Agroecological farming, flowering phenology and the pollinator-herbivore-parasitoid nexus regulate non-crop plant reproduction

Agroecological farming uses crop and non-crop plant biodiversity to promote beneficial insects supplying pollination and biocontrol services to crops. Non-crop plants (sown or weeds) are integral to supporting these beneficial insect species interactions. How the uplift of biotic complexity by agroecological management (crop diversification, ecological infrastructure) influences mutualistic and antagonistic insect interactions regulating the reproduction of non-crop plants remains less understood. 

Using a pesticide-free farm-scale (125 ha) agroecological experiment, we tested how the individual reproduction of pollinator-dependent, non-crop plant species with different flowering phenology (Cyanus segetum, Centaurea jacea) and their mutualistic (pollinator) and antagonistic (seed herbivore–parasitoid) insect interactions were affected by agroecological practices. 

Seed set and species interactions of replicate C. segetum and C. jacea randomly introduced to field margins was correlated with floral resource heterogeneity at focal plant (e.g., flower display size), local community (floral richness/abundance driven by sown wildflower or grass margins), and local landscape (crop diversification, area of semi-natural habitat or mass flowering crops) scales.  

At the seasonal peak of non-crop floral diversity and abundance, antagonistic interactions weakly regulated C. segetum seed set with gains from pollinator activity predominating. Conversely, C. jacea, which flowered past the peak of non-crop floral diversity/abundance benefited from the promotion of seed herbivore parasitism and pollinator activity by the local landscape cover of semi-natural habitat and mass flowering crops.

Synthesis and applications. Agroecological management produced spatial and-temporal gradients in crop and non-crop floral resources that interacted to modify pollinator or seed herbivore-parasitoid interactions and seed set of Cyanus segetum and Centaurea jacea plants. The degree of phenological overlap between C. segetum and C. jacea flowering and floral resources in the local community or landscape dictated the type and level of exposure to insect interactions influencing reproduction. Design of agroecological practices to deliver pollination and biocontrol services must consider how effects will vary with species traits and the ensemble of mutualistic (pollination) and antagonistic (herbivory, parasitism) interactions governing non-crop plant reproduction. Agroecological management supporting beneficial insect interactions may feedback to help restore functional non-crop plant populations and associated biodiversity, potentially reducing the frequency of management interventions (e.g., re-sowing wildflower strips).