The role of host species, age and defensive behaviour on ovipositional decisions in a solitary specialist and gregarious generalist parasitoid (Cotesiaspecies)

The main objective of this study was to determine the extent to which host acceptance behaviour as related to host species, age, and defensive behaviour might explain the differences in host use that exist between two congeneric and sympatric species of parasitic wasps.Cotesia glomerata(L.) (Hymenoptera: Braconidae) is gregarious and generalist on several species of Pieridae, whereasC. rubecula(Marshall) is solitary and specific toPieris rapae(L.).Cotesiaspecies differed in their responses to host species (P. brassicae (L.),P. napi(L.) andP. rapae) and developmental stage (early and late 1st, 2nd and 3rd instars). In no‐choice tests, host acceptance byC. rubeculawas higher forP. rapaeand females did not distinguish among the 6 host ages. In contrast, when foraging forP. brassicaeandP. napi, C. rubeculafemales more readily attacked early first instar.Cotesia glomeratashowed a higher degree of behavioural plasticity towards acceptance ofPierishost species and host age than didC. rubecula. Cotesia glomeratafemales parasitized the threePierisspecies and showed higher acceptance of first and second instars over third instar. Oviposition success was also influenced by host defensive behaviour. The frequency and the effectiveness of defensive behaviour rose with increasing age of the host,P. brassicaebeing the most aggressivePierisspecies. Furthermore, the mean duration ofC. glomerataoviposition was significantly reduced by the defensive reactions ofP. brassicae, which would likely affect parasitoid fitness as oviposition time is positively correlated to clutch size inC. glomerata.Acceptance frequencies corresponded well to field reports ofPieris‐Cotesiaassociations and to patterns of parasitoid larval performance, suggesting that the acceptance phase might be used as a reliable indicator ofCotesiahost‐specificity.