposed contact with the young gallæcola. Similarly, there are many varieties, especially of Labrusca, which, in this country, suffer in the roots, and never show a gall on the leaves.

The precise conditions which determine the production and multiplication of gallæcola cannot now, if they ever can, be stated; but it is quite evident that the nature and constitution of the vine are important elements, since such vines as the Herbemont often bear

  1. "Practical Entomologist," vol. i., p. 17.