The population dynamics and insecticide resistance of the diamondback moth, Plutella xylostella (L.), were investigated on

cruciferous vegetables in a Hohhot suburb, and on rapeseed in Wuchuan County, Inner Mongolia from 2009 to 2011. In the

suburb of Hohhot city, adult moths began to occur in early April and had four peaks of abundance during the year: late April

or early May, mid June, late August and early October. Larvae were continually present during the growing season of

cabbage and Chinese cabbage and had four peaks of abundance: early midJune, late June or early July, early September and

early October. In Wuchuan county, adults first appeared in late April or early May and there were more adults in spring and

summer than in autumn. In 2009 two peaks occurred in late May and midJune but in 2010 peaks occurred in early June and

early to midAugust. Larvae began to appear in late June and had three peaks during the rapeseed growing season in early

July, late July and late August. There were more larvae in autumn than in spring and summer. The toxicity of eleven

insecticides to larvae was tested using the leaf dipping bioassay method. The most toxic was Chlorfenapyr with a LC50 value

of 022 mg·L-1, followed by spinosad，chlorantraniliprole, fipronil and BTCrvlAc, with LC50 values of 082-083、109

、 136 and 341 mg·L-1, respectively, indoxacarb and abamectin with LC50 values of 1574 mg·L-1 and 603-2347 mg·

L-1, respectively, diafenthiuron, tebufenozide and chlorfluazuron with LC50 values of 5130、5266 and 6191 mg·L-1,

respectively, and finally alpha\|cypermethrin with LC50 values of 46266-67326 mg·L-1. A resistance bioassay of P.

xylostella larvae to 11 insecticides indicated no resistance to chlorfenapyr, tebufenozide, chlorantraniliprole and

diafenthiuron, low resistance to spinosad and fipronil, moderate resistance to indoxacarb and BTCrvlAc and high resistance

to alpha\|cypermethrin, chlorfluazuron and abamectin.