مشخصات پژوهش

Combination of pesticides with different mode of actions, may synergistically affect insect pests. Beet army worm, Spodoptera exigua, is a polyphagous insect that targeted by different pesticides in different crops. Intensive use of insecticides increased resistance potential of this pest. In this study, synergistic effects of Bt Var. Kurstaki in combination with ethanolic extract of Delphinium linearilobum were assayed against first larval instar of S. exigua. Insects were reared on artificial diet at 25 ± 2 °C, 50 ± 5 % RH and a photoperiod regime of 16: 8 (L: D) h. Plant materials collected, well washed with distilled water and were air dried under shadow. Extraction was carried out by maceration method and rotary evaporator was used (120 rpm, 50 °C) for solvent recovery. A standard bioassay method was used for dietary toxicity against first larval instars of S. exigua. Combination of different concentrations, including LC10, LC30 and LC50 values of plant extract and Bt, were used for determining the co-toxicity effects. A modified Chi-square test (developed by Koppenhöfer) was used for determining the type of co-toxicity effects including synergistic, additive or antagonistic. The results revealed that, toxicity of tested products increased as exposure time progressed. LC50 values of Bt during 2, 3, 4 and 5 days of dietary exposure, were calculated as 2.31 (1.64-3.33), 1.12 (0.83-1.49), 1.04 (0.77-1.39) and 0.85 (0.61-1.16) gr / L, respectively. Similarly, LC50 values of extract in same times were obtained as 26.41 (8.52- 56.35), 12.82 (3.62-46.92), 8.12 (3.34-27.30) and 3.62 (1.45-10.52) gr /L, respectively. Microbial insecticide (Bt), showed more toxicity comparing with ethanolic extract of D. linearilobum during similar exposure times. Co-toxicity of LC10 values of Bt with LC10, LC30 and LC50 values of plant extract, after 48 h, were recorded as 33.33, 42.31 and 53.33 %, respectively and their related synergistic indexes were calculated as 4.76, -10.35 and -1