Crop cultivar affects performance of herbivore enemies and may trigger enhanced pest control by coaction of different parasitoid species

Pest control by naturally occurring predators and parasitoids is an ecosystem service that may benefit from greater diversity of the pest's natural enemies. Co-occurrence of different enemy species, which act as service providers, can increase or stabilize pest control. Simultaneous cultivar diversity in varietal mixtures might foster such service providers, and the diverse cultivars might finally profit from occurrence of different natural enemies. Here, we tested how the ecosystem service pest control may be influenced by different crop cultivars and by the action of different parasitoid species. We investigated naturally occurring control of an apple pest, the apple blossom weevil Anthonomus pomorum (Curculionidae), by the parasitoid wasp species Scambus pomorum (Ichneumonidae) and Bracon variator (Braconidae) on five different cultivars (genotypes) of domestic apple Malus domestica in an organic orchard ecosystem. We quantified parasitism of the apple blossom weevil on the apple cultivars at three different times during the pest's development period. We evaluated whether apple cultivar and pest developmental status lead to comparable or to divergent parasitism patterns by different wasp species. We further assessed size and sex ratio of the wasps to estimate their performance and biocontrol potential on different cultivars. We found that pest parasitism was increased and homogenized across cultivars by coaction of the two wasp species, which belong to the same guild and contributed differently to parasitism - (i) with progressing time and (ii) on the different cultivars. Furthermore, female size differed significantly between cultivars for both wasp species, and it followed the preference-performance hypothesis for S. pomorum, where it was positively related to parasitism. Repeated sampling revealed that only the last sampling (shortly before adult weevil emergence) provides a complete estimate of effective parasitism by both wasp species. In conclusion, intraspecific variation among crop cultivars can result in divergent performance of parasitoids, and it may trigger enhanced pest control through different parasitoid species. The coaction of different species may thus contribute to maximization and stabilization of pest control in heterogeneous cropping systems.