INTERSPECIFIC COMPETITION BETWEEN GONATOCERUS ASHMEADI, G. TRIGUTTATUS, AND G. FASCIATUS FOR GLASSY-WINGED SHARPSHOOTER EGG MASSES Project Leader:

Interspecific competition between G. ashmeadi, G. triguttatus and G. fasciatus for GWSS egg masses was investigated in the laboratory using three experimental designs. Overall parasitism by G. ashmeadi was consistently higher (up to 76.0%) compared with G. triguttatus and G. fasciatus, for all three experimental designs. Exposing females to individual egg ages (1, 3 or 5 days) showed that G. ashmeadi parasitized a significantly higher (48.1%) proportion of GWSS eggs compared with G. triguttatus, whereas when females were exposed to all egg ages simultaneously, parasitism by G. ashmeadi and G. triguttatus was equivalent. Results from visual observations showed that G. triguttatus allocated a significantly higher (up to 36.4% and 22.9%, respectively) proportion of time to resting/grooming, compared with G. ashmeadi and G. fasciatus. Gonatocerus fasciatus spent a greater (19.6% and 9.6%, respectively) proportion of time off leaves with GWSS egg masses compared with G. ashmeadi and G. triguttatus. Gonatocerus triguttatus parasitized a significantly higher proportion (25%) of GWSS eggs compared with G. fasciatus, whereas time allocated to oviposition by each species was equivalent. Gonatocerus ashmeadi and G. triguttatus demonstrated aggressive behavior towards congeneric competitors for GWSS egg masses, whereas no aggressive behaviors were observed for G. fasciatus. The implications of these results for interspecific competition and biological control of GWSS by these three mymarid parasitoids are discussed. INTRODUCTION Gonatocerus ashmeadi has been resident in California since 1978 and genetic studies indicate it is native to the southeast U.S.A. and probably accompanied H. coagulata from its home range in the southeast U.S.A. Gonatocerus triguttatus has been imported from eastern Texas and released in California since 2001, while G. fasciatus was liberated in 2002 (CDFA, 2003). All three Gonatocerus species are mymarid parasitoids that attack GWSS eggs. The introduction of more than a single natural enemy to control a target pest as part of a classical biological control program may induce interspecific competition and result in either competitive exclusion or coexistence (Zwolfer, 1971; Myers et al., 1989). Coexistence of natural enemies can potentially lead to the disruption of biological control because interspecific competition can result in lower densities of the more efficient parasitoid which causes pest populations to be higher than what would occur with the more efficient species acting alone on the target population (Briggs, 1993; Rosenheim et al, 1995). A better understanding of the competitive interactions between G. ashmeadi, G. triguttatus and G. fasciatus for GWSS egg masses may provide insight into predicting and interpreting field outcomes following the establishment and proliferation of G. triguttatus and G. fasciatus in California, as they compete with the already well established and widely distributed G. ashmeadi. Irvin and Hoddle (2002) demonstrated that competition outcomes from laboratory studies can be contradictory and results greatly varied depending on the experimental designs used and the questions being addressed. Therefore, the following investigations were conducted to investigate which Gonatocerus parasitoid species is most competitive using three different experimental designs that varied GWSS egg age and egg density available for attack. OBJECTIVES Investigate interspecific competition between G. ashmeadi, G. triguttatus and G. fasciatus for GWSS egg masses of varying age and density. RESULTS Three experimental designs were used: the first involved exposing approximately 30 GWSS eggs of known age to one mated female G. ashmeadi, G. triguttatus and G. fasciatus (~24 hrs of age) for 24 hours in a 3 inch ventilated vial cage at 25C. This was repeated for eggs one, three and five days of age. The second experiment involved the above procedure, but exposed approximately 15 GWSS eggs of each age category (total of ~ 45 eggs), simultaneously to one mated female of each species. The third experiment involved exposing one egg mass (3-6 eggs and 24-48 hours of age) simultaneously to one mated female of each species searching together for one hour in a single 2 inch Petri-dish lined with moist filter paper. Visual observations for aggressive behavior were made every 5 minutes during the third experiment and behaviors at the