Legume integration for biological control deserves a firmer scientific base

Diversification measures are widely recognized for their potential to enhance the resilience of agri-food production systems under global change. Nitrogen-fixing legumes commonly feature in diversification schemes, where they exert positive impacts on soil structure, nutrient retention, carbon sequestration or fertilizer reduction. Though legumes are premier tools for conservation agriculture, their impacts on (invertebrate, microbial) biological control are far less clear. As legumes secrete copious amounts of energy-rich (floral, extra-floral) nectar and provide alternative host or prey items for resident natural enemies, their deployment is expected to benefit biological control. Yet, its mechanistic basis is poorly understood and scientific underpinnings weak. In this Perspective piece, we collate data from the published literature and open-access databases on the extent to which different natural enemy guilds interact with 25 legume species that are commonly used as intercrops in the (sub)tropics. Our work unveils how natural enemies regularly forage on legumes, but the portrayal of those interaction linkages -or "biostructure" data- is profoundly incomplete and their ensuing implications for biological control are under-investigated. For instance, merely 4 (out of 167) parasitoids of the globally invasive fall armyworm Spodoptera frugiperda have been recorded in association with legumes. We hope that this Perspective piece will stimulate further thought, drive a new wave of biodiversity x ecosystem function research, and ultimately result in the design and implementation (or preservation) of more resilient, resource-frugal and pest-suppressive farming systems.