Seedling resistance and chemical defenses against Psylliodes chrysocephala : the roles of seed age and sinapinic acid in Sinapis alba and Brassica napus

BACKGROUND The cabbage stem flea beetle, Psylliodes chrysocephala , is a major pest of winter oilseed rape ( Brassica napus ) in Europe. Adults feed on cotyledons and young leaves, threatening the establishment and vigor of the crop. With no insect‐resistant cultivars of B. napus currently available, farmers must rely primarily on synthetic insecticides for pest control. This study investigated the resistance of Brassica napus and Sinapis alba seedlings to adult P. chrysocephala feeding and explored the underlying chemical defense mechanisms. Seedlings from three B. napus accessions, including two with contrasting seed ages, and three S. alba accessions were evaluated for feeding damage in controlled laboratory conditions, while a subset of two B. napus and two S. alba accessions was further evaluated in semi‐field assays. Central metabolomic and glucosinolate profiling was conducted to identify resistance‐linked compounds.

RESULTS Under controlled conditions, S. alba seedlings exhibited substantially less feeding damage compared to a standard B. napus cultivar under controlled conditions. Notably, older B. napus seeds stored for 6 or 9 years produced seedlings with significantly reduced susceptibility to herbivory, coinciding with changes in central and specialized metabolite profiles, compared to freshly harvested seeds (younger than one year). Glucosinolate analyses revealed species‐specific profiles, while dual‐choice bioassays identified sinapinic acid as a feeding deterrent to P. chrysocephala adults.

CONCLUSION We successfully identified variation in cotyledon‐stage resistance to P. chrysocephala among Brassicaceae species. These findings highlight the potential of S. alba as a source of resistance traits and suggest that seed physiology influences seedling resistance, offering new avenues for breeding insect‐resistant oilseed crops, which are needed for integrated pest management. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.