Effects of cotton fruiting on bollworm, Helicoverpa zea (Boddie) and tobacco budworm, H. virescens (F.) larval growth, survivorship, length of pupal developmental period, pupal weight, susceptibility to H. zea nucleopolyhedrovirus (HzNPV) and production of virus in HzNPV-infected larvae were studied. Larval weight of both species after 10 days of rearing, varied significantly (P <0.05) and was in the order of artificial diet > flower-anthers > leaves > squares > sq. bracts. The length of larval developmental period, larval survivability, pupal weight and length of pupal developmental period was affected similarly. All of these were positively correlated with larval weight, except that the length of larval developmental period was negatively correlated. Mortality of HzNPV-treated bollworm or tobacco budworm larvae fed on square, sq. bracts and flower anthers ranged from 64.8 to 67.9% and 50.5 to 60.1%, respectively, and did not differ significantly among the tissues, indicating that dietary variation in cotton fruiting structures did not affect larval susceptibility to HzNPV. The number of viral occlusion bodies (OBs) produced by a larva fed square, sq. bracts or flower-anthers, respectively, were 2.0 X 10⁷, 2.2 X 10⁷and 6.6 X 10⁷ for bollworm and 2.0 X 10⁷, 1.0 X 10⁷, and 9.3 X 10⁷for tobacco budworm. In both species, larvae fed on flower-anthers produced significantly (P<0.05) higher number of OBs than those on squares or sq. bracts. In both species, the number of OBs produced by an infected larva fed different fruiting structures was positively correlated with the weight gained by a healthy larva fed on the same type of tissues. These results indicate that dietary variation in different reproductive structures of cotton directly affects bollworm and tobacco budworm larval growth and development, and indirectly affects the production of virus by HzNPV-infected larvae.