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The GWSS - A serious new PD vector for California Vineyards
by Phil A. Phillips, PhD, BCE, CCA
One thing California doesn't need is another pest in agriculture or the urban landscape. However, the glassy-winged sharpshooter, Homalodisca coagulata Say (Homoptera: Cicadellidae), fits the bill on both counts. Likely introduced around 1990 from the southeastern United States, where it is a known agricultural pest vectoring several bacterial diseases, this insect was first observed as a novelty on eucalyptus windbreaks in Ventura County lemon orchards near Santa Paula. By 1992 adults were observed on the stems of young lemon trees. In 1993 the first egg masses were observed in lemon tree foliage by this author. Adults were collected in the early spring of 1994 and sent to Jerry Davidson, entomologist with the Santa Barbara County agricultural commissioner's office. Formal identification was made by Dr. Ray Gill, CDFA. The glassy-winged sharpshooter (GWSS) is a large insect (~0.5 inch) whose general color is brown to black when viewed from the side or above. The underside of the abdomen is whitish. The upper aspect of the head and thorax are brown or black with numerous ivory to yellowish spots. These spots allow this sharpshooter to be easily distinguished from one of its close relatives in southern California, the native smoke tree sharpshooter (H. lacerta Fowler), which has pale, wavy lines instead of the spots.
Since the early 1990's, this insect has proceeded to expand its numbers and its range to where it can now be found in San Bernardino, Riverside, Orange, Los Angeles, Ventura, and southern Santa Barbara Counties. It has rapidly gone from novelty status to potentially a very serious pest. Sharpshooters are xylem feeders, generally accessing the water conductive xylem tissue of their host plant through the stem or major leaf veins using their strong stylet-like piercing mouth parts. As xylem feeders, sharpshooters as a group can be effective vectors of bacterial plant pathogens, particularly Xylella fastidiosa. This bacterium is the causal organism for Pierce's disease of grapes, phoney peach disease in the southern US, variegated chlorosis of citrus (Brazil), and most recently oleander leaf scorch in southern California. Once injected by the sharpshooter vector into the host plant's xylem tissues, this bacterium multiplies and produces a gel-like material that in combination with the multiplying bacteria blocks the water conductive xylem tissue. This initially causes die-back of leaves and shoots distal to the point of infection and eventually causes the entire plant (eg. grape vines) to collapse and die within a year or two (several years for mature citrus trees) as the infection becomes systemic.
Considering the GWSS's long association with Pierce's disease of grape in the southeastern US as a vector of the causal bacterium, this insect's eventual migration from southern California northward into the Central Valley and coastal viticulture regions is of great concern. Unlike the current sharpshooter vectors associated with Pierce's disease here in California (blue-green, green and red-headed), the GWSS is much larger, appears to fly much further and certainly in greater numbers into commercial agricultural plantings than our native sharpshooters. The ease with which GWSS moves into the middle of agricultural plantings extends the threat of PD infections from a primarily vineyard boarder problem to potentially a vineyard-wide problem, even on the largest plantings. Populations of GWSS have already moved into Temecula vineyards in northern San Diego County and have been associated there with new confirmed findings of PD in vineyards where PD was previously undocumented (unlike vineyards a few miles to the south which are closer to riparian areas and have had a long history of PD vectored by blue-green sharpshooters). Because of its wide host plant range, the GWSS is not confined to riparian areas. It can easily develop large populations on dooryard ash , eucalyptus, macadamia, or stone fruit trees. The large numbers this insect generates in crop or non-crop plantings increases the likelihood of bacterial transmission from even the smallest source. Linking our existing PD situation in California vineyards with the currently spreading populations of GWSS could spell a serious disaster- in-the -making for our grape industry.
GWSS has an extensive host range, attacking at least 73 species of plants in 35 different families. In Ventura County it has been observed breeding on native plants such as laurel sumac, tree tobacco, sycamore, and oak. It also attacks numerous ornamental hosts such as box wood, crape myrtle, philodendron, Chinese elm, ash, macadamia, apricot, birch, eucalyptus, hibiscus and many more. GWSS utilizes its broad host range to attain very large populations by the second generation during the summer. Its shear numbers in the urban environment have caused it to become a considerable nuisance to the urban and suburban communities .The copious amounts of liquid excreted during its feeding into the water conductive tissues of host plant stems of city park and backyard trees make sitting beneath such trees a very wet and miserable experience. Sidewalks and street edges are actually wet during the early morning hours form the previous night's feeding high in the canopy of host trees growing above these surfaces. In infested citrus orchards, the tree canopies take on a white-washed appearance by mid-summer due to the buildup of mineral salts left after the watery excretions have evaporated day after day for several months.
GWSS produces two generations a year in southern California. After a peak in adult activity during the winter months, oviposition begins in late winter and early spring (Feb./Mar.), peaking in May. Adults live about 2 months. They lay their small, sausage-shaped eggs side-by-side in masses averaging 10 to 11 eggs each. These eggs are laid just under the lower leaf epidermis of host plants. Egg masses can range in size from single eggs to masses containing as many as 27 eggs. Egg masses almost appear to be greenish water blisters on the undersides of the leaves. They are elongated with the entire mass being sausage-shaped itself with the individual eggs running transversely across the mass. From above, their location may be marked by the appearance of a yellowish or chlorotic elongated blotch. Nymphs hatch in about two weeks and proceed to feed into leaf petioles or small stems while they progress through four molts before becoming winged adults. A second peak in adult activity occurs in the summer during the months of July and August. Peak oviposition from these first generation adults occurs in August. After the eggs have hatched, the old egg mass blister appears as a tan to brown scar. Oviposition into laurel sumac and macadamia is about 2 to 3 times that of oviposition into citrus.
Only one biological control agent of any significance has been noted to date. A small egg parasite, Gonatocerus ashmeadi Girault (Hymenoptera: Mymaridae), attacks the GWSS egg masses starting in the spring. Its activity peaks in May, July, and again in early October, with the latter peak being the greatest when as much as 80-95% of the GWSS eggs can be parasitized. Parasitized eggs are evident by the small circular hole left by the emerged parasite at one end of the egg. Although parasitism rates have been quite high over the last two years, there have been ample numbers of over-wintering adult sharpshooters to produce problematic populations the following season. Another biological control agent, a predatory wasp, has been observed in a limited area of San Diego County. This wasp provisions its nests with adult GWSS which it has paralyzed.
In the future, should the GWSS become distributed throughout California grape growing regions, the glassy-winged sharpshooter may play a pivotal role in PD's rapid spread throughout California vineyards with potentially devastating results. More work on the biology, management and biological control (including foreign exploration for BC agents) of this important vector is needed in California.