First report of resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) to lambda-cyhalothrin from Pakistan: baseline susceptibility, selection, occurrence of cross-resistance, realized heritability, and inheritance mode of resistance.

Fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a global pest causing damage to several crops. However, its management using chemical control is a challenge due to its capacity to evolve resistance to insecticides. After 6 generations of selection with lambda-cyhalothrin, the LC50 for the insecticide-resistant strain (Lambda-Sel) was 486 ppm, higher than that of the field strain (FAW-MUL) (7.5 ppm), susceptible laboratory strain (Lab-PK) (0.46 ppm), and laboratory unselected strain (UNSEL) (5.26 ppm). Laboratory selection with lambda-cyhalothrin increased resistance from 16.3- to 1056.52-fold and 1.43- to 92.4-fold to lambda-cyhalothrin compared to Lab-PK and UNSEL strains, respectively. The selected strain of S. frugiperda (Lambda-Sel) presented low cross-resistance to chlorpyrifos, moderate to deltamethrin and indoxacarb, very low to spinosad, and no cross-resistance to emamectin benzoate. The realized heritability (h2) of lambda-cyhalothrin resistance in the Lambda-Sel strain was very high (0.88). The reciprocal cross progenies of F1 (Lambda-Sel ♀ × Lab-PK ♂), F1' (Lambda-Sel ♂ × Lab-PK ♀), BC1 (F1 ♀ × Lambda-Sel ♂), and BC2 (F1 ♀ × Lab-PK ♂) showed high resistance ratios of 545.64-, 396.52-, 181.18-, and 146.54-fold, respectively compared to Lab-PK. The degree of dominance values for lambda-cyhalothrin in F1 and F1' indicates incompletely dominant resistance. The difference between observed and expected mortality in backcross populations (BC1 and BC2) revealed a polygenic resistance. In conclusion, the resistance to lambda-cyhalothrin was autosomal, incompletely dominant, and polygenic. These findings provide new insights for insect resistance management strategies to mitigate the occurrence of resistance in this global pest.