| The diamondback moth, Plutella xylostella(L.), is one of the most destructive pests of cruciferae vegetable and has been reported to produce severious resistance to various convenient insecticides, including organophosphate and pyrethroid insecticides, Bacillus thuringiensis (Bt), acylurea insect growth regulators, abamectin, and DDT, etc.Now tebufenozide has been introduced for control of this pest. Tebufenozide is developed by Rohm and Hass Co. It binds to the ecdysone receptor in insect body, like the steroidal molting hormone, but initiates a premature incomplete molt and results in death of the larva. Tebufenozide appears to be lepidoperan-specific and has been used as an environmentally friendly insecticide. In order to make better use of this insecticide and keep it from developing resistance quickly in diamondback moth, resistance selection has been carried out in laboratory to evaluate the risk, and cross-resistance and resistance mechanisms tested for setting up an effective resistance management strategy.The resistance selection revealed that the resistance to tebufenozide increased slowly at first and then speeded up, and reached 15.7 times at the 7th generation. At the same time, the selection with tebufenozide also resulted in the increase of the LD50 of phoxim, acephate, hexafiumuron, flufenoxuron by 2.051 2.302 1.765 and 2.963 times, respectively. The selected resistant strain even showed 13 time cross-resistance to abamectin. But no changes was found with the LD50 of deltamethrin, cypermethrin and trichlorfon during the selection.The detoxicating metabolism enzymes were compared between the susceptible and the resistant strain of diamondback moth. It was found that the activity of all the enzymes tested was higher in the resistant strain than in the susceptible strain. Some difference was also found between sexes. Comparing with the susceptible larvae, the activity of esterase was 1.69 times higher in the resistant females and 1.19 times higher in the males. The affinity of the esterase in resistant larvae decreased obviously, Km increase by 43.55 and 1.93 times, respectively. The activity of glutathione-s-transferases was 1.25 higher in the female and 1.57 times in the males. However, the mixed function oxidases showed no sex difference. The resistant larvae of both sexes had 1.3 times higher activity. Finally, it wasconcluded that the activity increase of the detoxicating enzymes should contribute to the resistance of diamondback moth to tebufenozide.The effects of tebufenozide resistance on fitness of diamondback moth were evaluated in terms of developmental and reproductive characteristics. Life tables of tebufenozide-resistant and susceptible strain were constructed and the relative fitness was determined by comparison of the net reproductive rate (R0). The results indicated that the resistant strain possessed reproductive disadvantages including decreased percentage of female copulation, lower egg production of female and decreased percentage of egg hatching when compared with the susceptible strain. However, no developmental defect was observed. The resistant strain was calculated to have a fitness value of 0.61 relative to the susceptible strain.In this investigate, it was found that tebufenozide have risk of developing resistance in this pest, though it has a special mode of action. We should pay attention on the rational use of insecticides and management of resistance. Because the resistance at developing stage depends on the enhancement of the activity of the detoxicating metabolism enzymes, as the results showed, we should consider of adding synergists to the insecticide formulations to increase the effect of tebufenozide on the resistant diamondback moth and delay the resistance development. Otherwise, the decreasing of the fitness of resistant strain indicate that limited use of this insecticide and alternate use it with some other insecticides without cross-resistance can be also a effective method for the resistance management.
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