Differential Mechanical Defense: Herbivory, Evapotranspiration, and Leaf-Hairs

The common mullein, Verbascum thapsus (Scrophulariaceae) is shown to possess an effective age-specific mechanical defense against insect herbivores, based upon branched leaf hairs. Correlative and experimental evidence indicate that patterns of herbivory observed in this plant, in which older, less pubescent leaves are more frequently attacked than are more pubescent young leaves, may be explained by differences in pubescence associated with leaf age. In the mullein, leaf-hairs perform a dual role, functioning both as a defense against herbivores and as an evapotranspirative barrier against water loss. Although tests are previously unreported in the literature, age-specific mechanical defenses may be both widespread and effective against generalized chewing insects such as grasshoppers.