EVIDENCE OF DENSITY-DEPENDENT OVIPOSITION BEHAVIOR BY LISTRONOTUS-BONARIENSIS (COLEOPTERA, CURCULIONIDAE) IN CANTERBURY PASTURE

Argentine stem weevil (Listronotus bonariensis) (Kuschel) is a pasture pest of Lolium spp. in New Zealand and is the target of a classical b iological control programme using the parasitoid Microctonus hyperodae Loan. However, laboratory experiments have indicated a strong density -dependent egg laying response which may have important implications f or the biological control effort. Two field experiments were conducted using a manipulated range of weevil densities within 2.0-2.4 m(-2) fi eld enclosures and oviposition activity measured by weekly sampling of grass tillers within the enclosures. In both experiments, density had no effect on general oviposition behaviour although the percentage of tillers found to be infested with eggs increased significantly at hig her densities. In experiment 1, L. bonariensis exhibited strong densit y-dependent oviposition with a significant decline in eggs/tiller/fema le with increasing adult density. An initial population of five L. bon ariensis m(-2) oviposited 32 times more eggs/tiller/female than 200 L. bonariensis m(-2). By comparison, density-dependent oviposition was a bsent in experiment 2 with eggs/tiller/female comparable for all densi ties. The dramatic difference between the two experiments in the expre ssion of density-dependent oviposition was attributed to prevailing sp ring temperatures during the sampling interval, which in the second ex periment were much cooler and delayed the onset of egg laying. It is p robable that in experiment 2, oviposition was not regulated by density -dependent cues but limited by temperature and age related mortality i n the adult population. The mechanism for density-dependent ovipositio n, its occurrence in the field and impact on the effectiveness of the parasitoid M. hyperodae are discussed.