Environmental context and herbivore traits mediate the strength of associational effects in a meta-analysis of crop diversity.
Crop diversification offers a promising solution to meet expanding global food demands while maintaining ecosystem services. Diversification strategies that use mixed planting to reduce pest damage (e.g. intercropping), termed 'associational effects' (AE) in the ecological literature, can decrease (associational resistance) or increase (associational susceptibility) herbivore abundance on a focal plant. While application of AE to agroecosystems typically reduces pest abundance, the range of outcomes varies widely. We conducted a meta-analysis using 272 estimates of insect herbivore abundance on crops neighbored by a conspecific or heterospecific from 44 studies undertaken on six continents. We focus on four agricultural crops well represented from sites across the globe to test hypotheses related to understanding how herbivore traits (diet breadth, feeding guild, origin), plant traits (crop type, phylogenetic distance to neighbour) and environmental context (climate, experimental design) contribute to variation in the outcomes of AE. Overall, bicultures provided a strong reduction of insect abundance on the focal crop. Climate and interactions between herbivore traits, particularly diet breadth and origin, and plant traits or environmental context mediated the strength of AE. Bicultures provided the strongest reductions in insect abundance at low latitudes, and this effect decreased at higher latitudes but only for insects with certain traits. Abundance of generalist herbivores and globally distributed pests tended to be most strongly negatively affected by bicultures, under certain contexts, whereas specialist herbivores and native pests were less affected by neighbours. Synthesis and application. This meta-analysis highlights that crop diversification schemes have an overall strongly beneficial effect of reducing pest abundance. However, there was also variability in the outcomes that is determined in part by the interactive effects of herbivore traits and environmental context. The results provide guidance for incorporating beneficial ecological interactions into integrated pest management strategies.