Different herbivore responses to two co-occurring chemotypes of the wild crucifer Barbarea vulgaris

According to coevolution theory, plant chemical defences are continually evolving in response to selection by herbivores. Unique to the Brassicales, a few species in the Barbarea genus produce triterpenoid saponins that are highly deterrent to some specialist insect herbivores. One species, B. vulgaris, has diverged into two chemotypes, the G- and P-type, of which the P-type seems to have lost the saponin-based insect resistance by producing different saponin structures; it also produces different glucosinolates and other potential defence traits. Here, we examined the preference and performance of a larger set of specialist and generalist herbivores on the two plant types, including three generalist mollusc (Arion vulgaris, Deroceras sp., Cepaea sp.) as well as three specialist (Phaedon cochleariae, Athalia rosae, Pieris napi oleraceae) and two generalist (Mamestra brassicae, Myzus persicae) insect herbivores. Five out of six herbivore species preferred leaves of the P-type for feeding, and most of them also survived and/or grew better on the P-type, or preferred it for oviposition. In contrast, larvae of M. brassicae showed no preference and performed equally well on the two plant types; the leaf beetle P. cochleariae preferred the G-type for oviposition, which was, however, not reflecting larval performance. Overall, the defences of the P-type against herbivores seem not to be as effective as those of the G-type, which is surprising given its large geographical distribution, overlapping with that of the G-type in Scandinavia and Finland. This suggests that additional ecological interactions determine the success of the two chemotypes.