Effects of a Nuclear Polyhedrosis Virus and a Fluorescent Brightener on Colonies of Spodoptera exigua (Hübner)

Abstract

The beet armyworm, Spodoptera exigua (Hübner), is an important pest which is difficult to control because it is resistant to nearly all registered insecticides. A new commercial formulation of a nuclear polyhedrosis virus that is pathogenic to beet armyworm larvae, SPOD-X®, was tested with and without a fluorescent brightener, Tinopal LPW. The effects on larvae from six colonies of the beet armyworm which had recently been established from field collections in Georgia, Alabama and Mississippi were compared with a laboratory colony which had been maintained in the laboratory for 2 years. The LC50 was lower in all bioassays with SPOD-X in 0.25% Tinopal LPW than with SPOD-X in water, although in 3 of 12 bioassays the difference was not significant based on overlapping 95% confidence intervals. The mean LC50 for SPOD-X in water was 376 polyhedral occlusion bodies per cup (surface area 800 mm2). The mean LC50 for SPOD-X in 0.25% Tinopal LPW was 30 polyhedral occlusion bodies per cup. The LC50 for SPOD-X in water was not significantly different between the laboratory colony and 5 of 6 field-collected colonies when bioassayed concurrently. There was no significant difference in LC50 for SPOD-X in 0.25% Tinopal LPW between the laboratory colony and any of the field-collected colonies. Thus, Tinopal LPW enhanced the infectivity of SPOD-X for beet armyworm larvae in laboratory tests and reduced the variability of response of the beet armyworm colonies to the virus.