| Abstract: | Abstract Two resistant strains of diamond back moth Plutella xylostella L. were treated with dimehy-po and cartap in succession, The susceptible strain had never contacted with any insecticieds since reared in the in sectary. The rearing method by using vermiculite and radish seedling was discribed by Chen et al. (1990) and Liuet al. (1993). Comparison between reared strains and field strain did not display any difference in biological characteristics. The resistance reached 178 fold in dimehypo resistant strain in F85, and 87 fold in cartap resistant strain in F80, Two high level resistant strains had formed. After termination of selection, the resistance declined from 167 to 57 fold in dimehypo resistant strain and from 74 to 16 fold in cartap resistant strain within five generations. The resistance of diamond back moth to the two insecticides was unstable at high level, but could be steady at quite lower degree for a long time. It seemed impossible to regain the same sensitivity as before selection for the two resistant strains after resistance declining. |
