How to Manage Pests
UC Pest Management Guidelines
Sharpshooters are members of the leafhopper family Cicadellidae. They have increased in importance over the last decade as a result of the introduction of the glassy-winged sharpshooter, which has been associated with increases in plant diseases caused by the bacterium Xylella fastidiosa that these sharpshooters vector. One such disease is alfalfa dwarf.
Historically, the green sharpshooter has been one of the two most important vectors of alfalfa dwarf in California. It is about 0.25 to 0.36 inch (6-9 mm) long and usually bright green (spring and summer) in color. Nymphs are brownish. In the eastern San Joaquin Valley, adults of the fall generation that overwinter are usually a dull brown color. The green sharpshooter usually has three generations per year in the Central Valley and requires grasses for breeding, favoring watergrass, bermudagrass, and fescues. It also feeds and reproduces on sedges. Like all sharpshooters, it prefers succulent plants. For this reason, it is most frequently found on perennial grasses in areas that are regularly irrigated.
The red-headed sharpshooter is the other sharpshooter of historical importance as a vector of alfalfa dwarf. It is about 0.25 inch (6 mm) long and similar in appearance to the green sharpshooter except that it has reddish coloration on the front tip of its head. The red-headed sharpshooter usually has four generations per year in the Central Valley and strongly prefers to breed on bermudagrass. It tolerates slightly drier conditions than the green sharpshooter.
The glassy-winged sharpshooter is a large leafhopper, about 0.5 inch (13 mm) long, that appears dark brown to black when viewed from the top or side. The abdomen is whitish or yellow, and the head is brown to black and covered with numerous ivory to yellowish spots. The glassy-winged sharpshooter has two generations per year, with peak adult populations in July and August. It feeds on over 150 genera of plants, but prefers feeding on crops such as citrus, grapes, and ornamentals. Eradication or areawide management programs are currently underway for this pest wherever it is found in California. There is currently no evidence that the glassy-winged sharpshooter breeds in alfalfa or spreads alfalfa dwarf.
The blue-green sharpshooter is about 0.25 inch (6 mm) long with blue wings. It is primarily found in riparian vegetation in cooler coastal regions and the foothills of the Sierra, where it spreads Pierce's disease of grapes but has not been documented as a significant vector of alfalfa dwarf disease.
Sharpshooters feed on a wide range of host plants by inserting their proboscis into the xylem tissue and extracting xylem sap. They are vectors of the bacterium Xylella fastidiosa, which causes alfalfa dwarf disease, but only the green and red-headed sharpshooters are known to spread this disease in alfalfa. Other diseases in California caused by this bacterium include Pierce's disease of grapes and almond leaf scorch. Sharpshooters acquire the bacterium while feeding on an infected host plant and spread it to noninfested host plants through subsequent feeding. Sharpshooters continue to be able to transmit the bacterium to plants until they molt. Feeding by the leafhoppers themselves causes little or no damage.
The primary symptom of alfalfa dwarf disease is stunted regrowth after cutting. The stunting may not be apparent for many months after initial infection. Leaflets on affected plants are smaller, often a slightly darker (bluish) color but not distorted, mottled, or yellow. The taproot is normal sized, but slicing it diagonally or horizontally down the root reveals abnormally yellowish colored wood with fine dark streaks of dead tissue. In recently infected plants, the yellowing is mostly in a ring beginning under the bark, with a normal white-colored cylinder of tissue in the center. The inner bark is not discolored, and there are no large brown or yellow patches as is the case with bacterial wilt caused by Clavibacter insodiosum. Dwarf disease progressively worsens over 1 to 2 years after first symptoms and eventually kills the plant.
Alfalfa dwarf has rarely been reported since the 1950s and is primarily distributed only in southern California and from Madera County south in the San Joaquin Valley. It is currently unknown what effect the introduction of the glassy-winged sharpshooter will have on disease incidence in the San Joaquin Valley.
Sharpshooter management in alfalfa depends on the species of pest that is present, and the potential for developing alfalfa dwarf. In the absence of the bacterium Xylella fastidiosa, there is no need to manage sharpshooters. If the bacterium is present, green and red-headed sharpshooters can be reduced by removing weed hosts they require for breeding. The best approach is to prevent grass weeds from establishing when alfalfa fields are planted. Grasses along field margins that receive irrigation water also need to be controlled.
If glassy-winged sharpshooters are found in areas of the state not previously known to be infested, notify the county agricultural commissioner or cooperative extension personnel. In many areas, especially major grape-growing regions, where glassy-winged sharpshooters are known to occur, area-wide management programs are in place, and this pest is being controlled with biological control or insecticide use in neighboring crops where its populations tend to build up.
Biological control is important for management of the glassy-winged sharpshooter. Egg parasitic wasps (family Mymaridae) in the genus Gonatocerus are commonly found wherever glassy-winged sharpshooter occurs in California and are most effective during summer. Parasitized glassy-winged sharpshooter eggs can be recognized by a small, round hole chewed through one end of the egg by the emerging adult wasp. Information is currently being developed on the role of biological control in the natural regulation of green, blue-green, and red-headed sharpshooters.
Control green and red-headed sharpshooters by removing grass weeds from within the field and along ditches, ponds or roads. Green and red-headed sharpshooters require grasses such as bermudagrass, watergrass, cultivated fescues and perennial ryegrass to breed. Annual grass weeds or cover in orchards and vineyards do not seem to develop significant populations of sharpshooters if weeds are removed at least annually. Cultural controls for the glassy-winged sharpshooter have not been developed.
Biological and cultural controls (i.e., weed removal) are acceptable for use in an organically certified crop.
Treatments of insecticides to alfalfa for sharpshooters are not recommended.
UC IPM Pest Management Guidelines:
UC ANR Publication 3430
L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County