author(s)
Ranvir Singh, KS Jagadish and K Murali Mohan

abstract
The relative toxicity of eight geographical isolates of Helicoverpa armigera nucleopolyhedrosis virus (HaNPV) to gram pod borer, Helicoverpa armigera (Hubner) was assessed in the laboratory. The median lethal concentration (LC₅₀) values of different HaNPV isolates ranged from 4.3 x 10⁶ to 1.9 x 10⁷ PIBs/ml. Among the eight isolates, the Gulbarga (Karnataka) isolate was highly toxic, with the LC₅₀ value of 4.3 x 10⁶ PIBs/ml, while the Dhule (Maharashtra) isolate was least toxic (1.9 x 10⁷ PIBs/ml). The median lethal time (LT₅₀) for the eight isolates ranged between 117.54 to 127.69 hrs and 128.56 to 154.61 hrs at higher (1.3 x 10⁹ PIBs/ml) and lower (2.6 x 10⁸ PIBs/ml) concentrations, respectively. Among the tested isolates, Gulbarga isolate was faster in causing mortality of the test population with the LT₅₀ value of 117.54 hrs and 128.56 hrs at higher and lower concentrations, respectively. Maximum time for causing mortality was noticed in Dhule isolate (127.69 hrs and 154.61 hrs). Results suggested a direct relationship between concentration of NPV and time taken (LT₅₀) to cause mortality in H. armigera. Assessed HaNPV isolates showed considerable variability in their virulence and time taken to cause mortality. Therefore, it is critical to carefully choose suitable virulent commercial formulations of HaNPV for management of H. armigera in field.