Shigeyuki Mukawa

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.

Biological activity and identification of nucleopolyhedroviruses isolated from Mythimna separata and Spodoptera litura in Japan

We compared the infectivity of two nucleopolyhedroviruses (NPVs), MyseNPV G isolated from Mythimna separata (Walker) (Lepidoptera: Noctuidae) and SpltNPV S isolated from Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). MyseNPV G was more pathogenic against M. separata than against S. litura. Although SpltNPV S was more pathogenic than MyseNPV G against S. litura, it did not infect M. separata. Restriction endonuclease (REN) analysis of viral genomic DNA revealed that the two NPVs have quite different REN profiles. Based on nucleotide sequences of the coding regions of polyhedrin, lef-8 and lef-9, SpltNPV S was closely related to other SpltNPV isolates, whereas MyseNPV G appeared to belong to the Mamestra NPV group, and was distinct from a Chinese isolate of Leucania (=Mythimna) separata NPV. The potential of MyseNPV G and SpltNPV S to control pest insects is discussed.

Mamestra brassicae Nucleopolyhedrovirus Infection and Enhancing Effect of Proteins Derived From Xestia c-nigrum granulovirus in Larvae of Mamestra brassicae and Helicoverpa armigera (Lepidoptera: Noctuidae) on Cabbage

ABSTRACT The insecticidal effect of Mamestra brassicae nucleopolyhedrovirus (MabrNPV) T5 against Mamestra brassicae (L.) and Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), important pests of various vegetables and ornamental crops in Japan as well as many other countries, and the enhancing activity of proteins derived from occlusion bodies of Xestia c-nigrum granulovirus (XecnGV) &agr;-4, which was named GVPs, on the infectivity of MabrNPV were evaluated in a bioassay with second-instar larvae fed on virus-applied cabbage, Brassica oleracea L. var. capitata, plants. The lethal concentrations of MabrNPV achieving 95% mortality (LC95) were estimated to be 7.7 × 105 and 1.8 × 105 OBs per ml for M. brassicae and H. armigera, respectively, with MabrNPV-alone treatment. Because the mean areas of cabbage leaf consumed by one larva in 60 h were not significantly different between M. brassicae and H. armigera, we conclude that MabrNPV is more infectious to H. armigera than to M. brassicae. When larvae were fed on cabbage plants treated with 104 OBs per ml MabrNPV and various concentrations of the GVPs, the mortality of the two noctuid larvae increased in relation to GVP concentration. The requisite concentrations of the GVPs achieving 95% mortality with the MabrNPV for M. brassicae and H. armigera were estimated to be 5.93–8.30 and 1.94–3,48 µg/ml, respectively, In a comparison of the MabrNPV-alone treatment with equivalent 95% mortality, addition of GVPs increased the rate of larval death at younger instars, especially in M. brassicae. Our results indicate that GVPs are a potentially useful additive for improving the insecticidal efficacy of MabrNPV.

Two Year Field Study to Evaluate the Efficacy of Mamestra brassicae Nucleopolyhedrovirus Combined with Proteins Derived from Xestia c-nigrum Granulovirus

Japan has only three registered baculovirus biopesticides despite its long history of studies on insect viruses. High production cost is one of the main hindrances for practical use of baculoviruses. Enhancement of insecticidal effect is one possible way to overcome this problem, so there have been many attempts to develop additives for baculoviruses. We found that alkaline soluble proteins of capsules (GVPs) of Xestia c-nigrum granulovirus can increase infectivity of some viruses including Mamestra brassicae nucleopolyhedrovirus (MabrNPV), and previously reported that MabrNPV mixed with GVPs was highly infectious to three important noctuid pests of vegetables in the following order, Helicoverpa armigera, M. brassicae, and Autographa nigrisigna. In this study, small-plot experiments were performed to assess concentrations of MabrNPV and GVPs at three cabbage fields and a broccoli field for the control of M. brassicae. In the first experiment, addition of GVPs (10 µg/mL) to MabrNPV at 106 OBs/mL resulted in a significant increase in NPV infection (from 53% to 66%). In the second experiment, the enhancing effect of GVP on NPV infection was confirmed at 10-times lower concentrations of MabrNPV. In the third and fourth experiments, a 50% reduction in GVPs (from 10 µg/mL to 5 µg/mL) did not result in a lowering of infectivity of the formulations containing MabrNPV at 105 OBs/mL. These results indicate that GVPs are promising additives for virus insecticides.