Selection of nuclear polyhedrosis viruses as biological control agents ofSpodoptera exigua [Lep.: Noctuidae]

The virulence of 5 nuclear polyhedrosis viruses infectious for larvae of beet armyworm,Spodoptera exigua, was studied and their potential as biological control agents of this accidentally introduced pest in Dutch greenhouse crops is discussed. Three of the virus isolates were collected from deceased beet armyworm larvae found in Dutch greenhouses. Based on restriction endonuclease patterns of their DNA they appeared to be closely related toMamestra brassicae nuclear polyhedrosis virus (MbMNPV) and therefore were named MbMNPV-NL80, MbMNPV-NL82 and MbMNPV-NL83. These isolates were not related toAutographa californica MNPV (AcMNPV) or toSpodoptera exigua MNPV (SeMNPV), both originating from the USA. Comparison of the oiological activity of these 5 isolates showed that the SeMNPV was more virulent against beet armyworm than the other isolates. There was no significant difference in virulence between MbMNPV-NL80, NL82, NL83 and AcMNPV forS. exigua. The LD-50 values of the 5 isolates for 2nd instar larvae were 3, 26, 14, 17 and 18 polyhedra, respectively. Despite compensating qualities of the other MNPVs, such as a broader host range and potential production in alternate hosts or cell-lines, SeMNPV is considered to be the most suitable candidate as biological control agent of beet armyworm.