Susceptibility of Asian corn borer (ACB),
Ostrinia furnacalis (Guenée) to
Bacillus thuringiensis (Bt) Cry1Ab protein was studied between 2015 and 2016 with 11 ACB populations, collected from various geographical regions in Vietnam. A concentration range of Cry1Ab from 0.20 to 26.10 ng/cm
2 of diet was evaluated against F
1 ACB neonates using diet surface-overlay bioassays. Mortality data was recorded daily until seven days after infestation. Growth inhibition was recorded at the end of seven days. The median lethal concentration (LC
50) varied ≈3-fold among the different populations, ranging from 0.58 to 1.83 ng/cm
2 of diet with an overall mean of 0.86 ng/cm
2 of diet. Even the lowest concentration of 0.20 ng/cm
2 caused 73.53% growth inhibition. >90% growth inhibition was achieved at 0.82 ng/cm
2 or higher concentrations. The results reflect natural variation in Bt susceptibility among ACB populations rather than variation caused by prior exposure to selection pressures. LC
99 value (17.26 ng/cm
2) was generated by pooling mortality data across different populations. The upper fiducial limit of LC
99 (24.38 ng/cm
2) could be a potential diagnostic dose for future resistance monitoring programs. The findings from this study suggest that ACB populations in Vietnam are highly susceptible to Cry1Ab protein. This is the first report of Cry1Ab susceptibility of different ACB populations in Vietnam and will serve as a baseline for future resistance monitoring work.
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