Bacillus thuringiensis (Bt.) insecticide has since been used as an essential insecticide in commercial oil palm plantations to manage insect pests. Resistance against Bt. toxins has been reported in many pest insects and threatening the effectiveness of Bt. insecticide. However, the insecticide resistance capability of Tirathaba mundella (T. mundella) was not well documented due to its finite molecular data. In this study, the potential of insecticide resistance development in T. mundella was examined through bioassay and cadherin gene expression analysis. Cadherin gene which often associated with the resistance against Bt. toxin was amplified and its relative expressions were studied using quantitative polymerase chain reactions. T. mundella collected from the plantations with frequent exposure to Bt. insecticide was found to develop resistance to Cry1A with a significantly lower cadherin gene transcript level and lower mortality compared to the susceptible strain. Besides, a relatively lower level of cadherin gene expression was found in the early development stages of the resistant strain. The adaptation of larvae to the Bt. insecticide may be caused by the irregular application of insecticide in the field.

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