Grape phylloxera, Daktulosphaira vitifoliae (Fitch), is a serious pest of commercial grapevines worldwide. This tiny insect forms galls on leaves and roots of grapevines. It is believed that this insect originated in the Eastern United States, where damage is now most prevalent on leaves of French-American hybrid grapevines. High populations of foliar phylloxera can result in premature defoliation, reduced shoot growth, and reduced yield and quality of the crop.
Grape phylloxera has a complex life cycle (Figure 1). They overwinter either as a winter egg under the bark of older canes or trunks or as nymphs on grapevine roots. The winter egg (Figure 1A) gives rise to the fundatrix, or stem mother, which moves to a nearby shoot tip and begins feeding. Feeding by the phylloxera elicits gall formation, and the female becomes enclosed within a small, spherical gall on the underside of the grape leaf. This parthenogenetic female is capable of producing several hundred eggs. First instar nymphs, or crawlers, emerge and move out of galls to nearby shoot tips where they begin feeding and thereby initiate formation of new galls. There are three to five generations of foliar phylloxera per season in eastern North America (Figure 1B). Throughout the summer a certain portion of the foliar crawlers move actively or passively to the soil surface. These crawlers may move through cracks in the soil and eventually reach grapevine roots.
Phylloxera may also overwinter on grapevine roots as first or second instar nymphs. As soil temperatures increase, crawlers resume feeding. Feeding by phylloxera on grapevine roots results in two types of galls. Nodosities are galls formed on small, apical rootlets which are generally thought to result in little damage to the vine. Tuberosities are galls formed on larger, older portions of the root which, if sufficiently abundant, may eventually result in death of the vine.
From July through October, some root-infesting phylloxera develop wing buds and eventually become fully winged adults. Alates emerge from the soil and deposit two types of eggs, a larger egg which results in a female and a smaller egg which gives rise to a male. These sexual forms mate and the female deposits a single overwintering egg under the bark of older canes or trunks, thus completing the complex life cycle.
|Figure 1. Life cycle of grape phylloxera, Daktulosphaira vitifoliae (Fitch), (after Williams, 1938). (A) Winter egg, (B) foliar form (Gallicola), (C) root form (radicola), (D) winged adult, (E) sexual stage.|
This aphid-like insect is very small and difficult to see with the unaided eye. Galls formed by phylloxera are more readily identified than the insects. Galls on leaves are small, spherical growths on the underside of the leaves (Figure 2), with the opening being on the top. Phylloxera feeding on roots also results in small galls either on apical rootlets (Figure 3) or on older portions of the root. These insects generally occur in groups on both leaves and roots and the yellowish color of the insect and their eggs is useful in recognition.
Fig. 2. Phylloxera galls on the underside of grape leaves are quite conspicuous.
Fig. 3. Phylloxera galls on the rootlets of 'Concord' and other American cultivars are not uncommon.
Feeding by root phylloxera on European grapevines, Vitis vinifera L., is potentially devastating and nearly destroyed the French wine industry in the late 1800's. The epidemic was eventually brought under control by grafting V. vinifera scions onto resistant American, Vitis labruscana Bailey, rootstocks. A major resistance breeding program conducted in Europe against grape phylloxera resulted in grape cultivars commonly referred to as French-American hybrids. French-American hybrids are important in eastern North America for wine production, but they are particularly susceptible to foliar grape phylloxera. Widespread planting of French-American hybrids in eastern North America has resulted in a heightened awareness of foliar phylloxera. Foliar phylloxera reduce net photosynthesis of grape leaves. Leaf galling by grape phylloxera causes distortion, necrosis, and premature defoliation of French-American vines. Premature defoliation may delay ripening, reduce crop quality, and predispose vines to winter injury. Grapevines heavily infested with foliar phylloxera may contribute to root infestations. Research indicates that high population densities of foliar phylloxera can result in a reduction in yield and quality of the crop. Populations must reach very high densities before yield is affected, and in most years yield will probably not be affected. It is not known, however, what impact infestations by the insect year after year have on the overall health and vigor of the vine.
Few chemicals are registered for control of foliar grape phylloxera. Thiodan (endosulfan) is the standard for commercial growers, but no compounds are registered for homeowner use against grape phylloxera. (Endosulfan does burn some cultivars.)
Phylloxera crawlers can be spread on vineyard equipment. Therefore, when mechanical operations are performed, equipment should not be moved from an infested block to a noninfested block.
Infestations may also originate from wild grapevines near the vineyard, so these areas should be monitored carefully. Wild grapevines near the borders of vineyards should be destroyed if possible.
A number of natural enemies feed on grape phylloxera, but none are commercially available for use in biological control programs.
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