Toxicity of Organophosphate Insecticides and Biochemical Mechanisms Associated With Resistance in Field Populations of Plutella xylostella (L.) From Kashmir, India

ABSTRACT The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a globally important pest of cruciferous crops, for which intensive insecticide use has accelerated the evolution of resistance. However, the magnitude and biochemical basis of organophosphate resistance in P. xylostella populations from the Kashmir Valley remain poorly understood. Here, we quantified susceptibility to chlorpyrifos, dimethoate and quinalphos in nine field populations using leaf‐dip bioassays and investigated associations between resistance phenotypes and activities of acetylcholinesterase (AChE), cytochrome P450 monooxygenases (CYP450) and glutathione S‐transferases (GST). Resistance ratios (RR) varied substantially among populations, ranging from 8.48 to 34.67 for chlorpyrifos, 6.83–76.61 for dimethoate and 3.79–14.19 for quinalphos. Pulwama and Budgam consistently exhibited the highest resistance, whereas Ganderbal was the most susceptible population. Dimethoate resistance was particularly pronounced in Pulwama (RR = 76.61; LC 50 = 17.90 mL L −1 ). At the highest concentrations tested, corrected mortality remained below 50% across populations, although LC 50 estimates exceeding the tested concentration range should be interpreted cautiously. Enzyme assays revealed markedly elevated AChE and CYP450 activities in resistant populations, with increases of up to 34‐fold and 10‐fold, respectively, whereas GST activity showed no significant population‐level difference. Spearman analysis further indicated significant positive associations of AChE, CYP450 and GST activities with quinalphos resistance ( ρ = 0.79–0.91; p < 0.05), but not with chlorpyrifos or dimethoate resistance. These findings demonstrate substantial spatial heterogeneity in organophosphate susceptibility and indicate that biochemical resistance mechanisms are insecticide‐specific rather than uniformly expressed across organophosphate resistance phenotypes. The pronounced dimethoate resistance, despite weak associations with the measured detoxification enzymes, suggests the involvement of additional mechanisms, including target‐site insensitivity, alternative detoxification pathways, or reduced insecticide penetration. The results provide a baseline for resistance surveillance in Kashmir and support mechanism‐informed insecticide rotation and resistance‐management strategies for P. xylostella .