From midsummer until the close of the growing season, some of the eggs deposited by the root-feeders develop into nymphs which acquire wings and emerge from the soil to form new colonies from eggs deposited on the under side of the leaf. An individual insect deposits from three to six eggs of two sizes, from the larger of which come the females and these, after fertilization, move to the rough bark of the vine and deposit the winter egg for the renewal of the cycle.
Several methods of control have been employed in Europe and California, as treatment by carbon bisulfide injected in the soil; flooding in vineyards that can be irrigated; confining the vines to sandy soils; and, most important, planting vines grafted on resistant stocks, there being great variation in immunity of species of American grapes to phylloxera. The subject of stocks resistant to this pest has been discussed in Chapter IV and need not be taken up again. East of the Rockies, treatment is not necessary with American grapes.
The grape root-worm is the most harmful of the insect pests of grapes in the grape-belt along the shores of Lake Erie in Ohio, Pennsylvania and New York. This root-worm (Fig. 37) is the larva of a grayish-brown beetle (_Fidia viticida_), shown in Fig. 38. The worms feed at first on the rootlets and later on the bark of the larger roots of the vines so that the injured plants show roots devoid of rootlets and bark channeled by the pest. So plain is the work of the root-worm that the grower never need be at a loss as to the cause of vines injured by this pest. The worms feed during the latter part of the growing season, reaching full growth at this time. The next June they transform into pupæ and in late June or early July emerge as adult beetles.
[Illustration: FIG. 37. The grape root-worm.]
[Illustration: FIG. 38. Root-worm beetle.]
The presence of the adult beetles is more easily detected on the foliage than is that of the larvæ on the roots, for the feeding beetles ravenously devour the upper sides of the leaves, leaving chain-like markings, shown in Fig. 39, their destructiveness decreasing somewhat after a few days from their first appearance. A fortnight after the beetles begin their attack on the foliage the female begins laying her eggs, to the number of 200, placing them under the rough bark of trunk and cane. These hatch in late July or August and the young grubs at once seek the roots.