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Varied responses by specialist herbivores to glucosinolates could be a function of glucosinolate concentration or other correlated resistance factors, Herbivory by the specialist flea beetle Phyllotreta cruciferae (Goeze), and diamondback moth, Plutella xylostella (L.), varied curvilinearly with natural levels of glucosinolates in Brassica rapa (syn.campestris) (L.) such that maximum herbivory occurred at intermediate glucosinolate levels, Although the pattern was weak, decreases in herbivory at high concentrations of glucosinolates were enhanced by inoculating plants with the fungal pathogen Leptosphaeria maculans(Desm.) Ces. et de Not.(Ascomycetes), in laboratory experiments. This enhancement effect may have been caused by other correlated induced resistance factors produced by the plant or by resistance factors produced by the pathogen (independent of the host plant), Although specialist herbivores of mustards may have overcome glucosinolates, the apparent dose-dependent effect of glucosinolates suggest this herbivore counteradaptation may not have been complete, perhaps because of correlated resistance factors. Because a component of glucosinolate variation in B. rapa is heritable and because P. cruciferae can negatively affect plant fitness, the vried responses of specialist herbivores with glucosinolate concentrations reported here further suggest the possibility of disruptive selection on glucosinolates.
Siemens et al. (Sun,) studied this question.