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Abstract
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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
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