Environmental context and herbivore traits mediate the strength of associational effects in a meta-analysis of crop diversity

1. Crop diversification offers a promising solution to meet expanding global food demands while maintaining ecosystem services. Diversification strategies that employ 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. 2. 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 neighbor), and environmental context (climate, experimental design) contribute to variation in the outcomes of associational effects. 3. 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 associational effects. 4. 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 neighbors. 5. Synthesis and application. This meta-analysis highlights that variability in AE is determined in part by the interactive effects of herbivore traits and environmental context and provides guidance for incorporating beneficial ecological interactions into integrated pest management strategies.Data were processed for analysis using R. The R scripts for processing the data are made available here.Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: DEB-1901552Funding provided by: U.S. Department of AgricultureCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000199Award Number: FLA-ENY-006018Data for this meta-analysis were collected by extracting data from published primary research articles. No new data were collected for this study. Methods for the literature search, screening of papers, extration of response variables, moderators, and effect sizes are described in detail in the associated paper and supplemental materials