Macronutrient Content of Cotton (Gossypium hirsutum) Tissues Across Genotypes and Environments: An Indicator of Resource Quality for Insect Herbivores

Understanding the nutritional preferences and constraints on insect pests is integral to predicting when and where they will occur in agricultural systems. Equally as important, is determining what plants have to offer as a nutritional resource for these pest species. The historical consensus among ecologists and agronomists is that nitrogen (N) is often a limiting resource in terrestrial systems; hence, quantification of plant quality has long been achieved by measuring plant N content. While this method may be a useful proxy for indicating the relative quality of plants as resources of herbivores, it doesn’t delimitate the various non-nutritional compounds that contribute to the total N content of a plant. This is particularly important because both protein and certain allelochemicals are N-rich compounds, yet each has opposite effects on herbivore performance (growth, developmental time, fecundity, etc.). In this study we measured the digestible protein and carbohydrate content of true leaves, terminal growth, squares, and bolls across a range of cotton varieties, both in the field and greenhouse, to determine its nutritional content as a resource to insect herbivores. The goals of this study were to: (1) determine the macronutrient content of different cotton tissues, (2) to compare these concentrations across different genotypes (varieties) and environmental conditions (greenhouse and field), and (3) to determine whether published total N measures correspond to digestible protein concentrations.