Risk Assessment And Resistance Mechanism To Thiamethoxam In Frankliniella Occidentalis(Thysanoptera:Thripidae)

The western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an important pest of various crops in the worldwide. It was found first in Beijing in2003, and then spread throughout other nine provinces of China. In some of these provinces, western flower thrips has made abundant loss. Its particular biological characteristics, such as small size, high polyphagy and high fecundity make this thrips very difficult to control. And now, the thrips has developed resistance to many different insecticide classes. Thiamethoxam has been introduced into market for10years, and there were only a few kinds of pests have been recorded developed resistance to thiamethoxam in field. What’s more, thiamethoxam has high toxiy to western flower thrips, so thiamethoxam is an efficiency insecticide on controlling thrips.In order to use thiamethoxam scientifically, firstly, we assessed the resistance risk of thiamethoxam, secondly determined the cross-resistance with other sixteen insecticides, and then carried the resistance mechanism of thiamethoxam on western flower thrips. The results is essential for design the resistance management strategies.1. Risk assessment of F. occidentalis to thiamethoxamTo further assess the resistance risk for thiamethoxam, F. occidentalis was selected with thiamethoxam from27to50generations on the basis of Shan’s study. The resistance realized heritability (h2) of the resistance strain (TH-R), which followed23generations of selection and had a1.72-fold increasing on LC50value compared with the26th generation, was estimated at0.0225. When the selective pressure is50%-90%, the10-folds increase in LC50to thiamethoxam was expected in about44-98generations, suggesting that F. occidentalis has low resistant risk to thiamethoxam. 2. The fitness study of resistant F. occidentalis colonyExperimental population life tables of resistant and susceptible populations to thiamethoxam were compiled and their population number tendenecy index and fitness determined. The results showed that compare to the TH-S strain (60.17±5.20,75.37±2.82and47.69±1.80, respectively), the TH-R strain has a significant higher survival percentage of1st insrar (72.17±6.01), pupation percentage (83.68±2.04) and number of eggs laid per femal (54.03±3.18), while, and whose relative fitness compare to the susceptible was0.64. The results indicate that resistant population was less adaptable and had a competitive disadvantage with regard to the susceptible population. We could stop the use of thiamethoxam as F. occidentalis resistant to this insecticide to adverse resistant populations survive, thus delaying the development of resistance.3. Cross-resistance of F. occidentalis to thiamethoxam16insecticides belonging to different chemical classes were chosen to evaluate the cross-resistance against thiamethoxam because they had high efficacy against the larvae’ and adults of this thrips. Compared with the TH-S strain, the TH-R strain selected for54generations showed an extremely high level of cross-resistance to the neonicotinoid imidaclothiz (392.07-fold), significant cross-resistance to dinotefuran (5.74-fold). However, there was no significant cross-resistance to another fourteen insecticides. Emamectin benzoate, spinosad, spinetoram, acetamiprid, chlorpyrifos, phoxim, profenofos, butylene-fipronil, carbosulfan and benfuracarb could be chosen as alternated or replaced insecticides to thiamethoxam. Bifenthrin, imidacloprid and nitenpyram be used as alternatives or replacements to thiamethoxam when the field population is low. But imidaclothiz and dinotefuran were not suggested as replaced insecticides to thiamethoxam in field.4. Resistance mechanism of F. occidentalis to thiamethoxamBiochemical mechanisms of resistance were investigated after the moderate resistant colony was obtained by selecting with thiamethoxam. Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) exhibited significant synergism on thiamethoxam in the TH-R strain (2.58-and2.60-fold respectively), s,s,s-tributyl phosphorotrithioate (DEF) also exhibited synergism on thiamethoxam in the TH-R strain (1.51-fold) but not significant. And these three synergists were negligible in the TH-S strain. No synergy was showed for diethyl maleate (DEM) in either TH-S or TH-R strain. Biochemical assays showed that cytochrome P450monooxygenase activities, esterase and glutathione S-transferase of TH-R were2.83-,1.52-,1.14-flod higher than that of the TH-S strain, respectively. We concluded that P450-monooxygenases detoxification may play important role in this resistance colony for thiamethoxam resistance developing and esterase may also play some role in thiamethoxam detoxification.