Compensation for herbivory in cruciferous plants: Specific responses to three defoliating insects

Plant compensation may be specific to the defoliation of a particular insec t herbivore, or a generalized response to herbivory. These alternate hypoth eses were tested by measuring biomass and seed production of Brassica napus L. and Sinapis alba L. in response to 0, 25, or 75% defoliation of seedlin g plants. The herbivores were adults of Phyllotreta cruciferae (Goeze), lar vae of Plutella xylostella L., and larvae of Mamestra configurata (Walker). Although defoliated to the same extent, both B. napus and S. alba compensa ted most for defoliation by M. configurata and least for defoliation by P. cruciferae. Both plant species compensated better for 25% than for 75% defo liation, and S. alba compensated better than B. napus. Laboratory and held experiments showed similar patterns of compensatory leaf growth, but recove ry was more rapid in a controlled environment. Compensation was associated with changes in root biomass that were correlated with foliage biomass, ind icating that root-shoot ratios were maintained. Complete recovery of foliag e after defoliation did not assure complete recovery of plant fitness. For these three herbivorous insects, compensation by two plant species for defo liation was specific to the insect defoliator, and not a generalized respon se to herbivory. The compensatory responses of the two plant species explai n, in part, the differential impact the three herbivores have on the crops.