Insect baculoviruses are group of insect biological control agents that received considerable attention. The Helicoverpa armigera nucleo-polyhedrovirus (HaNPV) is a baculovirus that has the potential to be an effective biocontrol agent for the Helicoverpa pest species worldwide. In vitro production of baculoviruses on a commercial scale would be an advantage in the production of viral biological control agent. However, serial passaging of some baculoviruses in insect cell culture has been shown to lead to rapidly accumulating mutant, also know as "the passage effect" which a change from the wild-type (referred to as MP for 'many polyhedra') to the few polyhedra (FP) mutants can be detected. This resulted in poor production yield and virus efficacy that hinder the use of in vitro production of baculoviruses for use as biopesticide. To develop a large-scale in vitro production of the Th-HaNPV, it was essential to determine whether or not the passage effect was evident when this virus is serially passaged in cell cultures and what are the characteristics of the Th-HaNPV passage effect.

Figure 1. A) Electron micrograph of polyhedral inclusion bodies (PIBs) formed by NPV, which are stable in the environment. B) PIBs in the nucleus of the HaNPV-infected Hz insect cell. C) MP and FP phenotypes found in Th-HaNPV at passage 10.

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