Selection and Field Effectiveness of Nucleopolyhedrovirus Isolates Against Helicoverpa armigera (Hubner)
Helicoverpa armigera (Hubner) is a serious pest of tomato in hill and mountain ecosystem of North Western India. Larvae of this species are susceptible to a Nucleopolyhedrovirus (NPV) which has attracted interest as a potential biocontrol agent. Three strains of NPV were isolated from infected larvae of host insect. The comparative pathogenecity/speed of kill of various isolates was determined in term of median Lethal Dose (LD50) and median Lethal Time (LT50) in a bioassay by diet plug method to select a candidate isolate for field potential in tomato and chickpea against Helicoverpa armigera (Hubner). The order of virulence of each isolate was Samba>Udheywalla>Chenani on 3rd instar larvae with respective LD50 values of 89, 243 and 287 OBs/larva, For all the isolates, the LT 50 values were inversely related to doses and ranged from 134.4 to 175.2 h at 3 different concentrations of 1000, 5000 and 25000 OBs/larvae. The samba isolate was considered about 2.73 to 3.22 times more virulent than Udheywalla and Chenani, respectively. Existence of differential virulence was also revealed in pot experiments wherein the highest possible mortality (98.33%) in 3rd instar larvae was recorded in Samba isolate of HaNPV followed by Udhyewalla (86.11%) and Chenani (81.66%) after 9 days of inoculation, the difference being significant. The field effectiveness of the most virulent isolate (samba) was assessed on tomato and chickpea-the two major host crops of this pest. Application of virus alone @ 3.0x1012 OBs ha-1 in tomato and 1.5x1012 OBs ha-1 in chickpea resulted in significant suppression of pest, lowered the per cent fruit /pod damage and led to the desirable increase in marketable yields. The candidate virus was found compatible with the recommended insecticide endosulfan, Bacillus thuringiensis and egg parasitoid, Trichogramma pretiosum in combined as well as sequential manner. It was concluded that ecofriendly management of this pest on these crops is possible either by use of selected virus alone or their combined application with insecticides at reduced doses.
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