Pyrrolizidine alkaloid distribution in Senecio jacobaea rosettes minimises losses to generalist feeding

The within-plant distribution of pyrrolizidine alkaloids (PAs) was determined in the shoots of vegetative Senecio jacobaea L. Asteraceae plants. Also the distributions of feeding stimulants, soluble sugars and total nitrogen, were determined. PAs were found to decrease with leaf age, sugars were constant, and nitrogen decreased with leaf age. These results were compared with the within-plant distribution of feeding damage by the generalist herbivores Spodoptera exigua Hübner and Mamestra brassicae L. Both generalists showed a strong preference for older leaves. It was concluded, that the herbivores were forced to feed on sub-optimal parts of the plant.