REVIEW ARTICLE t Introduced parasitic wasps could control glassy-winged sharpshooter Leigh J. Pilkington Nicola A. Irvin Elizabeth A. Boyd Mark S. Hoddle Serguei V. Triapitsyn Bryan G. Carey Walker A. Jones David J.W. Morgan t The glassy-winged sharpshooter (GWSS) is an introduced pest that The glassy-winged sharp- spreads the bacterium Xylella fastidi- shooter, above, a native sharpshooter pest in the osa, which causes a variety of diseases southeastern United States such as Pierce’s disease in grapevines and northeastern Mexico, is now well established in and leaf scorch in oleanders. GWSS California, Hawaii, Tahiti and has been established in Southern Easter Island. Left, Pierce’s disease, which is transmitted California since about 1990 and has by bacterium vectored by the also successfully invaded French sharpshooter, is having an economic impact on California Polynesia, Hawaii and Easter Island. grape growers. Researchers from UC, the U.S. Depart- ment of Agriculture and the California Department of Food and Agriculture have introduced parasitic wasps for the biological control of GWSS. Four parasitoids from the southeastern United States have been released and appear to be establishing in Southern California. Parasitoids from Argentina are also being evaluated in quarantine but have not yet been released. he glassy-winged sharpshooter of coevolved natural enemies, coupled crease. Already the bacterium has been (GWSS) is a native insect of the with the irrigation of agricultural and found to cause two previously unrec- southeasternT United States that has be- urban areas in desert habitats that ognized diseases in olive trees and come extremely pestiferous in Southern would otherwise be too dry to support liquidambar, where it causes scorchlike California, where it became established GWSS populations (Hoddle 2004a). symptoms. The GWSS-Xylella combina- around 1990 (Sorensen and Gill 1996), The GWSS feeds exclusively on xy- tion has devastated or has the potential possibly imported as eggs on nurs- lem fluids, and its ability to spread the to devastate many agricultural crops, ery or private plant material. GWSS xylem-dwelling plant pathogenic bac- native vegetation, and urban ornamen- (Homalodisca coagulata Say [Homoptera: terium Xylella fastidiosa is at the core of tal and landscape plants. The potential Cicadellidae]) is now widely established its classification as a pest in California. effect of GWSS vectoring X. fastidiosa in several Southern California counties In California, the GWSS-X. fastidiosa into native California vegetation that (CDFA 2003) (fig. 1, page 224), and suc- combination causes Pierce’s disease previously had no association with the cessfully invaded Tahiti in 1999 (Cheou in grapes, almond leaf scorch, alfalfa bacterium may lead to new epidem- 2002), Hawaii in 2004 (Hoover 2004) dwarf disease and oleander leaf scorch. ics, and so is particularly worrisome. and Easter Island in 2005.* The insect’s In addition, the number and type of Consequently, the establishment of population growth in California has plant maladies caused by GWSS- GWSS in California has irrevocably been extraordinary, facilitated by a lack vectored X. fastidiosa is likely to in- changed the ecology of X. fastidiosa in http://CaliforniaAgriculture.ucop.edu • OCTOber–December 2005 223 *GWSS was first detected on Easter Island in August 2005. There appear to be no natural enemies attacking GWSS and it has been observed feeding on citrus, mangoes, guava and Bougainvillea (Sandra Ide, personal communication, October 2005). GWSS has shown strong invasive potential, having become established in a variety of places outside its native range, including California, French Polynesia, Easter Island and Hawaii. California’s wilderness, agricultural and in California are being pursued by urban landscapes. researchers at UC Riverside, the The presence of Xylella in Tahiti and U.S. Department of Agriculture’s Hawaii is currently unknown. It is pos- Agricultural Research Service (USDA- sible that the bacterium was introduced ARS) and the California Department to these South Pacific islands via the of Food and Agriculture (CDFA). importation of ornamental plants from Biological control involves identifying areas in the Americas where Xylella is natural enemies of the target pest. After native. These ornamental plants could the control agent is deemed safe and be silent Xylella reservoirs that harbor cleared from secure quarantine facilities, the bacteria without expressing disease they are released into the environment, Fig. 1. GWSS establishment in California symptoms. However, once a vector such where they utilize the pest as food and counties and infested parts of counties. as GWSS arrives, they could transmit thereby regulate its