Studies were conducted with the first aim to understand field evolved resistance to Bacillus thuringiensis and conventional insecticides. As an eco-friendly alternative, experiments were conducted to fully understand the nature of toxicity of Nucleopolyhedroviruses to insecticides resistant and susceptible Heliocverpa armigera Hübner populations and their impact on fitness of the insect. The most virulent strain synergised the activity with new chemistry insecticides such as spinosad and abamectin. Furthermore the most virulent HearNPV suppressed insecticides resistant H. armigra in the field. Helicoverpa armigera Hübner is one of the most destructive pests of several field and vegetable crops, with indiscriminate use of insecticides contributing to multiple instances of resistance. In the present study we first assessed whether H. armigera had developed resistance to Bt cotton and compared the results with several conventional insecticides. Furthermore, the genetics of resistance was also investigated to determine the inheritance to Cry1Ac resistance. To investigate the development of resistance to Bt cotton and selected foliar insecticides, H. armigera populations were sampled in 2010 and 2011. The resistance ratios (RR) for Cry1Ac, chlorpyrifos, profenofos, cypermethrin, spinosad, indoxacarb, abamectin and deltamethrin were 580-fold, 320-, 1110-, 1950-, 200-, 380, 690, and 40-fold, respectively, as compared with the population marked laboratory susceptible (Lab-PK). Selection of the field collected larval population with Cry1Ac in 2010 for five generations increased RR to 5440-fold. The selection also increased RR for deltamethrin, chlorpyrifos, profenofos, cypermethrin, spinosad, indoxacarb, abamectin to 125-fold, 650, 2840, 9830, 370, xxxi 3090 and 1330-fold respectively. The estimated LC50s for reciprocal crosses were 105 μg/ml (Cry1Ac-SEL female × Lab-PK male) and 81 μg/ml (Lab-PK female × Cry1Ac-SEL male) suggesting that the resistance to Cry1Ac was autosomal; the degree of dominance (DLC) was 0.60 and 0.57 respectively. Mixing of enzyme inhibitors significantly decreased resistance to Cry1Ac suggesting that the resistance to Cry1Ac and other insecticides tested in the present study was primarily metabolic. Resistance to Cry1Ac was probably due to a single but unstable factor suggesting that crop rotation with non-Bt cotton or other crops could reduce the selection pressure for H. armigera and improve the sustainability of Bt cotton. As an eco-friendly alternative to insecticides, HearNPV when tested for Lab-PK, field and Cry1Ac populations, there exist significant variation in LC50 values for these three populations with 0.91, 1.88 and 2.99x106 OBs, respectively. Field population required two times and Cry1Ac selected population needed about three times the concentration to kill 50% larval population. However, when SpltNPV was given to these three populations of H. armigera, there was insignificant variation in these values and their fiducial limits overlap each other. These revealed high efficiency of HearNPV against H. armigera than that of SpltNPV. There was an additive effect on the toxicity of co-infection of insecticides when mixed with HearNPV. This addition impact shows to be beneficial because insecticides vary in their mode of action and speed to kill as compared to that of HearNPV. However, this might be synergistic if observed after time enough for HearNPV to show its actual kill. Abamectin and spinosad are most commonly used insecticides against H. armigera and they showed lower LC50 xxxii values proving their high toxicity. However, their mixtures with HearNPV at sublethal concentrations showed more response of these insecticides than HearNPV. For UNSEL H. armigera population, development period of larvae and pupae was insignificantly variable for all the treatments when compared with control. However, total development period was less in control and more in abamectin with 5 days increase. Least adult life was on HearNPV mixture with spinosad and abamectin. Preovipositon period was of 3.8 days for UNSEL H. armigera population. For HearNPV SEL H. armigera population, development period of larvae and pupae was also insignificantly variable for all the treatments with total development period less in control and more in abamectin and spinosad. Least adult life was on HearNPV mixture with spinosad and abamectin. However, preovipositon period was insignificantly different for all the treatments. Three different bacteria were observed in dissected guts of H. armigera larvae with and without HearNPV treatment and compared with control, whose presence was observed in both treatments. The impact of HearNPV treatment on the production of occlusion bodies and liquefaction of H. armigera larvae was observed. HearNPV treatment increased the rate of occlusion bodies production up to 1000 times as compared to untreated which seems to be due to positive impact of these virions on gut flora activities. Survival rate comparison showed almost similar population trend in all treatments before application of insecticides. Spinosad was the most effective insecticide 24 hr post application followed by abamectin which remained statistically non-significant with HearNPV alone and or xxxiii mixed with insecticides. Data after 48 hours showed spinosad, abamectin and HearNPV with spinosad the most toxic. Third and fourth days after application proved both spinosad and abamectin as most toxic and did not differ from their mixtures with HearNPV. 5th, 6th and 7th days observations proved all the single and mixtures as the most toxic to H. armigera. The highest mortality was in spinosad followed by other single and mixtures when compared with control. Present studies revealed the efficacy of NPVs against Lepidopteran pests especially Helicoverpa armigera and the results proved it as alternate eco-friendly control and resistance delaying strategy for Helicoverpa armigera.