Toxic and Synergistic Action of α-Amyrin Acetate with Bacillus thurigiensis Berliner Aganist the Diamonback Moth, Plutella xylostella Linn. (Lepidoptera: Plutellidae)

Abstract

The diamondback moth, Plutella xylostella L. is the most destructive insect pest attacking crucifers. Toxic and synergistic activity of α-amyrin acetate with Bacillus thuringiensis Berliner (Bt) were evaluated by adopting drench and leaf dip bioassays for 24, 48 and 72 hours. Bt treatments showed most of the larval mortalities occurred after 24 hours. The LC₅₀ values were 5.77µg/ml, 6.63µg/ml,6.80 µg/ml, 7.22 µg/ml and 122.35 µg/ml, 124.70 µg/ml, 134.28 µg/ml, 145.97 µg/ml for 1^st, 2^nd, 3^rd, 4th larval instars at 72hrs of α-amyrin acetate and Bt treatment respectively. α-amyrin acetate was toxic to all the larval instars. Higher concentration of α-amyrin acetate caused high mortality of larvae even though only small portions of the leaf disc was consumed and larval death was due to a combination of poor feeding and midgut membrane disruption of α-amyrin acetate. Susceptibilty was observed to decrease with increasing larval age. The α-amyrin acetate had an enhancing synergistic influence on the Bt with synergistic factors ranging from 104.81(1:1), 129.47(1:2) to 142.41 (1:4) for first instar larvae; 89.36(1:1), 118.35 (1:2) to 146.83 (1:4) for second instar larvae; 76.96 (1:1), 112.57 (1:2) to 175.30 (1:4) for third instar larvae and 65.31 (1:1),117.01(1:2) to194.73(1:4) for fourth instar larvae at 72 hrs. The combination treatments being more efficient than the individual α-amyrin acetate or Bt treatments alone. Such integrated approach has important implication in the environmentally safe control of pests.