In vivo characterization of two granuloviruses in larvae of Mythimna separata (Lepidoptera: Noctuidae)

The pathogenicity of two granuloviruses (GVs), Xestia c-nigrum GV (XecnGV) and Pseudaletia unipuncta GV (PsunGV), was examined in Mythimna separata. Partial sequencing of the genome of PsunGV indicated that it is related closely to XecnGV, but considered to be a different species. PsunGV and XecnGV showed similar pathogenicity in terms of dose-mortality response and pattern of host mass changes following infection. Both GVs killed infected larvae in 2-3 weeks. Temporal changes in the concentrations of GV-specific DNA in the larval haemolymph were measured by using a real-time quantitative PCR. Viral DNA concentration increased quickly and reached a plateau at 60-72 h post-inoculation. Rates of budded virus (BV) production of each GV were estimated on the basis of viral DNA concentrations by a modified Gompertz model. The slopes of the estimated BV growth curves of both XecnGV and PsunGV in M. separata larvae were equivalent to that of Mamestra brassicae nucleopolyhedrovirus (NPV) in its original host, reported in our previous study. This suggested that BV production is not a major factor in the slower killing speed of GVs in comparison to NPVs. The GV-infected larvae survived for an additional 10 days or more after reaching a maximum level of BV concentration, and kept growing without pupation. These findings also suggested that the GVs have a unique mechanism to regulate the growth of host larvae.