Page:Popular Science Monthly Volume 5.djvu/18
THE POPULAR SCIENCE MONTHLY.
witness, by their leaves covered with abortive galls, to the futile efforts the lice sometimes persist in making to build in uncongenial places.
Yet other elements come into play, and nothing strikes the observer as more curious and puzzling than the transitory nature of these galls, and the manner in which they are found—now on one variety, now on another.
I was formerly inclined to believe that gallæcola was a necessary phase in the annual cycle of the insect's mutations; in other words, that it was essential to the continuance of the species, and was probably the product of the egg laid by the winged and impregnated female. On this hypothesis I imagined that gallæcola was probably the invariable precursor of radicicola in an uninfested vineyard, and that, if galls were not allowed to develop in such a vineyard, it would not suffer from root-lice. More extensive experience has satisfied me that the hypothesis is essentially erroneous, and that, while the first galls may sometimes be produced by lice hatched from the few eggs deposited above-ground by the winged female, they are more often formed by young lice hatched on the roots, and which, wandering away from their earthy recesses, are fortunate enough to find suitable leaf conditions. It is barely possible that under certain circumstances, as, for instance, on our wild-vines, where the soil around the roots is hard and compact, gallæcola may become more persistent, and pass through all the phases belonging to the species without descending to the roots—the eggs wintering on the ground, or the young under the loose bark, or upon the canes. For a somewhat similar state of things actually takes place with another plant-louse (Eriosoma pyri Fitch), which in the Western United States normally inhabits the roots of our apple-trees, and only exceptionally the branches; while in the moister Atlantic States, and in England and moister parts of Europe, where it was introduced from this country, it normally infests the branches, and more exceptionally the roots. But there are no facts yet known to prove such to be the case with the Grape Phylloxera, even on our wild-vines, and I do not believe that it ever is the case in our cultivated vineyards.
As already indicated, the autumnal individuals of gallæcola descend to the roots, and there hibernate. There is every reason to believe also that, throughout the summer, some of the young lice hatched in the galls are passing on to the roots; as, considering their size, they are great travelers, and show a strong predisposition to drop, their natural lightness, as in the case of the young Cicada, and of other insects which hatch above but live under ground, enabling them thus to reach the earth with ease and safety. At all events, I know, from experiment, that the young gallæcola, if confined to vines on which they do not normally, and perhaps cannot, form galls, will, in the middle of summer, make themselves perfectly at home on the roots.