Resistance Of Bemisia Tabaci To Chlorpyrifos And Its Mechanisms

The common pest, Bemisia tabaci, is a worldwide pest with an extensive host range and especially infests some important crops including cotton, tobacco, soybean, tamato and many other vegetables. It formerly broke out only intermittently in China. Since the early 1990s, however , Bemisia tabaci has reached large number every year and large amount of insecticide has been used for combating its infestation on crops. Though some related prevention work has been done, there is still not a suitable method and enough toxicity baseline data for monitoring of the resistance in the pest. Lacking of the knowledge of resistance mechanism is also the barrier for implementation of resistance management and efficient control. In this paper, We made efforts to develop a suitable resistance monitoring method and declare its resistance mechanism. At the same time, We select the chlorpyrifos resistance by Bemisia tabaci, and further to investigated the biochemical machanismas of the resistance. The main results are as follows:1 Resistance selecting with chlorpyrifosWe selected bemisia tabaci with chlorpyrifos in the laboratory, And after 13 generations, its resistance increased 33.9 times compared with relatively susceptible populations; Also, the agar with 1.3% concentration is the best, and after 48h the livability is 95%. When we transferred bemisia tabaci, the worms can come round from 30s to 60s with anaesthesia in 10s-20s, and its livability can achieve 100%. This research indicated the status of the worm's resistance increase.2 Resistance measurement of field populations to common used insecticidesIn order to know the relative resistance with different insecticides from different area, we determined the toxicity of bemisia tabaci from Nanjing, Beijing, Wuhan and Guangzhou with chlorpyrifos, fipronil, beta-cypermethrin, carbosulfan, abamectin and imidacloprid. The result indicated that fipronil and abamectin displayed the highest toxicity to bemisia tabaci, while chlorpyrifos the lowest. However, there are great differences in toxicity of same insecticide to different Bemisia tabaci populations. Guangzhou population is most susceptible to fipronil, while Beijing population is the lest (20 times less); however, Beijing population showed the highest susceptibility to abamectin, while Nanjing populations is lest susceptible (20times less); and to chlorpyrifos, Guangzhou population is most susceptible, while Nanjing and Wuhan populations are the lest (7times less). As to other three insecticides, there is no obious difference in the susceptibility among different Bemisia tabaci populations. With the differences in the toxicities among Bemisia tabaci populations, it should be wise to choose different insecticides in different areas when controling the Bemisia tabaci.3 Synegism of bimixture of fipronil and beta-cypermethrinWe determined the Synegism of bimixture of fipronil and beta-cypermethrin with the ratio of 1:1, 1:3, 1:5, 1:7 and 1:9, respectively. The common toxic coefficients with all ratios are higer than 100%, and the CTC achieves 1031.69 with the ratio 1:7. It suggests that using this bimixture could increase the control efficiency, and that the ratio of 1:9 is recommended for use to decrease the control cost.4 Mechanisms for chlorpyrifos resistance of bemisia tabaciSynergism experiment in vitro with TPP, PBO and DEM was performed to see the roles of detoxicating enzymes in chlorpyrifos resistance. The result revealed that TPP had little synergism effect on chlorpyrifos in Wuhan and Guangzhou populations; and DEM had no synergism effect in all populations. Enzyme activity mearsuremt results showed that esterase activity of Wuhan populations was much higher than susceptive population (SR=4.06); The activity of microsomal O-demethylase were obviously different among populations. Compared with susceptive population, the other three populations were obviously different in both Vmax and Km. Furthermore, the I₅₀ of chlorpyrifos on AChE of resistance population was 8.53 times than in susceptive population. The molecular machanisms for this AChE insensitivity by chlorpyrifos need to be further explored.