Plant diet strongly impacts the fitness of insect herbivores. Immediately, we think of plant defensive compounds that may act as feeding deterrents or toxins. We are, probably, less aware that plants also influence insect growth and fecundity through their nutritional quality. However, most herbivores respond to their environment and select the diet which optimizes their growth and development. This regulation of nutritional balance may occur on many levels: through selecting and ingesting appropriate plant tissue and nutrient digestion, absorption and utilization. Here, we review evidence of how nutritional requirements, particularly leaf protein to digestible carbohydrate ratios, affect caterpillar herbivores. We propose a model where midgut endocrine cells assess and integrate hemolymph nutritional status and gut content and release peptides which influence digestive processes. Understanding the effects of diet on the insect herbivore is essential for the rational design and implementation of sustainable pest management practices.