Grape phylloxera (family Phylloxeridae) is an aphidlike, root-sucking insect. It feeds on the roots of Vitis vinifera and certain other grape rootstocks, stunting growth of the vines or killing them due to the phylloxera's plant-toxic saliva.
Phylloxera are tiny and feed hidden underground. To determine whether this pest is present and abundant, examine grape roots and see the damage section below.
Grape phylloxera are oval to pear shaped with adults 1/25 inch (1 mm) or less in length. Nymphs resemble small adults. These insects are barely visible to the naked eye but are readily seen with a 10X hand lens. Adults and nymphs are bright yellow when young and feeding on a healthy or young root. When feeding on older or weakened roots, the phylloxera are light brown, olive green, orange, or yellowish green. Dead phylloxera become abundant in the fall and are black or brown. Overwintering occurs only as first instars (young nymphs) that do not feed and become brown, gray, or tan.
During the growing season, most adult grape phylloxera are wingless females. Males can be present in the fall and winged adults are occasionally seen in the summer and fall but are relatively uncommon in California.
There are also leaf-galling forms of grape phylloxera that are abundant in the Eastern United States. In California they are found in at least Solano and Yolo Counties. They cause warty green swellings on leaves. There can be several dozen galls per grape leaf and sometimes a high proportion of a grapevine's leaves become galled.
Phylloxera eggs are oval and 1/50 inch (0.5 mm) or less in length. They are bright yellow when recently laid and darken to yellowish gray as they age. In older eggs, a pair of red eyespots and a dark line that is the strawlike mouthparts become visible through the shell.
Grape phylloxera develops through 3 life stages: egg, nymph, and adult. First instars (nymphs) overwinter on roots. In spring, when soil temperatures exceed 60°F, nymphs start feeding, growing, and maturing into adults. The adult females then lay up to 300 eggs per female. After hatching, the mobile first instars (crawlers) may move between grapevines by walking through crevices in soil, on the soil surface, or up vines where they are blown by the wind. Crawlers can also be spread on boots, equipment, and tools that contact infested vines and on grape cuttings, such as those used for propagation.
In the fall when soil temperatures fall below 60°F, all life stages die except the first instars that hibernate and overwinter. Where soil temperature does not go below 60°F, feeding and reproducing may occur throughout the winter. There are 3 to 5 generations each year, the greater number in warmer growing regions.
Grape phylloxera is a serious pest in the heavy clay soils that are common in cooler grape-growing regions such as Lake, Mendocino, Monterey, Napa, and Sonoma counties as well as the Sacramento River Delta and the Sierra foothills. Grape phylloxera is present but is generally less damaging in soils of the San Joaquin Valley. It is not a pest in sandy soils.
Aboveground symptoms of an infestation include grapevines that grow slowly, remain undersized, and have shortened internodes. Infested plants may have foliage that is somewhat more pale colored than normal and exhibit symptoms of various nutrient deficiencies and water stress. For example, in North Coast growing areas, infested vines commonly exhibit symptoms of potassium deficiency.
Grape phylloxera feeding causes young rootlets to swell and turn yellowish. Infested root tips become bent and swollen (clubbed). On mature hardened roots the swellings are often hard to see. Necrotic spots (areas of dead tissue) develop at the feeding sites on the roots. The necrotic spots are a result of secondary fungal infections that can girdle roots and kill large sections of the root system. Such root injury causes vines to be stunted and produce less fruit. The severity of infestation will differ with the vigor of the grapevine as well as with soil texture and drainage. Soil that drains slowly and poorly favors damage by grape phylloxera.
Planting grapes on phylloxera-resistant rootstocks or where soil is sandy are the only completely effective methods for controlling grape phylloxera. Insecticides for phylloxera are believed to not be very effective on home-grown vines. In commercial vineyards it may take years of applications of a systemic neonicotinoid such as dinotefuran or imidacloprid to remedy severe damage. Although many phylloxera may be killed by insecticide, their abundance can rebound rapidly.
Avoid rootstocks that have any V. vinifera parentage because virulent biotypes of phylloxera commonly damage these rootstocks. Choose resistant Vitis species such as V. riparia, V. rupestris, and hybrids between these or various species of native American Vitis rootstocks. For example, V. rupestris rootstocks are commonly called Rupestris du Lot, Rupestris St. George, or St. George. Note that in soil where heavily infested grape roots have grown, even resistant rootstocks may support a low abundance of phylloxera and the grapevines may be somewhat stunted.
In hot growing areas such as California's Central Valley, phylloxera damage may be reduced by providing optimal irrigation and fertilization and other good cultural practices that help to limit plant stress. Effectively controlling other insect pests and diseases of grapevines can increase their tolerance to phylloxera feeding.
If root-feeding phylloxera are suspected to be causing unhealthy vines, dig near their trunk under the drip emitters or where water is otherwise applied. Inspect young roots looking for those that have whitish yellow, hooked feeder roots that are bent and swollen. Heavily infested roots may appear yellow because of the numerous yellow insects. Phylloxera-killed roots can be blackish.
Examine any galled roots with a hand lens looking for the presence of phylloxera. Live phylloxera are most easily viewed in the spring and late summer. Soil is easier to dig if this is done shortly after irrigation, but wait until topsoil has drained and is not muddy before digging to examine roots.
If grape phylloxera and their damage to roots are found to be abundant and vines are not performing as desired, seek grapevines with phylloxera-resistant rootstock and replant with those.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources also available as an eBook and Pest Management Guidelines: Grape, University of California Statewide Integrated Pest Management Program (UC IPM).