Morphological, ultrastructural and pathological studies of the nuclear polyhedrosis virus of Spodoptera exigua.

Abstract

Nuclear polyhedrosis virus (NPV) of the beet armyworm, Spodoptera exigua found in Thailand, was multiple-embeded type. Polyhedra varied greatly in shape and size. Mature polyhedra often angular in shape measured 1.4 µm in diameter. Polyhedra with cavities, protrusions and rough surface structure were frequently observed. In addition unusually huge globular polyhedra and adherent polyhedra not previously described were present in the nuclei of infected fat cells. Virions, contained 2-6 virus particles, measured 132 x 365 nm. Rodshaped nucleocapsids or virus particles with the average size of 51 x 325 nm, commonly occurred in bundles outside polyhedra. Histopathological studies revealed that heavily infections occurred in the nuclei of the fat body, hypodermis, and tracheal matrix cells. Mid gut epithelium, malpighian tubules and the epithelium and connective tissues associated with tests were apparently susceptible for virus replication. Infection process of the virus was characteristically similar to those of other nuclear polyhedrosis virus in Lepidopterous hosts. Electron microscope investigations revealed some cytopathologic changes directed by the virus in the nuclei of infected cells. Fibrillar strand, as the prominent masses, occurred in most nuclei. Curved and curled membranes known as membranous profiles are commonly found associated with fibrillar strand and virogenic stroma. Thick electron dense filamentous membranes were also observed in this study. Results from this study would suggest that the NPV of exigua was different from the NPV of Heliothis armigera, Trichoplusiani and Bombyx mori found in Thailand. This virus was highly effective and promising to be developed as microbial agent for pest control programs.