The diamondback moth (DBM), Plutella xylostella, is considered as the most notorious insect pest species of crucifers due to its rapid ability to develop resistance against various kinds of insecticides, including diamide – a novel class of insecticide that targets the insect ryanodine receptor (RyR). Recently, a diamide-resistant DBM population from Sudlon, Cebu was reported to exhibit an irreversible G4946E mutation in the RyR. Thus, in order to monitor and design an efficient insecticide resistance management strategy for diamide, the relative potency of flubendiamide to DBM populations from various cabbage-growing areas in the Philippines was estimated using the leaf-dip bioassay method. The DBM populations were then genotyped for the presence of target-site mutation G4946E in the RyR using pyrosequencing technology. The bioassay results revealed the continuing presence of high levels of flubendiamide resistance in the DBM Sudlon strain (LC₅₀ = 270.6 ppm). The DBM populations from Majayjay, Laguna were also found to be flubendiamide-resistant (LC₅₀ = 218.195 ppm), as well as strains from Calauan, Laguna (LC₅₀ = 200.3 ppm), and Buguias, Benguet (LC₅₀ = 135.02 ppm). Genotyping results confirmed the presence of the RyR target-site mutation G4946E in these resistant populations. Both Calauan and Cebu strains were 100% homozygous for the resistance allele while the Majayjay strain is composed of 90% and 10% homozygous and heterozygous for the resistance allele, respectively. On the other hand, the Buguias strain exhibited 33% homozygosity and 44% heterozygosity for the resistance allele and 22% homozygosity for the susceptible allele. The data in this study suggest that diamide resistance allele for RyR mutation G4946E is still spreading, and therefore, requires immediate attention to design efficient insecticide resistance management programs on diamide use against DBM.