Nucleopolyhedrovirus (NPV) was isolated from infected Hyposidra talaca (Walker) caterpillars. This defoliating pest was sampled from the Terai and Dooars tea plantations along the foothills of Darjeeling, India. Phase contrast and transmission electron microscopy revealed polyhedral occlusion bodies (OBs) typical of NPVs. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis showed that the molecular weight of the major protein of the OBs is 32.17 kDa. Dose–mortality bioassays conducted for OBs on the second instar caterpillars of H. talaca resulted in a median lethal concentration (LC50) of 2.8 × 103 OBs/ml. The median lethal time (LT50) was 5.45 days for 1 × 104 OBs/ml, 4.15 days for 1 × 105 OBs/ml and 4.05 days for 1 × 106 OBs/ml concentrations. These results indicate the potential of using this NPV as a microbial pesticide against H. talaca.

The effects of pH, sunlight and rearing temperature on infectivity of nucleopolyhedrovirus (SpltNPV) were investigated against larvae of Spodoptera litura (Fabricius). The infectivity of virus increased with increased larval rearing temperature. The LT50 values were significantly reduced from 24 to 6 days when rearing temperature was increased from 20 to 35 °C. The pH of the substrate significantly affected the infectivity of the virus. The LT50 values increased to about fourfold when the pH of suspension was raised from 7 to 11. Infectivity of the virus was more adversely affected in alkaline than in acidic suspension. Sunlight also deleteriously affected infectivity of the virus. The LT50 values increased with the increase in the period of sunlight exposure. A complete viral inactivation occurred after 12 h of direct sunlight exposure.

In situ field level mass production of Amsacta albistriga nucleopolyhedrovirus (AmalNPV) was achieved at Pavagada (Karnataka, India) during September 2004. The yield of virus mass produced in situ in the field was 5.05 times greater than that when produced in the laboratory. This technique is farmer-friendly, easy to execute and much cheaper than laboratory production, having the potential to be used for the mass production of NPV of gregarious pests that occur in large numbers.

Two isolates of nucleopolyhedroviruses (NPVs) from Kenya and South Africa were compared to Gemstar® (a commercial NPV) for their pathogenicity against the first four larval instars of Helicoverpa armigera (Hübner). The larvae were fed on droplets with the three virus products in concentrations of 0 (control), 6×102, 6×103, 6×104 and 6×105 occlusion bodies/μl. The bioassays showed that the median lethal dose (LD50) values of 23 and 631 occlusion bodies for the first and second instars, respectively, were comparable to those of Gemstar®. The LD50 values for the third and fourth instars were 3981 and 39,810 for the Kenyan isolate and 1288 and 25,119 for the South African isolate. There was a linear relationship between the log LD50, the larval age and the lethal time (LT50), which appeared to be dose dependent. This correlation constitutes a useful index for estimating susceptibility of larval populations. The LT50 increased from 2.8 to 11.9 days and 2.8 to 6.8 days, respectively, for the Kenyan and South African isolates, suggesting a slight increase of resistance with age within infected larvae.

The production of the nucleopolyhedrovirus (NPV) of the tobacco cutworm, Spodoptera litura was studied to determine the optimum conditions for in vivo production using homologous host larvae reared on artificial diet. We compared NPV production in 7, 8, 9 and 10-day-old larvae and found that 9-day-old larvae gave the highest virus yield. The optimum dose was determined by infecting 9-day old larvae (weighing 125–155 mg) with a range of doses. The maximum NPV production achieved was 3.91 × 109 polyhedral inclusion bodies (PIBs) per larva using an inoculum of 4.8 × 106 PIBs. A 7-day incubation period was found to be optimum for NPV production. Moribund larvae were found to be more suitable for handling and for the mass production of virus than dead ones.

The effects of neem seed kernel extract (NSKE), neem oil (NO) and nucleopolyhedrovirus (NPV) of the Spodoptera litura Fabricius (Lepidoptera: Noctuidae) on the mortality, feeding and development of S. litura were evaluated in the laboratory. Dose-dependent mortality was evident after NPV treatment, which also depended on larval age at treatment. Upon combining NPV with neem products, the percentage mortality was enhanced threefold even at reduced concentrations. The NSKE and NO significantly inhibited the growth and development of S. litura, and extended the duration of its larval and pupal stages. Oviposition period, adult longevity and fecundity were significantly reduced following treatment with neem products and with neem + NPV treatment. The efficiencies of conversion of ingested food (ECI) and digested food (ECD) were also significantly decreased by treatment with NSKE, NO and NPV, either alone or in combination.