An interactive system made of Solanum melongena L. and Tetranychus cinnabarinus (Boisduval) was used to study the relationship between T. cinnabarinus population dynamics and S. melongena physiological changes in this paper. Continuous measurements of PAL (a key enzyme in propylbenzene metabolism pathway) and G6PD (a representative enzyme in plant primary metabolism) activity shed new light on S. melongena's stress reaction caused by T. cinnabarinus piercing-sucking. Results showed that changes in enzyme activity was correlated with the strength of T. cinnabarinus stress and both were fluctuating during the interaction. It was concluded that the defence of S. melongena when attacked by T. cinnabarinus was an entire stress reaction coordinated by primary and secondary metabolism, and was a self-regulated, fluctuating, normal physiological process.