Effective control of insect pests through intercropping of cabbage with barley and buckwheat

Barley intercropping is an effective strategy for managing multiple insect pests in cabbage fields. We hypothesized that combining barley with flowering plants can increase pest suppression by enhancing parasitism and predation effects in cabbage fields. This study aimed to evaluate whether the spatial arrangements of flowering plants in a cabbage–barley intercropping system can enhance pest control. We also assessed the roles of predation by ground-dwelling predators, such as carabid beetles, and flight disruption, using Pieris rapae crucivora Boisduval (Lepidoptera: Pieridae) as the target pest. Over two years, planting flowering plants (buckwheat, lacy phacelia, and coriander) around barley-intercropped cabbage showed inconsistent pest suppression compared with those around barley intercropping alone. However, a significant increase in multiple pest suppression and parasitism by two species of parasitoid wasps was observed when cabbage was intercropped with a mixture of barley and buckwheat. This suggests that the proximity of cabbage, barley, and flowering plants plays a key role in pest control. In planter experiments, the carabid beetle Chlaenius micans (Fabricius) (Coleoptera: Carabidae) was able to prey on larval P. rapae crucivora on cabbage plants. Additionally, the flight activity of P. rapae crucivora seemed to be disturbed by the intercropped barley in cabbage fields. These findings highlight the importance of both top-down and bottom-up effects in suppressing pest populations in cabbage–barley–buckwheat intercropping systems.