Resistance Monitoring And Resisance Mechanism Of Rhopalosiphum Padi(Linnaeus) To Beta-cypermethrin

The bird cherry-oat aphid, Rhopalosiphum padi(Hemiptera: Aphididae), is a serious pest which damages the wheat by sap-sucking. This pest can transmit a serious virus(barley yellow dwarf virus, BYDV), and caused enormous economic loss of wheat yield annually in the world. R. padi distributes in wheat-growing region of China. Pyrethroids are important synthetic insecticides and are widely used in agriculture and health pest control. Extensive use of insecticides has contributed to the development of insecticide resistance in the target insects. To evaluate the resistance level of R. padi to various insecticides and detect the mutations of voltage-gated sodium channel related to pyrethroids resistance, we collected 12 field populations of the pest from different provinces of China, analyzed the regional susceptibilities of the field populations to 10 insecticides by the leaf dip method and amplificated the IIS4–IIS6 region of the voltage-gated sodium channel gene. Furthermore, we cloned the full length of the R. padi sodium channel gene. Additionally, a beta-cypermethrin-resistant strain was set up, the biochemistry and the molecular biology mechanism of resistance was analyzed.1. The toxicity of different insecticides to the susceptible strain of R. padiThe leaf dipping method was adopted for the toxicity bioassay of 10 insecticides. Among the 10 insecticides tested, bifenthrin was found to be the most toxic to R. padi, with the LC50 value of 0.2959 mg/L, followed by decamethrin(0.4647 mg/L), imidacloprid(0.7342 mg/L), thiamethoxam(0.7433 mg/L), acetamiprid(0.9809 mg/L), malathion(1.0654 mg/L), beta-cypermethrin(1.103 mg/L), chlorpyrifos(1.138 mg/L), abamectin(3.0293 mg/L) and pymetrozine(10.7687 mg/L).2. Resistance monitoring of field populations to diffenrent insecticidesWe analyzed the resistance of 12 R. padi field populations to 10 insecticides. The R. padi populations showed susceptible or minor resistant to chlorpyrifos, malathion, thiamethoxam, beta-cypermethrin and pymetrozine, whilst showed minor resistant to moderate resistant to acetamiprid, imidacloprid, bifenthrin, decamethrin and abamectin. Correlation analysis indicated positive and significant correlations between R. padi resistant level to thiamethoxam and beta-cypermethrin, between R. padi resistant level to chlorpyrifos and four other insectides(decamethrin, abamectin, acetamiprid and thiamethoxam), as well as R. padi resistant level between acetamiprid and three other insectides(decamethrin, thiamethoxam and beta-cypermethrin). A slightly negative correlation was found between chlorpyrifos and bifenthrin. There was a lack of significant correlation for malathion, pymetrozine and imidacloprid in populations of R. padi from China.3. Mutations in IIS4–IIS6 region of the R. padi sodium channel geneMutations in IIS4–IIS6 region of sodium channel contains many mutation sites that have previously been shown to confer resistance to pyrethroids in other species. In this study, corresponding fragments in IIS4–IIS6 region of R. padi sodium channel were subsequently amplified from R. padi field populations and susceptible population. HNN population showed a single base difference in their sequencing traces, which demonstrated the presence of the super-kdr mutation(M918T) in these samples, but the other known mutation sites were not detected in all of the test samples.4、Cloning of voltage-gated sodium channel genes from R. padiIn this study, VGSC of R. padi was cloned which was a unique heterodimeric VGSC coded by two genes, unusually for insect channels, and contains the highly conserved MFM motif. However, there was a channel’s selectivity filter motif(DENS) which was different from the standard sodium selective DEKA filter found in other eukaryotes. The open reading frame(ORF) of Rpvgsc1 was 3450 nucleotides long, encoding 1150 amino acids, and ORF of Rpvgsc2 was 2874 nucleotides long, encoding 957 amino acids. Sequence analysis shows that VGSC of R. padi shares high(64.1 %(DI–DII); 64.0%(DIII–DIV)) amino acid identity with equivalent domains in Drosophila melanogaster VGSC.5、Selection of beta-cypermethrin resistant strain of R. padiAfter 20 generations selection with beta-cypermethrin, the resistant level of R. padi to beta-cypermethrin was 9.89-fold and slope(b) value of toxicity regression lines have decrease tendency.6、Resistance mechanisms of R. padi to beta-cypermethrinBiochemical assay showed that the activity of Car E in the resistant strain was 1.63 fold of which in susceptible strain, and the activity of MFO in the resistant strain was 1.90-fold of which in susceptible strain. The previous reported mutations on other insects were not detected in all of the test resistance samples. The results showed that the Car E and MFO might played an important role in the resistance development of resistance of R. padi to beta-cypermethrin.7、Sublethal effects of beta-cypermethrin and indoxacarb on R. padiEffects of sublethal concentrations(LC10 and LC30) of beta-cypermethrin and indoxacarb on fecundity and longevity of two successive generations of R. padi were studied under laboratory using life tables. The results showed that two insecticides could significantly reduce the fecundity and longevity of two successive generations of R. pad. Sublethal concentrations of the two insecticides significantly prolonged the development of nymph on subsequent generation.