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Two types of Barbarea vulgarisvar. arcuata, the G-type and the P-type, differed in resistance to larvae of the diamondback moth (DBM) Plutella xylostella. Rosette plants of the G-type were fully resistant to the DBM when grown in a greenhouse or collected in the summer season, but leaves collected during the late fall were less resistant, as previously found for flea beetle resistance. The P-type was always susceptible. Extracts of resistant leaflets inhibited larval growth in a bioassay, and a growth-inhibiting fraction was isolated by activity-guided fractionation. A triterpenoid saponin (1)was isolated from this fraction and identified as 3-O-β-cellobiosyloleanolic acid from spectroscopic data and analysis of hydrolysis products. The decrease in resistance of the G-type in the fall was correlated with a decrease in the level of 1, from 0.6–0.9 to <0.2 μmol/g dry wt. Compound 1was not detected in the susceptible P-type. We conclude that 1is correlated with the variable resistance of B. vulgarisfoliage to the DBM.

One essential aspect of the study of the evolution of host-plant use by insects is (variation in) its genetic basis. The genetic basis of the ability of a flea beetle (Phyllotreta nemorum) to use the crucifer Barbarea vulgarisssp. arcuata(G type) as a host plant was studied in a Danish population (Kvaerkeby) occurring naturally on this atypical host plant. Evidence was found that this ability was determined by a single, major, autosomal gene, although the presence of genes at additional loci at lower frequencies could not be excluded. No evidence was found for sex-linked inheritance, which was common in a second population in Denmark (Ejby) using Barbareaas a host plant. All beetles in the Kvaerkeby sample were homozygous ‘resistant’ to Barbareadefence. After crossing resistant F1offspring from pairs consisting of a field-collected beetle and a susceptible one amongst each other, genotyping the F2(reared on radish) showed a 1:2:1 ratio of homozygous resistant, heterozygous and susceptible beetles. No evidence was found for a reduction in the viability of beetles that were homozygous resistant at the autosomal locus, in contrast to what had been found earlier for two backcrossed lines founded by beetles from Ejby. The results show that there is variation in the genetic basis of host-plant use across local populations and imply that population structure should form part of the study of the interaction between P.nemorumand its host plants.