Production Of Spore-δ-entotoxin Of Bacillus Thuringiensis Strains And Studies On Cross-resistance Of Cry1Ac And Bt-based Biopesticide Resistant Helicoverpa Armigera

The present study elaborates on the results of cross-resistance of Cry1Ac andBt-based biopesticide resistant Helicoverpa armigera (Hübner) to various Btstrains (HD-1, HD-4, HD-8, HD-57, HD-133, HD-234, HD-263, HD-282, HD-551,HD-744 and Bt-based biopesticide. In this study, all the bioassays were done onlarvae of 55th, 60th and 61st Cry1Ac and 58th, 64th, and 65th Bt-based biopesticideselected generations of H. armigera. The resistance ratios (RR) among Cry1Acand Bt-based biopesticide selected generations varied from 417 fold in 46thgeneration to 172 fold in 55th generation of Cry1Ac selection and from 21 fold in58th generation to 15 fold in 67 generation of Bt-based biopesticide selection.Cry1Ac selection exhibited minimum resistance ratio (2.722, 2.756) against HD-1and HD-8 among all the HD strains tested and the slopes for HD-1 and HD-8were highly significant (1.49 and 1.31 respectively). The test insect also showedlow resistance ratios 3.959 and 4.385 for HD-551 and HD-57 respectively withhighly significant slopes 1.18 and 2.36 accordingly. The spore-crystal toxins ofstrains of HD-1, HD-8 and HD-263 and Bt-based biopesticide caused significantreduction in larval wt.(%) with regressions y=0.011x+93.912, R~2=0.7269, df=4,p＜0.05, and y=0.03x+85.332, R~2=0.6982, df=4, p＜0.05, y=0.0458x+79.58,R~2=0.7271, df=4, p＜0.05, and y=0.0776x+59.169, R~2=0.8273, df=4, p＜0.05,respectively against Cry1Ac selection. Significant slopes 1.49, 1.31, 2.36, 1.67,1.18, 1.12 and 2.09 were observed for HD-1, HD-8, HD-57, HD-263, HD-551, HD-744 and Bt-based biopesticide respectively which indicated fast interactionbetweenspore-crystal toxins of Bts and the Cry1Ac resistant insect. Bt-based biopesticideselection of H. armigera showed low resistance ratios (1.12, 1.10, 1.82, 1.95,2.09, and 1.55 for spore-crystal toxins of HD-1, HD-8, HD-133, HD-263, HD-282,and HD-744 respectively). The linear regressions between larval wt. reduction(%) and toxin concentrations were significant for HD-263, HD744 and Bt-basedbiopesticide with equations y=0.0937x+82.891, R~2=0.7832, df=2, p＜0.05, y=0.0421x+92.930, R~2=0.8869, df=4, p＜0.05, and y= 0.01163x+44.382, R~2=0.7975, df=2, p＜0.05) respectively. Previous studies conducted by otherworkers indicated synergistic impact between spores and crystals of Bt strainsagainst resistance insects. Our results suggest that the spore-crystal complex(perhaps due to synergistic interaction between or among the spore and crystalcomponents) may have caused synergism due to which a Cry1Ac selection wasknocked down and spore-crystal complex-based formulations of Bts can bedeveloped for use as an effective tool to manage insect populations that havedevelopment resistance to single toxin (Cry1Ac) produced in transgenic crops(e.g. Bt-Cotton).