R. J. Rabindra

Characterization of five Indian isolates of the nuclear polyhedrosis virus ofHelicoverpa armigera

Five Indian geographical isolates of the nuclear polyhedrosis virus ofHelicoverpa (Heliothis) armigera were studied for the variation in their size, virulence, and the restriction and electrophoretic patterns of the polyhedrin. The diameter of the polyhedra varied from 1.6 to 2.4 μm (mean 1.933 μm), with no significant differences among the five isolates. The most virulent isolate was that from Ooty (OOT), with the lowest LC50 value of 2.538 × 103 purified occlusion bodies (POB)/ml, followed by the isolate from Coimbatore (CBE). Genomic variability as determined by the HindIII digestion pattern of the DNA, showed that isolates OOT and CBE were similar and that the three other isolates,viz., APK, GUJ, and RAJ represented another group which had a similar restriction pattern. The SDS-PAGE electrophoretic pattern of the polyhedrin of all the isolates was the same.

Genetic and biological variations amongPlutella xylostella granulovirus isolates

Success of a baculovirus in the field depends on the selection and utilization of effective isolates of viruses with increased pathogenicity and virulence. Isolates ofPlutella xylostella granuloviruses collected from Kenya and India were compared for genetic variations by restriction endonuclease enzyme analysis and for biological activity based on their median lethal concentration and time-to-kill early- and late-third instars. TheP. xylostella granulovirus isolates in Kenya were more diverse than those in India and were considerably different from the latter, with only 53–61% similarity. The Kenyan and Indian isolates did not differ significantly for their median lethal concentration, but the Kenyan isolate collected from Kibwezi was significantly faster than other isolates in killing early- and late-third instarP. xylostella. Cluster analysis of restriction fragment marker profiles of theP. xylostella granulovirus with other granuloviruses revealed that it grouped into a cluster with Spodoptera litura granulovirus, whereasHelicoverpa armigera andChilo infuscatellus granuloviruses grouped into another cluster. Results of the present study indicate the existence of genetically and biologically diverse isolates ofP. xylostella granulovirus which could be further developed into effective biopesticides for the management of diamondback moth.

Economic threshold for the management of Plutella xylostella with granulovirus in cauliflower ecosystem

Resistance to chemicals and some biological control agents by Plutella xylostella has resulted in the search for alternatives, such as P. xylostella granulovirus (PlxyGV), for the management of P. xylostella. However, use of PlxyGV would be economical only if it is applied at the right crop stage and at economic threshold levels. Hence, we evaluated the field level economic threshold for application of PlxyGV at the reproductive phase of cauliflower taking into consideration both the economic threshold model and the economic optimization model. PlxyGV was found to be very effective in management of the pest and was comparable to weekly applications of quinalphos 25EC. However, due to the higher cost of PlxyGV application than quinalphos, the cost benefit ratio with quinalphos application was much higher than the virus application. The net income from the virus application decreased both at lower thresholds—due to the higher cost of plant protection, and at higher thresholds—due to the increased damage by the pest. Based on the economic threshold model, the economic threshold for application of PlxyGV was estimated at 9–11 larvae per ten plants. Considering the net income per hectare and the cost of plant protection, it was found that at a threshold of 12–14 larvae per ten plants net income was just equal to the cost of plant protection. Based on the economic optimization model, the optimum threshold for application of PlxyGV was 15 larvae per ten plants when profits are maximum.

Biological activity and characterization of nucleopolyhedrovirus isolates ofSpodoptera litura

Geographical isolates ofSpodoptera litura (Fabricius) (Noctuidae: Lepidoptera) nucleopolyhedrovirus (SpltNPV), collected from different parts of India and maintained at Tamil Nadu Agricultural University, were compared for their biological activity and subjected to Restriction Endonuclease (REN) analysis. Neonate and second instar bioassay studies revealed similarity in biological activity as shown by the overlapping fiducial limits of LC50 values. However, there were differences in yield among isolates: significantly higher yields were obtained from isolates UAS and CBE than from the BARC isolate. REN analysis of the four isolates withPst I,Hind III,Bam HI andEco RI enzymes indicated genotypic variation among the isolates. Based on the commonality of the bands, the isolates could be broadly divided into two groups: isolates AU and CBE formed one group, and the other group comprised UAS and BARC based on genetic relatedness.