This thesis explores some of the components of the interactions between herbivorous insects and their host plants. Two cruciferous plants, Brassica napus L. and Sinapis alba L., and several insects which feed on these plants, were used to test the following hypotheses: (1) that the specific impacts vary for different insects feeding on a particular species of plant; (2) that plant compensation for defoliation by insects is specific to a particular herbivore, and not a generalized response to a certain level of injury; and (3) that the specificity of plant compensation for herbivory is related to the pattern of defoliation. Specific impact was measured as the reduction in plant biomass per unit gain in insect biomass for moth larvae, Mamestra configurata (Walker) and Plutella xylostella (L.), a beetle larva, Entomoscelis americana Brown, beetle adults, Phyllotreta cruciferae (Goeze) and Phyllotreta striolata (F.), and aphids Myzus persicae (Sulzer) and Lipaphis erysimi (Kaltenbach). Specific impact was different for different insects, and was highest for Phyllotreta spp., intermediate for Pl. xylostella and the two aphids, and lowest for E. americana and M. configurata. There was more than an order of magnitude difference between specific impacts of some insects. Laboratory and field experiments were performed to determine the level of compensation of B. napus and S. alba for equal amounts of injury to seedlings by M. configurata, Pl. xylostella, and Ph. cruciferae. Plants compensated most for injury by M. configurata and least for injury by Ph. cruciferae. Sinapis alba recovered more from the feeding than did B. napus. Seedling injury alone caused reductions in the number of seeds and average weight of seeds in both plants. Seedlings of B. napus and S. alba were artificially injured in ways that simulated feeding by M. configurata, Pl. xylostella, and Ph. cruciferae to determine the effect of pattern of injury on a plant's compensatory ability. Some of this artificial injury resulted in patterns of compensation and levels of recovery similar to those from the insect injury that artificial injury was meant to simulate. Plants generally compensated most for concentrated injury, characteristic of feeding by M. configurata, and least for perforations of cotyledons and injury to the apical meristem, characteristic of feeding by Ph. cruciferae. Injury to the apical meristem resulted in S. alba producing multiple stems, and resulted in damage to true leaves in both plant species. (Abstract shortened by UMI.)