Introduction: Host-plant selection by phytophagous insects highly depends on plant chemistry, from volatiles influencing distance recognition to surface and internal metabolites for contact recognition.The balance between simulant and deterrent metabolites finally determines why certain plants are accepted while others are rejected. In the present study, we try to understand the chemical bases of host plant rejection in a pollinivorous insect, the pollen beetle. The pollen beetle is a major pest of oilseed rape (Brassica napus), for which no source of resistance has been identified in B. napus cultivars. Previous studies have shown that a related species, the white mustard (Sinapis alba), has variable and high levels of resistance to this insect. However, the mechanisms underlying this rejection were still unknown. 

Methodology: To investigate the chemical traits responsible for this rejection, a bioguided fractionation approach of internal bud compounds was performed using artificial substrate feeding assays. Untargeted high-resolution metabolomics was then performed on active fractions to progress in the identification of compounds responsible for the rejection. 

Results: We identified active fractions of different polarities involved in the rejection of white mustard. A common active fraction between the two resistant genotypes was identified and prioritized. Using untargeted high-resolution metabolomics three candidate ions responsible for the rejection of the S. alba to the pollen beetle were identified. Using MS/MS fragmentation, the structure of these ions were elucidated and revealed that all of them belong to the sphingosine family. 

Conclusion: Overall, our results provide a better understanding of the mechanisms underlying the rejection of S. alba by the pollen beetle and  offer promising perspectives for breeding programs aimed at introducing resistance into oilseed rape cultivars.