Study the interaction effects of combined application of some native strains of Bacillus thuringiensis and Nucleopolyhedrovirus on control of Helicoverpa armigera

Helicoverpa armigera is one of the most important tobacco pests in all over the world. Today, due to the problems of using chemical pesticides such as environmental pollution and endangering the health of humans and other organisms, high costs and the problems of pest resistance, has led to more attention to use of IPM method and important biological control agents such as Bacillus thuringiensis (Bt) and Nuclear Polyhedrosis Virus (NPV). In this research, the interaction effects of combined use of 5 superior Bt isolates and one superior NPV isolate that in previous research of authors, were isolated from different parts of the northern of Iran, the investigation was conducted by combining two concentrations of Bt (2 × 107 – 1 × 107 spore/ml) and two concentrations of NPV (103 – 104 POB/ml). The mortality of 2th larvae of H. armigera was compared among the pathogen treatments on a completely randomized factorial design with 20 treatments and 3 replications under laboratory conditions in plant protection department of Tirtash Research and Education Center through 2016. The results showed that Bt isolates (12, 73, 72, 50) with (2 × 107 spore/ml) concentration in combination with NPV (104 POB/ml) had the most lethal effect, so that on 7 and 14 days after treatment, they had 100% lethal effects. The lowest mortality related to Bt-58 with (1 × 107 spore/ml) in combination with NPV (103 POB/ml), which on the 4Th. day after applying the treatment, it caused only 42/2% mortality. LC50 of superior isolate (Bt-72) was equal to 7 × 106 spore/ml and superior isolate (NPV-10) was equal to 8/1 × 104 POB/ml. The results revealed that 1 × 107 spore/ml concentration of Bt in combination with both two NPV concentrations and also Bt (2 × 107 spore/ml) in combination with 104 POB/ml showed Additive effects. Synergistic interaction effects were observed in combination of the highest concentration of Bt (2 × 107 spore/ml) and the lowest concentration of NPV (103 POB/ml). Generally, the results of this research showed that the combination of (Bt + NPV) leads to increase their pathogenicity and mortality.