Studies On The Resistant Mechanism Of The Diamondback Moth Plutella Xylostella (L.) To Chlorfluazuron

The results of toxicity test showed that the diamondback moth Plutella xylostella (L.) had developed high resistance to chlorpyrifos, but was sensitive to chlorfluazuron. Toxicity of chlorfluazuron and chlorpyrifos to the different instar larvae of Plutella xylostella (L.) increased with the instar development, it indicated great difference among different instar.The activity of amidase enhanced following the quantitative addtion of amidase. The optimum pH for the enzyme reation was 7.5. The increase of amidase activity was linear in 60min. The amidase activity of the larvae increased with the rise of temperature at different temperatures. While 2 days after treatment at different temperatures, the activity of amidase were all lower than the control, the inhibitive function of amidase increased with the rise of temperature. At different hours after chlorfluazuron treatment, the activity of amidase changed at different degree, the activity of amidase in the first, 2nd and 3rd instar larvae all decreased after treatment for 1-5 hours and dropped to the lowest point after treatment 1 hour. While 10-60 hours after treatment, the activity of amidase in the first, 2nd and 3rd instar larvae fluctuated, the inhibitive function of amidase activity of the 2nd instar larvae was bigger thaflthat of the 3rd and 4th instar larvae. Amidase was found to exist in the head, midgut, fatbody and epidermis of the diamondback moth Plutella xylostella (L.), the total activity of amidase was highest in the fatbody, then in the head, and lowest in the midgut. At different hours after chlorfluazuron treatment with the 4th instar larvae, the amidase activity was all lower than the control, this indicated that chlorfluazuron had different inhibitive function to amidase activity in the different tissues.Selection of larvae of the diamondback moth Plutella xylostella (L.) with chlorfluazuron illuminated the profile of resistance development. The resistancemechanism was investigated.The resistance development of the diamondback moth Plutella xylostella (L.) to chlorfluazuron showed that the velocity of resistance development was form slow to rapid. It was slower among Fo-Fs generations, however, it was suddenly high form Fia-Fis generations. When laboratory-selected FU generations, the resistance ratio was 27.52 fold, as compared with the F0 generation, the diamondback moth Plutella xylostella (L.) had become resistant strains to chlorfluazuron. The susceptibilities of susceptible strains to chlorfluazuron did not reduce.Synergic action of chlorfluazuron with PB> TPP and DEM against larvae of resistant and susceptible strains showed that PB had effective synergic activity in resistant strain, but no synergism in susceptible strain, TPP and DEM had no synergism in both resistant and susceptible strains. The studies on synergism of chlorfluazuron+PB showed that resistance was principally due to the metabolic (multiple function oxidase) mechanism. Toxicological data suggested that more than one factors (genes) were responsible for resistance to chlorfluazuron in the diamondback moth Plutella xylostella (L.).There were no significant differences in the specific activities of amidase among the various developmental stages of the diamondback moth Plutella xylostella (L.), but the total activity of amidase per insect increased sharply with the growth of the developmental stages, and reached the peak at the fourth instar and pupal stage. The activity of amidase in resistant diamondback moth Plutella xylostella (L.) was obviously higher than that in susceptible strain. It was suggested that amidase was an important factor in resistance to chlorfluazuron.The Km was similar in the resistant and susceptible strains, but the Vmax in resistant larvae was markedly higher than that in susceptible larvae, this indicated that the quality of amidase in resistant and susceptible strains was similar. The difference was mainly amidase quantity, not quality. It indicated that the resistance was related to the augment of amidase quantity, not ap...