Baseline Susceptibility of Rice Leaf Folder <i>Cnaphalocrocis medinalis</i> (Guenee) to Cartap and Emamectin Benzoate

The baseline susceptibility of the rice leaf folder, Cnaphalocrocis medinalis (Guenee) (Lepidoptera: Crambidae), was determined during 2023–24 for populations collected from major rice-growing regions of Kerala, including Palakkad, Kuttanad, Onattukara, Wayanad and Thrissur. This assessment was conducted against the nereistoxin analogue cartap hydrochloride and the glutamate-gated chloride channel allosteric modulator emamectin benzoate, with comparisons made to the susceptible population from Wayanad. The LC50 values for cartap hydrochloride in the populations from Palakkad, Kuttanad, Onattukara, and Thrissur were 633.87 mg/ l, 583.42 mg/ l, 60.47 mg/ l, and 393.87 mg/ l, respectively, while for emamectin benzoate, the values were 81.09 mg/ l, 52.69 mg/ l, 24.97 mg/ l, and 34.94 mg/ l. These results highlight variations in the susceptibility to these insecticides among the populations. This marks the first reported occurrence of emamectin benzoate resistance in rice leaf folder field populations in India. Enhanced enzymatic activities of carboxylesterase (1.11- to 1.63-fold), glutathione-S-transferase (2.33- to 2.97-fold), and cytochrome P-450 monooxygenase (1.52- to 2.49-fold) noted in these populations were validated through synergism assays. Cytochrome P450 enzymes were identified as a primary mechanism contributing to cartap resistance in most populations, with esterase activity also playing a role in Kuttanad. In the case of emamectin resistance, cytochrome P450 was involved in populations from Palakkad, Kuttanad, and Onattukara, whereas no evidence of metabolic resistance was observed in Thrissur, indicating the possibility of alternative resistance mechanisms that warrant further study.