Of glucosinolates and saponins - Pollen beetles can cope with general but not with specific plant defence compounds in crucifers

The pollen beetle (Brassicogethes aeneus) is one of the major insect pests of oilseed rape (Brassica napus L.; Brassicaceae) causing significant seed yield losses. Since pollen beetles have become increasingly resistant to pyrethroids, alternative control strategies within the framework of integrated pest management are urgently needed. To develop new strategies against the pollen beetle we studied the natural chemical variation in brassicaceous plant species and identified plant compounds mediating reduced feeding damage. Reduced feeding damage has rarely been observed in oilseed rape and B. napus resyntheses, but could be demonstrated for Barbarea vulgaris, Eruca sativa and Sinapis alba (Austel et al., 2021). The beetles’ sex and the plant accession affected the feeding response of the pollen beetle to these three species. To identify feeding deterrent secondary plant compounds, we compared semi-polar metabolite fraction profiles of B. vulgaris, E. sativa, S. alba and B. napus by a non-targeted approach. Here we selected the discriminating features showing negative correlation with the beetles’ feeding behaviour and identified several glucosinolates, flavonoids and saponins (Austel et al., 2021). We tested the direct influence of single metabolites achieved as commercial standards or from fractionated plant extracts in a newly designed dual-bud-choice assay. Whereas we observed no effects for the flavonoids, all five saponins from B. vulgaris deterred the beetles from feeding. From the six tested glucosinolates only the B. vulgarisspecific glucobarbarin deterred the beetles from feeding. We conclude that pollen beetles are well adapted to defence compounds from a wide range of cruciferous species, but not to specific compounds from B. vulgaris. Furthermore, we highly recommend securing correlations between metabolites and feeding behaviour by performing dose response experiments with single putative marker compounds to unravel the chemical signals mediating plant-insect interaction in crop plants.