3Department of Entomology
Louisiana Agricultural Experiment Station
Baton Rouge, LA 70803, U.S.A.

The glassy-winged sharpshooter, Homalodisca coagulata (Say), is native to the southeastern United States. It has existed in southern California since about 1990 (Sorensen & Gill 1996), and has become an economic problem as a vector of the pathogenic bacterium Xylella sp., that is the cause of leaf scorch disease in oleander. H. coagulata also feeds on citrus and many other plants.

In October 1995, the mymarid wasp Gonatocerus ashmeadi Girault was reared from egg clusters of H. coagulata collected on citrus leaves in Santa Paula, California (Triapitsyn & Phillips 1996). This finding prompted us to conduct a limited survey of egg parasitoids of H. coagulata in California and also in the southeastern United States to assess the feasibility of a biological control program against this pest. Prior to this study, published information concerning natural enemies of H. coagulata was limited to the work by Turner & Pollard (1959) in Georgia.

Egg masses of H. coagulata are rather conspicuous and thus easy to locate on the underside of leaves. The female leafhopper covers its eggs with a white, chalky material, which may have a bactericidal effect, and spreads this material over the eggs using the tarsi. Parasitized eggs turn black before parasitoids exit through characteristic emergence holes. To obtain parasitoids, leaves with H. coagulata egg masses were collected and held in plastic containers. Upon emergence, parasitoids were placed directly in 70% ethyl alcohol and later identified by the senior author.

In California (1996-1997), parasitized H. coagulata eggs were collected on various ornamental plants on the University of California, Riverside campus. Eggs collected early in spring were not parasitized. During summer, G. ashmeadi was found in all samples from southern California and in rather large numbers (up to 80% of eggs were parasitized in Riverside in the July 1997 samples). Three other species of Gonatocerus—G. capitatus Gahan, G. incomptus Huber and G. novifasciatus Girault—were collected in very small numbers, and only early in spring in Fillmore by University of California Cooperative Extension (Ventura Co.) researchers. Gonatocerus capitatus and G. novifasciatus have not been previously reared from eggs of H. coagulata whereas G. incomptus was a known parasitoid of this sharpshooter species in Georgia (Huber 1988).

Earlier studies in Monticello, Florida (R. F. M., unpublished data) estimated average parasitism of H. coagulata eggs there at ca. 80%. During July and August 1997, we collected egg masses of H. coagulata from crape myrtle (Lagerstroemia indica L.) and citrus trees grown at the University of Florida’s North Florida Research and Education Center. In July samples, more than 90% of ca. 300 emerged parasitoids were G. ashmeadi. One female was identified as G. morrilli (Howard), and the rest belonged to an undescribed Zagella sp. (Trichogrammatidae). In August samples, however, Zagella sp. was by far the dominant parasitoid of H. coagulata eggs. The genus Zagella has never been previously reported from a cicadellid host. The minute size of Zagella individuals relative to the large size of the egg of their host suggests that this species may be gregarious.

In urban Baton Rouge, Louisiana, sharpshooter egg parasitoid collecting was conducted during a five week period in April and May 1997. Adult H. coagulata were captured at a 250 watt mercury vapor light trap set on a white sheet at ground level. They were held overnight in a container with leaves of host plants and then placed on live host plants for oviposition. During the first two weeks of screening, sharpshooters were confined within mesh sleeves around branches of crape myrtle or elderberry (Sambucus canadensis L.). During the latter three weeks of screening, sharpshooters were placed in collapsible screen cages containing potted sunflowers (Helianthus sp.). Sharpshooters confined within mesh sleeves were held for approximately one week for oviposition. The sleeves were then removed and the egg masses exposed for an additional week. Egg masses were then harvested along with adjacent plant tissue, held individually in covered, one-ounce plastic cups and monitored daily for parasitoid emergence. Sharpshooters were added continually to the cages containing sunflowers. When multiple egg masses were visible on a sunflower plant, it was removed and placed in the yard adjacent to the cage. After one week of exposure, the egg masses were harvested and held in plastic cups for parasitoid emergence. Parasitization of H. coagulata eggs in Baton Rouge area was ca. 50%. The samples collected using the method described above revealed two species of Gonatocerus: G. ashmeadi (ca. 69% of the parasitoids collected) and G. fasciatus Girault (ca. 31%). The latter species was reported by Turner & Pollard (1959) from eggs of H. coagulata in Georgia but it has not been known from Louisiana or California (Huber 1988).

