orf18 (ac18) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved gene in lepidopteran nucleopolyhedroviruses, but its function remains unknown. In this study, an ac18 knockout AcMNPV bacmid was generated to determine the role of ac18 in baculovirus life cycle. After transfection of Sf-9 cells, the ac18-null mutant showed similar infection pattern to the parent virus and the ac18 repair virus with respect to the production of infectious budded virus, occlusion bodies, or the formation of nucleocapsids as visualized by electron microscopy. The deletion mutant did not reduce AcMNPV infectivity for Trichoplusia ni in LD(50) bioassay; however, it did take 24 h longer for deleted mutant to kill T. ni larvae than wild-type virus in LT(50) bioassay. Our results demonstrate that ac18 is not essential for viral propagation both in vitro and in vivo, but it may play a role in efficient virus infection in T. ni larvae.
