Abstract: The resistance selection and patterns of cross-resistance of diamondback moth (DBM),Plutella xylostella (L.), to chlorfluazuron, a benzoylphenylurea (BPU) insect growth regulator(IGR) that interferes with chitin synthesis, were investigated during 1995-1996. Laboratory selection of a Shenzhen strain (SZ-S) of DBM, having been reared in laboratory for 6 years since itscollection in 1990, by chlorfluazuron for 6 generations resulted in 23.78-fold resistance to chlorfluazuron, and quick reversion to susceptibility was observed when the culture was kept without exposure to the chemical pressure. Cross-resistance to conventional insecticides cyhalothrin, cypermethrin, phoxim, quinalphos, methomyl, microbial insecticide Bt, abamectin and diafenthiuron wasnot apparent. Negative cross-resistance was demonstrated to two nereistoxin derivatives, cartap(0.21) and shachongdan (0.23) . The results of in vivo tests showed that addition of PBO or TPP had no effect on the toxicity of chlorfluazuron in the SZ-S strain, while PBO or TPP significantly increased the toxicity of chlorflu-azuron in both SH-R and CH-R strains, with synergism ratios ranged from 15.42 to 29.71-fold and 2.74 to 2.87-fold based on comparison of LCs0 and PBO restored the effectiveness of chlorfluazuroncompletely, indicating that MFO is the major resistance mechanism.

Key words: Plutella xylostella, chlorfluazuron, resistance selection, cross-resistance, resistance mechanism