Isolation, Identification, and Determination of Virulence of a Nuclear Polyhedrosis Virus from the Beet Armyworm, Spodoptera exigua (Lepidoptera: Noctuidae)

A multinucleocapsid nuclear polyhedrosis virus (MNPV) was isolated during an epizootic in California's Merced County from a single larva of the beet armyworm, Spodoptera exigua (Hübner). Occlusion bodies produced by this virus isolate, designated Se MNPV-608, measured 1.52 ± 0.11 µm ( $$\bar x$$ ± SD) in diameter and most contained virion; with two to four nucleocapsids per envelope. Restriction enzyme profiles of viral DNA indicate that this virus is markedly different from the Autographa californica (Speyer) MNPV ( Ac MNPV) and from Se MNPVs isolated from Egypt and the Netherlands, but is; similar to other previously described Se MNPV isolates from California. Synchronous per o5 bioassays of the virus against neonate beet armyworm larvae demonstrated that Se MNPV 608 was highly virulent, with an ST ₅₀ of 57.39 ± 1.59 h ( $$\bar x$$ ± SD), and more virulent for larvae of S. exigua than Ac MNPV (ST ₅₀ = 70.06 ± 2.66 h). Additionally, limited host range studies indicated that this isolate is specific for S. exigua no activity was apparent against larvae of the noctuids Heliothis virescens (F.), Trichoplusia ni (Hübner), or Spodoptera frugiperda (J. E. Smith).