Grapevine phylloxera, which threatened to devastate the European wine industry in the mid-19th century, remains a serious threat to vineyards. Hearty American rootstocks were successfully grafted to European varieties to control the devastation. However, the “survival of the fittest” rule ensures that each surviving phylloxera population will be more adaptable to new host plants. After a description of this insect’s taxonomy and morphology, the complicated reproductive strategy will be detailed. Then, following a brief review of the phylloxera in viticulture history, this article will discuss (1) the relationship between the insect and the Vinis
species hosts, (2) recent attacks on Napa and Sonoma County vineyards, and (3) associated pest control
measures, as well as economic impact.
Viteus vitifoliae, also known as Dakulosphaira vitifoliae or Phylloxera vitifoliae, is found in the order Hemiptera, suborder Homoptera, superfamily Aphididae, and family Phylloxeridae. There are species of phylloxera specific to oak and pecan trees as well. This report will concentrate on the phylloxera whose entire life cycle is confined to Vitis species hosts. For ease of reading, this paper will refer to the insect as V. vitifoliae or simply phylloxera. The aerial leaf-feeding form of this insect is called gallicolae, and the root-feeding form is called radicicolae. V. vitifoliae may resemble to scale insects to those outside the entomology world, but are closer in morphology to aphids. They hold their wings horizontally when at rest, however, and lack an aphid’s cornicles. Adults are pear or oval-shaped, minute (less than 1 mm), and may be winged or wingless. Color ranges from yellowish green to brown.
V. vitifoliae has a life cycle that’s relatively typical of Aphididae, but certainly more complicated than most species. The reproductive strategy includes parthenogenic and sexual stages, aptertous and alate offspring, and periods of development on leaves, stems, and roots. In spring, an overwintering egg on the grape stem develops into a fundatrix: an apterous, or wingless, viviparous parthenogenic female. The fundatrix migrates to a developing leaf and forms a “pouch gall” characterized by a depression on the leaf surface. The gall is formed in the meristematic tissue, which is the young, unspecified cellular leaf material. As soon as the “stem mother” aphids mature, they lay 400-600 eggs inside each gall. The wart-like galls are about Ã?Â¼ inch in diameter, and characteristic of V. vitifoliae. These eggs produce wingless offspring like their mothers. Most of these are the leaf-feeding gallicolae, but some are the root-feeding radicicolae. The radicicolae are able to survive the winter by traveling underground. They form swelling-type galls on young roots. In the fall, alate, or winged, females that can produce both sexes of offspring fly from the root galls to the vines. The alate females then lay eggs for wingless non-feeding sexually reproductive offspring. These offspring mate, and each female produces a single egg, which will diapause in winter. The entire cycle takes cycle takes two years to complete. It is important to note that the vine species, climate and other conditions play a role in the development of the gallicolae form. There is a marked difference in the stages and success of the phylloxera in Europe and the United States due to these discrepancies.
In European vineyards, the phylloxera seldom creates leaf galls. It is the root gall that causes rot and the eventual death of the vine. “The grape phylloxera and the American species of grapes, coexisting for millennia, have co-evolved and no doubt continue to do so,” notes Gilbert Wauldbauer in Insights from Insects. “In other words, they have a long history of reciprocal responses, the grapes evolving chemical and other defenses and the insects reacting by evolving ways of circumventing these defenses.”
The wild labrusca grapes of the United States, while considered by European vintners to be inferior in flavor, did have a practical advantage. V. vitifoliae did not attach to the American rootstocks, or was tolerated by the plant at low levels, or the plant had a resistance to the insect in the form of a naturally produced compound which inhibited gall making. In the mid 1800’s the American rootstock was imported to Europe, and the phylloxera attacked the native Vinis vinifera. The European grape species had no resistance to the pest. Within 25 years, phylloxera destroyed nearly one-third of French vineyards, nearly 2.5 million acres. In 1871, American entomologist Charles Riley was able to identify the complete life cycle of V. vitifoliae and recognized that the insect survived, but did not thrive, on the American varietal. By 1873, Charles Riley and French entomologist Jules Planchon laid plans to replant the affected vineyards with hybrid rootstock. Over 11 trillion grapevines in France were replaced with French-American grafts. “The Great Wine Blight” was conquered, but phylloxera remains a serious threat.
The primary damage is caused when V. vitifoliae feed on the root fluids. Necrotic spots develop at the feeding site, girdling the roots. Fruit productivity decreases and both vines and roots die. The popular Vinis vinifera grape species is particularly susceptible to damage. To control the pest, resistant rootstocks (such as those from the Eastern United States) are planted and the desired cultivar is grafted to the root. The process generally produces grapes that are both high quality and phylloxera-resistant. Damage may still be sustained if the roots are weakly resistant due to “impure” parentage. These roots can allow the phylloxera to colonize, as well as lead to the development of more virulent pests.
In the last century, phylloxera has inflicted damage on vineyards worldwide. Napa Valley grapes were attacked by 1880, after spreading through scattered Sonoma County vineyards over fifteen years. The leaf-galling form of V. vitifoliae is not seen in California. Terroir which mirrors that of Europe allows for ideal vinifera conditions, but also encourages the same type of root infestation. It is possible that the pest was introduced to California before the Civil War with the transport of the “Catawba” vines from the Eastern United States. Vintners attempted, and failed, to quell phylloxera by replanting many types of rootstock. Of 577 vineyards in Napa County, 244 showed marked evidence of phylloxera by 1893. By 1900, the Napa valley industry was nearly wiped out. Chemist Georges de la Tour was the savior of the industry, succeeding with vine Rupestris St. George. California began reconstruction with grafted vines in 1896.
In the 1960’s, UC Davis scientists recommended the use of rootstock AXR-1 for higher yields than other vinifera hybrids. California vintners spent millions replanting with AXR-1, but were also able to update other elements in the upheaval. Efforts were concentrated on unique handling of vineyards in regards to specific trellising, varietals, microclimates, and methods, instead of uniform treatment. Unfortunately, the phylloxera had mutated. A new biotype of the pest vigorously attacked the rootstock that made up most of the post-1960 plantings. AXR-1 was not resistant to the new hardier strain of phylloxera and a plague faced California vineyards in the 1980’s and 1990’s.
It is theorized that the flood of 1986 may have washed the pest from one vineyard to another. Areas still planted with St. George remained resistant. The “type B” phylloxera, identified in 1983, has been infecting more than 2,000 acres of AXR-1 in Northern California each year since 1990. The rootstock, which had resisted the pest for over 50 years, was not resistant to “type B.” According to trade publication Wines & Vines: “In December 1989, the Phylloxera Task Force, made up of U.C. researchers, Ã¢Â?Â¦ urged growers to avoid planting AXR-1 rootstock and to plant other rootstocks that did not contain Vitis vinifera in their parentage. Starting even before 1989, growers in Napa and Sonoma counties who had found phylloxera infestations in their vineyards had begun the task of removing infested AXR-1 acreage and replanting with a much broader range of alternative rootstocks.”
V. vitifoliae has caused over one billion dollars in damage to California vineyards in the past decade. Replanting with resistant stock may be organically sound, but it can be prohibitively expensive. Current estimates range from $12,000 to $15,000 per acre. The vintner must be prepared for a substantial investment of time, labor, and money. V. vitifoliae are as yet unsusceptible to insecticides or predators. Therefore, new ways to control the pest are a matter of vested interest to entomologists. Alternative pest management that could supplement or replace rootstock grafts would prove a boon to the industry. The University of California IPM Program is currently exploring such options as mutating the vine physiology, using cover crops, and establishing pathogens to the phylloxera.