Distribution and habitats: The blue-green sharpshooter (Graphocephala atropunctata) is considered to be the most important Pierce’s disease vector for coastal California, from San Diego in the south through Mendocino counties to the north. It is reported as occurring from Central America through British Columbia and is often abundant in coastal Oregon and Washington. In California, it can be found in the foothills of the Sierra Nevada mountains along streams or less abundantly in riparian (stream bank) vegetation along rivers in the Sacramento and San Joaquin Valleys. Along the foothills and mountains of the Pacific coast, the blue-green sharpshooter occurs mostly along streams or springs, the margins of forest openings and in a variety of ornamental landscapes.
Host plants: Perennial plants, including grapevines, are favored for feeding and reproduction. The list of plants on which it regularly feeds is enormous, but it favors some plant species over others, especially for laying eggs. The most common riparian plants on which it is found in California include grape, blackberry, elderberry, mugwort, stinging nettle, and mulefat. In ornamental landscapes in residential areas or parks, it favors roses, fuschia, ivy and a variety of ornamental shrubs or trees. Unlike other important Pierce’s disease vectors in California, the blue-green sharpshooter commonly occurs on commercial grapevines near riparian vegetation. Like other xylem-feeding insects, it prefers new growth on plants that are in a succulent condition. This is probably the major reason why it prefers riparian areas in California.
Life cycle: Adult blue-green sharpshooters are long-lived. There is usually only a single generation per year. A few adults may lay eggs a few weeks after they mature, resulting in a partial second generation, but most females require a period of cool temperatures to mature reproductively and do not lay eggs until the following spring. In the Sacramento Valley a higher percentage of first generation blue-green sharpshooters reproduce in the summer as well as the following spring. Many adults survive the winter, but not much is known of their behavior during winter.
Transmission of X. fastidiosa: Like other vectors, adult blue-green sharpshooters retain infectivity with X. fastidiosa for an indefinite period. Thus adults that acquire the bacterium during the autumn can introduce X. fastidiosa into plants during the following spring. The spatial pattern of Pierce’s disease in north coast California vineyards reflects the spring dispersal pattern of blue-green sharpshooter adults. To reduce the spread of Pierce’s disease near riparian source areas of blue-green sharpshooter, growers must be able to reduce the number of adults entering vineyards in the spring months (see Goodwin and Purcell, 1992). As mentioned, blue-green sharpshooters have been collected from many plant species, and there are undoubtedly many more not yet recorded. Surprisingly however, just removing the relatively few favored egg-laying hosts of blue-green sharpshooters has greatly reduced the populations of this insect in riparian vegetation in Napa and Sonoma Valleys.
For further details of the biology of the blue-green sharpshooter and its relation to Pierce’s disease, see DeLong and Severin, 1949; Frazier, 1966; Freitag and Frazier, 1946; Goodwin and Purcell, 1992; Hewitt et al., 1942; 1949; Purcell, 1975; 1976; 1979; 1990; 1993; 1994; Severin, 1949.