Although adult H. coagulata appear to prefer new growth and can feed on many hosts, females choose to oviposit onto a somewhat narrower range of plants that includes citrus, crape myrtle, Euonymus spp., okra (Abelmoschus esculentus [L.] Moench), passion vine (Passiflora spp.), sunflower, etc. Apparently, oleander is not a preferred host plant of H. coagulata for oviposition, probably due to different feeding requirements of the nymphs (Brodbeck at al. 1995). The fact that most of the damage to oleander is not due to direct feeding but is caused by a plant pathogen makes the prognosis for complete biological control against H. coagulata in southern California unlikely. Nevertheless, introduction of several additional species of egg parasitoids from the southeastern United States may be warranted. The obvious candidates are G. fasciatus and Zagella sp. If established, these parasitoids may enhance the overall natural control of H. coagulata in southern California. Partial suppression has already been achieved there during the summer months by the local species G. ashmeadi.

Material Examined: G. ashmeadi: CALIFORNIA. Riverside Co., Riverside: 18-VI-1997, J. Bethke, 6//, 2?? (on Passiflora sp.); 6-VII-1997, M. Gates, 8//, 8?? (on Passiflora sp.); 18-VII-1997, S. Triapitsyn, numerous //, ?? (on Annona cherimola Miller); 21-VII-1997, S. Triapitsyn, 5//, 7?? (on Cocculus sp.). Ventura Co.: Fillmore, 2-V-1996, P. Phillips, 4// (on citrus); Piru, 17-IX-1997, S. Triapitsyn, 1/ (on citrus). FLORIDA. Jefferson Co., Monticello: 2-VII-1997, S. Triapitsyn, numerous //, ?? (on crape myrtle); 20-30-VIII-1997, R. Mizell, 2?? (on crape myrtle). LOUISIANA. East Baton Rouge Co., Baton Rouge: J. Bossart: 23-30-IV-1997, 3//, 1? (on elderberry); 1-22-V-1997, 24//, 5?? (on sunflower). G. capitatus: CALIFORNIA. Fillmore, 4-V-1996, P. Phillips, 1/ (on citrus). G. fasciatus: LOUISIANA. Baton Rouge, J. Bossart: 23-30-IV-1997, 9//, 1? (on elderberry); 1-22-V-1997, 4//, 1? (on sunflower). G. incomptus: CALIFORNIA. Fillmore: 4-V-1996, P. Phillips, 2// (on citrus); 13-III-1997, J. Dyckes, 1/ (on Macademia sp.); 17-IV-1997, J. Dyckes, 1/ (on Platanus sp.). G. morrilli: FLORIDA. Monticello, 2-VII-1997, S. Triapitsyn, 1/ (on crape myrtle). G. novifasciatus: CALIFORNIA. Ventura Co., Bardsdale, 17-IV-1997, J. Dyckes, 2// (on Fraxinus sp.). Zagella sp.: FLORIDA. Monticello: 2-VII-1997, S. Triapitsyn, numerous //, ?? (on crape myrtle); 25-VII-1997, 27-VII-1997, 4-VIII-1997 and 20-30-VIII-1997, R. Mizell, numerous //, ?? (on crape myrtle, citrus and Betula sp.) [all in UCRC]. This is Florida Agricultural Experiment Station Journal Series No. R-06134.

Summary

A survey of egg parasitoids of the leafhopper Homalodisca coagulata was conducted in California, Florida and Louisiana. The mymarid wasp Gonatocerus ashmeadi was found to be the most common natural enemy in all locations sampled; G. fasciatus and the trichogrammatid Zagella sp. are recognized as potential biological control agents for introduction into southern California.

References Cited

Brodbeck, B., P. Andersen, and R. Mizell. 1995. Differential utilization of nutrients during development by the xylophagous leafhopper, Homalodisca coagulata. Entomol. Exp. and Appl. 75: 279-289.

Huber, J. T. 1988. The species groups of Gonatocerus Nees in North America with a revision of the sulphuripes and ater groups (Hymenoptera: Mymaridae). Mem. Entomol. Soc. Canada 141: 1-109.

Sorensen, J. T., and R. J. Gill. 1996. A range extension of Homalodisca coagulata (Say) (Hemiptera: Clypeorrhyncha: Cicadellidae) to southern California. Pan-Pacific Entomol. 72 (3): 160-161.

Triapitsyn, S. V., and P. A. Phillips. 1996. Egg parasitoid of glassy-winged sharpshooter. Citrograph 81 (9): 10.

Turner, W. F., and H. N. Pollard. 1959. Life histories and behavior of five insect vectors of phony peach disease. Tech. Bull. United States Dep. Agric. 1188, 28 pp.