Plants store chemical defenses that act as toxins to herbivores, such as toxic isothiocyanates (ITCs) in Brassica plants, hydrolyzed from the precursor glucosinolates (GLSs). Fitness of herbivorous larvae can be highly affected by these toxins, causing immature deaths. Theoretically, toxins can inordinately reduce larval fitness to death. We model this phenomenon by a set of ordinary differential equations and establish a direct relationship between feeding, toxin exposure and net energy of a larva, where the fitness of an organism is proportional to its net energy according to the optimal foraging theory. Our equations explain that toxin exposure can steeply reduce larval net energy to zero at an instar stage. Since herbivory needs energy, the only choice left for a larva is to stop feeding at the time point. If that time-point is significantly earlier than the end of the last instar stage, the larva dies.