Spatiotemporal changes in landscape crop composition differently affect density and seasonal variability of pests, parasitoids and biological pest control in cabbage

Declines in functional biodiversity, caused by changing structure and composition of agricultural landscapes, endanger the provision and stability of important ecosystem services, such as biological pest control. Understanding how spatiotemporal landscape changes affect the dynamics of pests, natural enemies and their activity can help safeguard and improve pest control in crops. Here, we tested the influence of spatial and temporal differences in landscape composition of crops (landscape-wide crop rotation diversity and % pest host crops) on the density and seasonal variability of cabbage pests, parasitoids and rates of aphid parasitism and predation. We also tested whether low seasonal variability of pests and parasitoids translates into stable and high pest control, reduced crop damage and higher yields. We show that landscapes with diverse crop rotations increased aphid parasitism, but also leaf-chewing pest density and seasonal stability. Leaf-chewing pest density decreased with increasing % host crops from year to year, suggesting a dilution effect. However, parasitoid density increased in these landscapes. We found no relationship between predation or parasitism and crop damage or yield. Further, stable parasitoid populations led to more stable parasitism rates. These findings highlight that impacts of spatially and temporally changing landscapes are pest- and enemy-specific, and tradeoffs for landscape management may occur. Appropriately managing the diversity and composition of arable crops in farmed landscapes may help influence species occurrences, without converting fertile land to non-crop habitat.