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

Abstract

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.

Interactions between large‐scale and local factors influence seed predation rates and seed loss

“…Thus, these findings suggest that less dense plants benefit by escaping host detection in LPRs whereas host plant detection seems similar regardless of density in the HPR. Although plant diversity might differ between LRR and HRR, evidence shows that most specialist herbivores are unlikely to be strongly influenced by heterospecific neighbors (Hahn & Cammarano, 2023 ). In sum, our results suggest that densities of host plant resources (seed heads per plant) for specialist herbivores, potentially in addition to other factors such as the overall population size, spatial distribution, and herbivore abundance are important for predicting herbivory levels.…”

Associational Effects of Desmodium Intercropping on Maize Resistance and Secondary Metabolism

“…There is growing evidence that increasing the biodiversity of agroecosystems through intercropping can increase overall agricultural yields (Chai et al 2021; Li et al 2021b; Li et al 2023) by increasing nutrient acquisition (Li et al 2014) and reducing pest pressure (Trenbath 1993;Poveda et al 2008). Diversi cation of agroecosystems through intercropping generally reduces herbivore and pathogen density, however the size and direction of these effects varies substantially between studies and systems (Letourneau et al 2011; Chad eld et al 2022; Hahn and Cammarano 2023). Intercropping could in uence resistance to pests and pathogens through at least several hypothesized mechanisms, such as numerical dilution of host plants (and host plant cues) (Tahvanainen and Root 1972;Root 1973), disruption of insect host-nding by non-host cues, increased accumulation of natural enemies like predators and parasitoids, or resistance-enhancing changes in the metabolism of the focal plant species (Poveda et al 2008; Barbosa et al 2009).…”

Diet specialization mediates drivers of Cucurbita herbivory in a semi-arid agroecosystem

“…Similarly, among natural enemies, predators often have wider diet breadths than parasitoids, which must evolve strategies to overcome host prey defenses and thus often specialize within a genus or family of prey ( Vorburger 2022 ). A recent global meta-analysis found that the decline of generalist herbivore abundance in response to crop resource dilution in bicultures was stronger than the decline in specialist herbivore abundance ( Hahn and Cammarano, 2023 ), and numerous other studies have observed variable responses to landscape-level resource concentration among herbivores and natural enemies with varying diet breadths ( Egerer et al, 2017 ; Perez-Alvarez et al, 2019 ). While general patterns emerge from a global perspective, regional variation in arthropod responses to local and landscape-level resource availability underscores the need for regional and crop-specific studies to understand herbivory risks within a given production system.…”

“…Further, studies of biocontrol by natural enemies in Ohio Cucurbita production found that predators were not influenced by the availability of alternative host plants and that control of target specialist herbivores was higher in sites with greater predator richness and in landscapes with more agriculture ( Phillips and Gardiner, 2016 ; Mabin et al, 2020 ). These and an additional three studies ( Lawrence and Bach 1989 ; Hooks et al, 1998 , and Frank and Liburd, 2005 ) included in a recent meta-analysis ( Hahn and Cammarano, 2023 ) all utilized additive polycultures to examine variation in the relative density of Cucurbita plants and test the Resource Concentration Hypothesis as a frequency-dependent process ( Kim and Underwood, 2015 ). Much less is known about how absolute density-dependent processes, derived from variation in local-level Cucurbita establishment and landscape-level Cucurbita production, influence herbivory.…”