Egg Parasitoids of Homalodisca coagulata (Homoptera: Cicadellidae)

Abstract

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 Giraultwere 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 Earlier studies in Monticello, Florida (R. F. M., unpublished data) estimated average parasitism of H. coagulata eggs there at ca. 80%.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