Does the “Enemies Hypothesis” operate by enhancing natural enemy evenness?

Root (1973) observed fewer pests in weedy Brassica oleracea plantings, and suggested this could be because greater plant diversity bolstered resources for natural enemies strengthened herbivore suppression (i.e., the Enemies Hypothesis) and/or disrupted host-location by specialist herbivores Resource Concentration Hypothesis). These proposed mechanisms have been exceptionally influential efforts to manage agroecosystems promote conservation biological control, yet their relative importance remains unclear. Because direct (plant-driven) indirect (enemy-mediated) drivers of pressure are difficult isolate experimentally, we compiled a multi-year observational dataset tracking arthropod pest predator densities B. plantings on over 50 farms varying weed cover within fields as well habitat surrounding landscapes. We then used structural equation models evaluate interaction links consistent with Hypotheses, while also considering landscape context enemy community structure. predicted that non-crop would limit indirectly supporting evenly distributed communities enemies. Our results revealed vegetation indeed benefitted evenness thus correlated enhanced densities, but instead limiting herbivores, seemed track both aphids caterpillars. Landscape had no effect evenness, appeared benefit density, having contrasting effects (positive) caterpillars (negative). found evidence negative between indicate an important role Hypothesis. Instead, plants across our network field sites, suggesting partial support More generally, suggest consideration predator-predator interactions local biodiversity help explain inconsistent relationships suppression.