Individual and combined impact of nuclear polyhedrosis virus and spinosad to control the tropical armyworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), in cotton in Pakistan

Abstract The tropical armyworm, Spodoptera litura Fabricius (Noctuidae; Lepidoptera), is among the most harmful pests causing economic loss in the quality and production of a variety of crops, particularly cotton. Entomopathogens play an important role in insect pest management. The nuclear polyhedrosis virus (NPV) isolate of S. litura (V-SpltNPV) was isolated from infected larvae in a cotton crop, and viral occlusion bodies were confirmed, using an inverted microscope. The pathogenicity of V-SpltNPV against 2nd, 3rd, and 4th larval instars of S. litura was evaluated at various concentrations (1 × 104 to 1 × 108 OBs/ml). Mortality rate was high (37.65–96.82%) in early instar larvae against tested concentrations. LC50 and LT50 values increased with increasing larval age. There was 689,865 times increase in LC50 value (1.35 × 102 OBs/ml) for 2nd instar larvae to LC50 value (6.90 × 105 OBs/ml) for 4th instar larvae. LT50 values enhanced from 4.99 days for 2nd instar larvae to 7.49 days for 4th instar larvae, due to a decrease in efficacy of NPVs with the increasing age of larvae. In a greenhouse experiment, a combined application of spinosad with V-SpltNPV (1 × 108 OBs/ml) caused (100%) mortality of 2nd instar larvae. A single application of V-SpltNPV (1 × 108 OBs/ml) resulted to mean mortality (52.63%) of tested larvae. The native isolate V-SpltNPV seems to have a potential to be used in integrated manner with other IPM tactics to significantly reduce the use of toxic chemical pesticides.