In recent years, Napa viticulture has been threatened by a pest no less serious than the grape phylloxera that wiped out vineyards in the United States and Europe in the mid and late nineteenth century: the Glassy-winged Sharpshooter (GWSS). A non-native Pierce’s disease vector, the Glassy-winged Sharpshooter has recently become established in most of southern California and in pockets of Tulare, Fresno, Santa Clara and Sacramento counties. If this insect becomes established in Napa County, it could mean the end of the vineyard industry here.
Since the early 1990s, the Glassy-winged Sharpshooter has been seen in high numbers in citrus along the coast of southern California. A few years later it became locally abundant farther inland in Riverside and San Diego counties. In 1998 and 1999, high populations on citrus and adjacent vineyards were seen in southern Kern County. The Glassy-winged Sharpshooter is expected to spread north along the citrus belt of the Central Valley and eventually become a permanent resident of various habitats throughout northern California. It feeds and reproduces on a wide variety of trees, woody ornamentals, and annuals in its region of origin, the southeastern United States. Crape myrtle and citrus are especially preferred. It reproduces on hundreds of different plants in southern California, including coastal trees such as Eucalyptus and live oaks. It appears that Glassy-winged Sharpshooter can sustain permanent breeding populations in natural vegetation of forested areas.
Blue-green Sharpshooters, like Glassy-winged Sharpshooters, are in the same insect family as leafhoppers (Cicadellidae). The Blue-green Sharpshooter feeds, reproduces, and is often abundant on cultivated grapevines. It also feeds and reproduces on many other plants but prefers woody or perennial plants such as wild grape, blackberry, elderberry, mugwort and stinging nettle The Blue-green Sharpshooter is most common along stream banks or in ravines or canyons that have dense growth of trees, vines, and shrubs. Because it feeds on succulent new growth in areas of abundant soil moisture and shade, it can sometimes be found in ornamental landscaping around homes. The Blue-green Sharpshooter is currently the most important vector of Pierce’s Disease in the Napa Valley. It has one generation a year in most of California and a second generation in some parts of the state. In late winter and early spring, adults become active, and a percentage begin moving into nearby vineyards for feeding and egg laying starting just after bud break until grape shoots are several inches long. Their dispersal into vineyards increases as natural vegetation dries up. Eggs hatch from May through July. Some of the nymphs become adults by mid-June, and the number of young adults continues to increase through July and August. In August when grape foliage is less succulent, Blue-green Sharpshooters begin to move back to nearby natural habitats. Populations of Blue-green Sharpshooter are always larger in natural vegetation than in vineyards, and still, by spreading Pierce’s Disease, they cause millions of dollars of damage to vineyards every year.
Glassy-winged Sharpshooter and Blue-green Sharpshooter vector the bacterium Xylella fastidiosa, which causes Pierce’s Disease in grapes. (This bacterium also causes alfalfa dwarf disease and almond leaf scorch in California.) The bacterium multiplies in the water-conducting vessels and eventually clogs them, causing water stress that becomes apparent in midsummer as marginal leaf burn and then later as leaf browning, drying, and premature dropping. The leaf petiole may remain attached after the leaf falls, and irregular brown areas may show up on otherwise green canes. If an infected vine bears grapes, they may shrivel when the weather warms. Some varieties of winegrapes are somewhat tolerant to Pierce’s Disease and may recover from the infection or survive for several years before succumbing. Early season infections are more likely to cause mortality since by season’s end the disease may have spread into wood that will not be removed by winter pruning. Mortality may occur the first year, but more often vines die 2 to 3 years after infection.
Experience with the Glassy-winged Sharpshooter as a Pierce’s Disease vector indicates that rather than the linear increase in Pierce’s Disease incidence over several years, Glassy-winged Sharpshooter may cause an exponential increase in the spread of this disease—due to “vine to vine” transmission. This possibility warrants the vigilant removal of vines with Pierce’s Disease symptoms as soon as they are seen in all vineyards subject to influxes of Glassy-winged Sharpshooter. Systemic insecticides (such as Imidacloprid) approved for grapevines are currently the most effective materials for control of Glassy-winged Sharpshooter in citrus orchards and vineyards.
With Napa growers’ efforts toward more sustainable farming practices and organic methods, establishment of Glassy-winged Sharpshooter here would be disastrous. Even with the abandonment of those progressive programs in favor of necessary pesticide applications, established Glassy-winged Sharpshooters would be impossible to eradicate, and vine losses from Pierce’s Disease would be huge. The repercussions would be felt throughout the Napa Valley as job losses in the vineyard, winery and tourism industries.