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.
Highlights
The tropical armyworm, Spodoptera litura Fabricius (Noctuidae; Lepidoptera), which is one of the important species of genus Spodoptera attacks a variety of agricultural crops, such as horticultural plants, fiber crops, vegetables, and miscellaneous wild plants as well as weeds (Zhou et al 2010)
Some commercial products have become available in the market. This current study aimed to evaluate effective control tools of S. litura and to provide basis for research and development on indigenous nuclear polyhedrosis virus (NPV)-based biopesticide in Pakistan
The increase in the Lethal concentration to kill 50% population (LC50) values, with an increase in the age of larvae, may be due to the dilution effect of virus inoculum associated with the increase in weight of larvae
Summary
The tropical armyworm, Spodoptera litura Fabricius (Noctuidae; Lepidoptera), which is one of the important species of genus Spodoptera attacks a variety of agricultural crops, such as horticultural plants, fiber crops, vegetables, and miscellaneous wild plants as well as weeds (Zhou et al 2010). The larvae of S. litura can cause 26–100% yield loss in field (Tuan et al 2014). It is distributed worldwide, especially in North America, Oceania islands, Africa, and. S. litura is an emerging insect pest of Pakistan as it causes a heavy loss in various regions such as the northern and southern districts of Punjab (Ahmad et al 2007). Its overlapping generations throughout cotton cultivated areas of Pakistan decimated this crop in 2003 (Ahmad et al 2007). The increase in cultivation of succulent crops like cotton, soybean, cabbage, mung bean, and vegetables provides ideal conditions for S. litura to vigorously reproduce, resulting in a rapid increase of generations and population size (Gao et al 2004)
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