The effects of plant species richness on the function and stability of ecosystems have been an area of focus in recent decades. Arthropod community is one of the most important components in agroecosystems and can provide multiple ecosystem services, including biocontrol and pollination. In particular, species composition and biocontrol function can be sensitive to changes in plant species richness. Here, we designed 50 plots with five levels of plant species richness to examine arthropod distribution and composition over 4 yr. Arthropod richness was found to be positively correlated with plant species richness. High plant species richness can enhance the temporal stability of the arthropod community but can also lead to a decline in the population stability of some species. The species richness and biomass of environmentally friendly insects (EFI), such as honeybees, ants and flies, were found to be positively correlated with those of the natural enemies. As such, high levels of EFI could sustain food web robustness by serving as alternative prey/hosts for natural enemies. The mediation of EFI in the interaction between crops and pests has implications for successful biocontrol practices using natural enemies. Planting diverse plant species with a certain level of spatial turnover could benefit the biocontrol function of natural enemies and safeguard multiple ecosystem services.