Relationship of Larval Phase and Susceptibility of the Armywoim, Pseudaletia separata WALKER(Lepidoptera : Noctuidae) to a Nuclear Polyhedrosis Virus and a Granulosis Virus

Larval susceptibility in solitary and gregarious phases of Pseudaletia separata to nuclear polyhedrosis virus (PsNPV) and granulosis virus (GVH) was examined under laboratory condition.As the rearing density was decreased, larvae became more susceptible at the 5th instar and died within a shorter period when perorally inoculated with PsNPV. The larvae reared in isolation were more susceptible than those reared in crowded conditon when the 2nd instar larvae were perorally inoculated with GVH. The larvae which had been transferred from isolated to crowded condition or vice versa after viaral administration had intermediate susceptibility to PsNPV infection relative to the susceptibiilty of larvae reared in isolated and in crowded condition throughout the larval stage. The pale larvae produced from crowded rearing were slightly more susceptible than the dark larvae. No differences in susceptibility and time of death were observed between the solitary and the gregarious larvae when PsNPV was injected into the 5th instar larvae.