Effect of prolonged storage on the virulence of polyhedral inclusion body from seven nucleopolyhedrovirus isolates infectious to the tobacco cutworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)

Polyhedral inclusion bodies (PIB's) of seven isolates of nucleopolyhedrovirus (NPV) (Spodoptera litura NPV Fukuyama, S. littoralis NPV Egypt, Mamestra brassicae NPV Geihoku, M. brassicae NPV Tokyo, Xestia c-nigrum NPV Nasu, Mythimna separata NPV Geihoku, and M. loreyi NPV Fukuyama) that have been known to infect the tobacco cutworm, S. litura, were stored as water suspensions at 4-6°C for more than 20 yr after preparation in cold storage. The virulence of PIBs was detected successfully from six NPV isolates, excluding M. loreyi NPV Fukuyama. PIBs of re-isolated viruses prepared from newly infected S. litura larvae were further used to estimate the initial infectivity of the stored PIBs. PIBs of S. litura NPV Fukuyama and S. littoralis NPV Egypt were relatively resistant to longterm storage and the infectivity decreased to 1/50 after 24-yr storage in distilled water. M. brassicae NPVs seemed highly sensitive to long-term storage under the conditions of the present study.