Larger olive fruit size reduces the efficiency of Psyttalia concolor, as a parasitoid of the olive fruit fly

The larval parasitoid, Psyttalia concolor (Szépligeti), has been released for biological control of the olive fruit fly, Bactrocera oleae (Rossi), in California. The effect of olive (Olea europaea L.) fruit size on parasitism efficiency was quantified within one cultivar (Sevillano) and across four different sized cultivars (in order of decreasing size: Sevillano, Ascolano, Manzanillo, and Mission). Parasitism was examined under two different host distributions: (a) variable distribution in which host density increased with fruit size; and (b) uniform distribution in which host density was similar across all fruit sizes. Regardless of host distribution and cultivar, parasitism by P. concolor was consistently higher on smaller fruit. Field cage studies also showed that olive fruit fly parasitism by P. concolor was lower within fruit of the largest olive cultivar (Sevillano) compared to fruit of the smallest cultivar (Mission). Results suggest larval B. oleae were protected in large fruit due to the relatively short ovipositor of P. concolor. By rearing P. concolor on the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), adult females had a larger body size and ovipositor length than when female wasps were reared on B. oleae. In turn, P. concolor females with longer ovipositors had higher levels of parasitism in similar sized olive fruit. We discuss the potential implications of these findings for biological control of B. oleae.