Effects of parasitoid spatial heterogeneity, sex ratio and mutual interference on the interaction between the olive bark beetle Phloeotribus scarahaeoides (Col., Scolytidae) and the pteromalid parasitoid Cheiropachus quadrum (Hym., Pteromalidae)

Abstract

AbstractInformation on parasitoid sex ratio, mutual interference and spatial heterogeneity, obtained from both field and laboratory experiments, was integrated into a model of the interaction between the solitary pteromalid, Cheiropachus quadrum, and the olive bark beetle pest, Phloeotribus scarahaeoides. Parasitism was predominately aggregated in its distribution within the beetle breeding logs, while percentage parasitism ranged from spatially random to spatially density dependent between logs. The effects of spatial density dependence in the model was, however, overshadowed by the effects of both parasitoid mutual interference and competition between female beetles for oviposition ‘space’. Searching efficiency (area of discovery) declined with increasing adult parasitoid density as a result of mutual interference. This density dependent 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 density‐dependent changes in the secondary sex ratio had very little effect on either stability or equilibrium levels. The model predicts that whilst C. quadrum has the potential to strongly regulate beetle populations, its contribution in practice is only to suppress P. scarahaeoides populations by a mere 20%, which is insufficient to reduce the beetle numbers to non‐damaging levels. A major constraint on the effectiveness of the parasitoid appears to be mutual interference, without which the emerging beetle population could be reduced by about 70%.