Nutritive Quality of Plant Protein: Sources of Variation and Insect Herbivore Responses

Protein quality has received comparatively little attention as a factor in host plant suitability for insects. It is argued here that plant protein quality is subject to considerable variation from genetic and environmental influences and thus may significantly impact herbivore performance. Furthermore, other phytochemicals that are ingested with protein may negatively impact protein utilization. There is a wide distribution of alkylating agents found in plants (e.g., quinones, phenolics, aldehydes, pyrrolizidine alkaloids, sesquiterpene lactones, isothiocyanates) that form covalent bonds with nucleophilic side chains of proteins (e.g., -SH, -NH, -NH₂) and potentially limit amino acid availability. The behavioral and physiological adaptations of insects to variation in protein quality are also discussed. Finally, preliminary evidence for physiological adaptation to low protein quality in Helicoverpa zea is provided. The potential role of protein quality in host plant specialization is summarized.