THE ROLE OF HOST SPECIES, AGE AND DEFENSIVE BEHAVIOR ON OVIPOSITIONALDECISIONS IN A SOLITARY SPECIALIST AND GREGARIOUS GENERALIST PARASITOID (COTESIA SPECIES)

The main objective of this study was to determine the extent to which host acceptance behaviour as related to host species. age, and defensi ve behaviour might explain the differences in host use that exist betw een two congeneric and sympatric species of parasitic wasps. Cotesia g lomerata (L.) (Hymenoptera: Braconidae) is gregarious and generalist o n several species of Pieridae, whereas C. rubecula (Marshall) is solit ary and specific to Pieris rapae (L.). Cotesia species differed in the ir responses to host species (P. brassicae (L.), P. napi (L.) and P. r apae) and developmental stage (early and late 1st, 2nd and 3rd instars ). In no-choice tests, host acceptance by C. rubecula was higher for P . rapae and females did not distinguish among the 6 host ages. In cont rast, when foraging for P. brassicae and P. napi, C. rubecula females more readily attacked early first instar. Cotesia glomerata showed a h igher degree of behavioural plasticity towards acceptance of Pieris ho st species and host age than did C. rubecula. Cotesia glomerata female s parasitized the three Pieris species and showed higher acceptance of first and second instars over third instar. Oviposition success was a lso influenced by host defensive behaviour. The frequency and the effe ctiveness of defensive behaviour rose with increasing age of the host, P. brassicae being the most aggressive Pieris species. Furthermore, t he mean duration of C. glomerata oviposition was significantly reduced by the defensive reactions of P. brassicae, which would likely affect parasitoid fitness as oviposition time is positively correlated to cl utch size in C. glomerata. Acceptance frequencies corresponded well to field reports of Pieris-Cotesia associations and to patterns of paras itoid larval performance, suggesting that the acceptance phase might b e used as a reliable indicator of Cotesia host-specificity.