EFFECTS OF PARASITOID SPATIAL HETEROGENEITY, SEX-RATIO AND MUTUAL INTERFERENCE ON THE INTERACTION BETWEEN THE OLIVE BARK BEETLE PHLOEOTRIBUS SCARABAEOIDES (COL., SCOLYTIDAE) AND THE PTEROMALID PARASITOID CHEIROPACHUS-QUADRUM (HYM., PTEROMALIDAE)

Information on parasitoid sex ratio, mutual interference and spatial h eterogeneity, obtained from both field and laboratory experiments, was integrated into a model of the interaction between the solitary ptero malid, Cheiropachus quadrum, and the olive bark beetle pest, Phloeotri bus scarabaeoides. Parasitism was predominately aggregated in its dist ribution within the beetle breeding logs, while percentage parasitism ranged from spatially random to spatially density dependent between lo gs. The effects of spatial density dependence in the model was, howeve r, overshadowed by the effects of both parasitoid mutual interference and competition between female beetles for oviposition 'space'. Search ing efficiency (area of discovery) declined with increasing adult para sitoid density as a result of mutual interference. This density depend ent change in searching efficiency had a strong stabilizing influence on the parasitoid-host interaction. Crowding of adult parasitoids also led to a decline in the proportion of female progeny emerging. Such d ensity-dependent changes in the secondary sex ratio had very little ef fect on either stability or equilibrium levels. The model predicts tha t whilst C. quadrum has the potential to strongly regulate beetle popu lations, its contribution in practice is only to suppress P. scarabaeo ides populations by a mere 20%, which is insufficient to reduce the be etle numbers to non-damaging levels. A major constraint on the effecti veness of the parasitoid appears to be mutual interference, without wh ich the emerging beetle population could be reduced by about 70%.