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Spodoptera litura (Fabricius) is a widely distributed worldwide insect pest, particularly of vegetable crops. To determine the sensitivity of S. litura to insecticides, the toxicities of beta-cypermethrin, chlorpyrifos, emamectin benzoate, methoxyfenozide, and indoxacarb were tested from 2014 to 2016 using a diet incorporation assay on populations of S. litura collected from different districts in Sichuan Province and a susceptible strain (Lab-HN). Compared to the Lab-HN strain, field populations showed high levels of resistance to beta-cypermethrin (267.3- to 1789.2-fold). Field populations exhibited low or moderate resistance to chlorpyrifos in 2014 (8.9- to 24.4-fold) and high levels in 2016 (440.5- to 950.5-fold). All populations collected in 2014 and 2016 were susceptible or had a low or moderate level of resistance to indoxacarb (2.2- to 31.0-fold). No field populations exhibited resistance to emamectin benzoate or methoxyfenozide (0.7- to 1.5-fold and 0.9- to 1.4- fold) in 2014, whereas the resistance in 2016 increased to 29.8- to 55.2-fold and 38.1- to 59.0- fold, respectively. A synergist experiment indicated that the Lab-HN strain and field-collected population from Shuangliu in 2016 (SL16 population) to beta-cypermethrin may be related to cytochrome P450 monooxygenases (P450s), with the highest synergism ratios (SR) of piperonyl butoxide (PBO) of 34.4- and 73.3-fold, but not to carboxylesterase (CarE) or glutathione-S- transferase (GST). We detected the activities of detoxification enzymes and found that the ethoxycoumarin O -deethylase activities were also the strongest in the Lab-HN strain and the SL16 population with the highest SR values of 2.6- and 4.1-fold, respectively. Non-denaturing polyacrylamide gel electrophoresis (native PAGE) indicated that the inhibition of in vivo P450 activity caused by PBO was due to the binding of the synergist to the E5 or E6 isozyme, which were absent in the Lab-HN strain. [ABSTRACT FROM AUTHOR]