The present paper deals with the problem of enhancement of plant fitness due to parasitization of herbivores. The experimental evidence for such situations is reviewed. Two mathematical models, plant-herbivore (two trophic) and plant-herbivore-parasitoid (three trophic) are considered to analyse the experimental observations. The effect of environmental fluctuation in the tritrophic system is also observed and optimum values of the inaccessible parameters involved in the system are estimated for purposes of biological control.