population growth the bacteria to susceptible host plants. and subsequent abundance. In the southeastern United States Economic impact of GWSS and northeastern Mexico, GWSS eggs The economic costs to California are parasitized by several species of attributed to GWSS are immense. For mymarid and trichogrammatid para- tance from human intervention). While example, oleander leaf scorch has sitic wasps. The most common natural G. ashmeadi may have established on caused damage in excess of an esti- enemies associated with GWSS eggs in early GWSS populations, it more likely mated $52 million on 2,000 miles of the southeastern United States are all established on the native smoke-tree freeway median plantings (Costa et al. mymarid wasps: Gonatocerus ashmeadi sharpshooter (Homalodisca liturata Ball) 2000). In 1998 and 1999 grape growers Girault, G. triguttatus Girault, G. mor- (Vickerman et al. 2004). in Riverside and San Diego counties ac- rilli Howard and G. fasciatus Girault While these two species were already crued estimated losses of $37.9 million (Triapitsyn and Phillips 2000). In the established, additional introductions of due to Pierce’s disease (Siebert 2001). In late 1990s, in an effort to use natural the same species may be more effective. 2000, researchers in cooperation with enemies to control GWSS popula- For example, the introduced southeast- affected grape and citrus growers spent tions in Southern California, UC and ern parasitoid populations may exhibit $6.9 million to apply spray applications CDFA imported these four parasitoids a greater ability to tolerate cool winter of Admire and Assail (acetamiprid) from southeastern states, cleared them temperatures and dry conditions, or be directly on GWSS habitat in Temecula through quarantine and introduced more aggressive reproductively. These and Bakersfield, in an effort to manage them into California urban and agricul- characteristics could help the parasit- populations migrating into vineyards. tural areas. oids become established and make them In 2002, primary producers incurred ad- G. triguttatus was first released more effective as control agents. The ditional economic costs resulting from in fall 2000 in Riverside, Ventura new parasitoid stock will likely increase GWSS containment activities such as and Kern counties. G. ashmeadi and the genetic variability of the already inspections of export nursery stock and G. morrilli releases started in 2001, and established G. morrilli and G. ashmeadi shipments of bulk grapes and citrus G. fasciatus in 2002. Anagrus epos (from populations, which could lead to im- from GWSS-infested counties (CDFA Minnesota) releases started in 2005. proved biological control. 2003). There are currently more than Over 1.2 million parasitoids have now CDFA has also made more than 70 state and federal research programs been released in 13 California counties 90 separate recoveries of egg masses studying GWSS or X. fastidiosa. where GWSS populations have been parasitized by G. triguttatus or G. fas- Traditional pesticide use and cultural reported. More than 1,900 releases ciatus at 23 sites in seven Californian practices to assist in reducing the popu- have been made at 373 release sites counties, suggesting that these control lations of GWSS may be augmented by covering agricultural, riparian and agents are becoming established. a long-term and cost-effective use of urban environments (see CDFA [2005] Biological control is a long-term biological control, an area heavily re- for release locations). strategy for the suppression of GWSS searched in California with a history of Two of these species (G. morrilli in areas where it has already become success. and G. ashmeadi) are already estab- established. To this end, CDFA has lished in California. G. morrilli is na- established two facilities based in Biological control strategies tive to California and G. ashmeadi is Riverside and Kern counties to mass Classical biological-control strate- self-introduced from the southeastern produce, release and monitor in- gies to reduce GWSS populations United States (with no direct assis- troduced biological control agents. 224 CALIFORNIA AGRICULTURE • VOLUME 59, NUMBER 4 t Parasitoids can aid in control of the glassy- G. ashmeadi G. triguttatus winged sharpshooter. Clockwise, from upper left: Gonatocerus ashmeadi is widespread in California and was most likely self-intro- duced from the southeastern United States and northeastern Mexico; G. triguttatus is a subtropical and tropical species introduced into California from Mexico and southeast- ern Texas; G. morrilli, native to Southern Cal- ifornia, is having its genetic pool augmented with conspecifics collected from other U.S. regions; G. fasciatus is native to the south- east and Midwest