Realized defense of artificially selected lines of Brassica rapa: Effects of quantitative genetic variation in foliar glucosinolate concentration

Herbivore species-specific responses to a plant secondary compound could result in variable selection on the concentration of that plant chemical. The purpose of this study was to determine if lines of a rapid-cycling Bressica rapa (L.) that had been artificially selected for quantitative differences in foliar glucosinolate content also differed in their realised defense, defined as the number of eggs laid on or level of damage incurred by a plant when exposed to herbivores. I asked whether defense was conferred at the oviposition or feeding stage, whether 2 herbivore species respond similarly to different foliar glucosinolate concentrations, and whether these same 2 species could impose selection in different directions on foliar glucosinolate concentration. I used an oligophagous herbivore, Pieris rapae (L.), and a polyphagous herbivore, Trichoplusia ni Hubner. Feeding was examined for both herbivore species, and oviposition was examined for P. rapae, P, rapae adult females did not favor any single selection line over another as a site for oviposition. However, P. rapae larvae did remove greater amounts of leaf area on plants from lines selected for low glucosinolate content when given a choice. In addition, T. ni larvae caused significantly greater amounts of damage on low glucosinolate line plants. Thus, quantitative variation in glucosinolate content among selected lines does confer differences in realized defense against these 2 herbivores. However, because P. rapae adult females do not appear to discriminate among plants that differ in glucosinolate content, defense is conferred at the level of feeding. Further, in contrast to previous studies, larvae of both the polyphagous and the oligophagous herbivore species responded similarly to quantitative variation in secondary compounds, causing more damage on low glucosinolate lines. This parallel response suggests that both herbivore species should impose selection on defense in the same direction in B. rapa for increased levels of glucosinolate production.