BACKGROUND: The cabbage stem flea beetle (Psylliodes chrysocephala) is a key pest of winter oilseed rape (OSR, Brassicanapus); larval stages develop by feeding on petioles and stems during winter. Larval infestation may be influenced by planttraits and cropping systems. This study investigates the effects of OSR genotype and cropping system on larval infestationthrough specific ecophysiological traits and their interactions. In a field trial with four replicates, three OSR varieties(Mambo, Feliciano, and Angelico) were cultivated as monocrops or were sown with faba bean (Vicia faba) companion plants.Ecophysiological traits (morphological and metabolic) and P. chrysocephala larval infestation were assessed using a two-phaseanalysis combining direct relationship analysis and a regularized structural equation model.RESULTS: The varietal factor had a greater influence on larval infestation than companion planting in this experiment. Larvalinfestation correlated positively with plant fresh weight, glucoraphanin, and butyl glucosinolate (butyl-GLS), and negativelywith glucobrassicanapin concentration. The ecophysiological traits of Angelico, with lower concentration of glucoraphaninand butyl-GLS, explained its moderate decrease in larval infestation compared to Mambo. The ecophysiological traits of Feli-ciano, with a higher concentration of glucobrassicanapin and lower concentration of glucoraphanin and butyl-GLS, partiallyexplained the resistance effect of this variety compared to Mambo.Contrary to expectations, companion planting slightly increased larval infestation, particularly in Mambo. This may be due togreater plant biomass under companion planting conditions rather than a direct effect of the companion plant on infestationrates. However, OSR grown with companion plants expressed greater concentration of glucobrassicanapin and a reduction inglucoraphanin, which may result in an indirect negative effect on the larval infestation.CONCLUSION: The variation in P. chrysocephala larval infestation in OSR plants may be partly explained by an interplay of spe-cific concentrations of GLSs and morphological traits such as plant biomass.