6 - Ecophysiology and Insect Herbivory

Abstract

This chapter presents an overview of the factors that determine nutrient levels in plant tissues, the effects that variation in plant nutrients have on insect herbivores, and how herbivory affects the acquisition and allocation of nutrients in plants. Conifers manufacture their basic food materials—such as carbohydrates, fats, and proteins—from N, carbon dioxide, oxygen, water and a dozen mineral elements using light energy through photosynthesis. Mineral nutrients function as components of plant tissues, regulators of osmotic potential, constituents of buffer systems, activators of enzymes, and regulators of membrane permeability. The factors determining the nutrient levels in plant tissues are primarily environmental stress—water stress, nutrient stress, and air pollution), plant tissue and age, plant genotype (this can be either interspecific or intraspecific—and tree stand age, density, and structure. Plants form the substrate on which herbivore insects feed but the problem of determining an herbivore's response to one specific nutrient in a plant is marred by strong intercorrelations among the many nutrients of the plant. Insect herbivores that feed on conifers require N and minerals for growth and reproduction. The factors that determine nutrient levels in plants also determine the effects nutrient intake will have on the herbivore. Herbivory can have both negative and positive effects on important physiological process in conifers. The variable effects of herbivory on ecophysiological processes in conifers make it difficult to predict how global change might alter insect–plant relationships.