Resistance Mechanisms Of Western Flower Thrips(Frankliniella Occidentalis) To Spinosad

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an important invasive peston horticultural crops worldwide. Great economic losses have been caused by F.occidentalis since itsfirst invasion to China in2003. Spinosyns (spinosad and spinetoram), which is a new class of naturalproducts for insect control with high-efficiency and low-toxicity, is one of the best kinds of insecticidesto control western flower thrips at present. However，it is of great risk for every insecticide to developresistance when used unreasonably. In order to delay the development of resistance to spinosad inwestern flower thrips, monitoring of the field resistance, resistance selection in laboratory andcross-resistance paterns, the activities of detoxicifing enzymes, cloning of the gene encoding nAChR α6subunit and its relative expression levels both in susceptible and resistant strains were carried out toevaluate the resistance risk and explore the resistance mechanisms of F.occidentalis.to spinosad.1. Field survey for insecticide resistance in F. occidentalisThe toxicity of7insecticides to4field F. occidentalis populations collected from Beijing andYunnan province had been tested using leaf-tube residue method in2011. Different resistant levels ofthe F. occidentalis populations were revealed. All of the populations were sensitive to abamectin andchlorpyrifos. Contrarily, higher resistance to emamectin-benzoate and spinetoram existed in2Beijingpopulations and1Yunnan population. Thiamethoxam had little effective on F. occidentalis in ourcountry. Though spinosyns was the best kind of insecticides to control F. occidentalis, times forspraying this kind of insecticides should be limited to avoid developing resistance.2. Resistance selection, cross-resistance and biochemical mechanisms of resistance to spinosad inF. occidentalisA spinosad resistant strain of F. occidentalis with the resistance ratio11999.34-fold compared tothe corresponding susceptible strain, was obtained by continuous selection with spinosad for about fiveyears. The resistant development could be divided into two stages:(1) slowness stage from the first to33rd generation, during which the resistance developed slowly with a resistance ratio30-fold atgeneration33.(2) fast ascending stage from33to46generation, during which the resistance developedrapidly and the resistance ratio was up to11999.34-fold at generation46.The resistant strain had an obvious cross-resistance to spinetoram and thiamethoxam, with theresistance ratio53718and84-fold, respectively. And its sensitivities to abamectin and chlorpyrifoswere also decreased slightly, the resistance ratio were3.33and2.28-fold respectively. But nocross-resistance to chlorfenapyr was observed, with the resistance ratio0.56-fold. The activities ofCarE、GSTs and MFOs of the resistant strain were all not significantly different from that of itssusceptible counterpart. It indicates that the metabolic mediated detoxification does not play a role inthe resistance of F. occidentalis to spinosad, suggesting that the altered nicotinic acetylcholine receptorwhich is the target site of spinosad may involve in the high resistance.3. Cloning of the cDNA of nAChRα6(Foα6) from F.occidentalisThe cDNA of nAChRα6was cloned by using RT-PCR and RACE techniques in spinosad-resistantand insecticide-susceptible F.occidentalis and was named Foα6. It is of1873bp and the open reading frame is1458bp which encodes485amino acid residues，the5’ and3’ UTRs are294and121bprespectively. It shares high identity with nAChRα6genes of other insects. There are two amino acidresidues deleted in the resistant strain compared to its susceptible counterpart. The prediction of3Dstructure about the deduced amino acid residues revealed that the deletion situated at the secondtransmembrane domain.4. Relative expression levels of Foα6in spinosad-susceptible and resistant strains of F.occidentalisReal time PCR was used to test the relative expression levels of Foα6in spinosad-susceptible andresistant strains of F.occidentalis, during which actin was acted as the reference gene. The resultsshowed that there was no significant difference in the relative expression levels of Foα6betweenspinosad-susceptible and resistant strains of F.occidentalis in different instars tested in the experiment.