Aphids are among the most economically significant pests in cereal crops worldwide. Despite high interest in the natural control of aphids by applying natural enemies, intercropping and companion planting, the concurrent effects of the combinations of these methods due to their synergistic or antagonistic interactions remain largely unknown for both aphids and their host crops. Here we identify the relative effectiveness of simultaneous bottom–up and top–down factors in controlling one of the most economically important species of cereal aphids, the bird cherry-oat aphid Rhopalosiphum padi L. For this, we conducted microcosm experiments using a full-factorial design of three aphid-control treatments including predator presence (i.e., lacewing larvae of Chrysoperla carnea), host-plant intercropping (i.e., barley alone vs barley in combination with wheat and rye) and companion planting with an aphid-repellent plant (garlic), and estimated their direct, indirect and interactive effects on aphid density. Our results show strong simultaneous top–down control of the aphid population by predation and bottom–up control by both host-plant biomass and companion planting with garlic. The use of garlic as a companion plant for cereal crops in our study neither altered crop biomass nor suppressed the efficiency of aphid predator. Our findings suggest that the simultaneous application of aphid predator and companion planting with garlic holds promise as a potential strategy for the natural control of cereal aphid populations on grain crops, without generating related agroecosystem disservices, such as loss in crop production and deterioration of the natural enemies of pests. However, given the controlled lab conditions and limited timeframe of our study, further research is needed to confirm its effectiveness in field conditions to ensure its broader applicability in sustainable agricultural practices.
