The life history of the phylloxera is very complex where the different forms of the insect appear and need not be entered into in detail here. East of the Rockies, the most evident indication of the presence of the pest is great numbers of leaf-galls on the under side of the leaves of the grape as shown in [Fig. 36]. These galls, however, are seldom to be seen in California and are not present on Concords and some other varieties in the East. The winter egg may be taken as the beginning of the life cycle of the phylloxera. From a single winter egg a colony may arise, the first insect after hatching making its way to the leaves where it becomes a gall-maker and gives rise to a new generation of egg-laying root-feeders. On varieties and in regions where the gall form is not found, the insect probably goes directly from the winter egg to the roots. Once the pest is established on the roots, generation follows generation throughout the growing period of the vines, as many as seven or eight occurring in one season.
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.
Fig. 37. The grape root-worm.
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.
