Abstract:The resistance to methamidophos and dichlorvos in the field populations of Plutella xylostella (Lepidoptera: Yponomeutidae), collected from Fuzhou, Fujian, China from October 1998 to January 2003, was determined using a leaf-dipping method with 3rd instar larvae. P. xylostella‘s resistance to insecticides, methamidophos and dichlorvos, was significantly higher in autumn and spring than in summer. Apparently, seasonal variations of the insecticide resistance existed in the field population of Plutella xylost...