Susceptibility to two insecticides (profenophos and cypermethrin) as influenced by host plants viz., cauliflower, soybean and castor in comparison to artificial diet and changes in detoxification enzymes in tobacco caterpillar, Spodoptera litura were studied under laboratory. In comparison to larvae reared on artificial diet, those reared on cauliflower were tolerant to profenophos and cypermethrin by, 1.41 and 6.6 respectively, while the larvae reared on soybean showed 1.32 and 2.00-fold more tolerance to these insecticides. However, the larvae reared on castor were found to be more susceptible to profenophos than those reared on artificial diet. The host plants had a significant influence on levels of detoxification enzymes in S. litura. Elevated levels of carboxylesterase (20.72-fold) and cytochrome P450 mono-oxygenase (3.10-fold) were observed in larvae reared on cauliflower as compared to diet fed larvae. Activity of glutathione S-transferase (1.56-fold) was higher in larvae reared on soybean. Enhanced activity of detoxification enzymes in larvae of S. litura reared on different host plants could be correlated with insecticide susceptibility. Positive and significant (p < 0.05) correlation was observed between the detoxification enzyme (CarE and GST) activity in different host fed larvae and LD50 of profenophos (r = 0.77; r = 0.93). Activity of cytochrome P450 mono-oxygenase also had positive correlation with LD50 of cypermethin (r = 0.98). The present studies have shown that differential susceptibility of S. litura population could be correlated with the diet/host plants and levels of detoxifications enzymes like carboxylesterases, glutathione S-transferase and cytochrome P450 mono-oxygenase.