Phylogenetic relatedness of a focal plant to weed species and field crop species affects its seed production through insect-mediated trophic interactions

Crops and weeds coexist in complex multispecies communities, whose biotic and abiotic interactions can limit or benefit crop performance and production. Co-evolutionary processes mean that closely-related plant species support similar insect herbivore communities, including pests. Comparatively, less is known about how phylogenetic relatedness between crops and weeds affects multitrophic interactions involving insect antagonists and mutualists affecting plant performance. We experimentally investigated how seed production of focal mustard plants, Brassica juncea L., and multitrophic interactions with insects (herbivores-predators-pollinators) were affected by the phylogenetic relatedness to a) co-planted focal weed species (phylogenetically close: wild mustard, Sinapis arvensis L. or phylogenetically distant: cornflower, Cyanus segetum Hill) and b) the crop species in the surrounding field (phylogenetically close: winter oilseed rape - OSR Brassica napus napus L. or phylogenetically distant: winter wheat Triticum aestivum L.). Brassica juncea attained the highest seed weight when planted in association with C. segetum in wheat fields. Interactions with pollinators, herbivores and predators did not affect B. juncea seed weight, however. Herbivore abundance was higher on B. juncea planted in OSR fields than in wheat fields, while there was no effect of the co-planted focal weed species. However, predator abundance was unaffected by the co-planted focal weed species and the field crop species. Pollinator visits on B. juncea focal plants was significantly more abundant when co-planted with S. arvensis in wheat fields compared to other treatments. Our results suggest that plant phylogenetic relatedness modifies antagonistic and mutualistic interactions from the plant to the field scale, with implications for crop management. • Phylogenetic relatedness between weeds and field crops interacted to increase Brassica juncea seed weight. • Neither herbivores, predators nor pollinators influenced B. juncea seed production. • Herbivore abundance increased on B. juncea planted in the closely-related crop field. • Predator abundance was unaffected by the plant phylogenetic relatedness. • Pollinator visits increased on B. juncea planted with closely-related weed plants in wheat fields.