Research Of The Sublethal Effects Of Nitenpyram On Brown Planthopper Nilaparvata Lugens(Stal)

Brown planthopper Nilaparvata lugens?Stal??Homoptera: Delphacidae?,is one of the most significant rice pests found throughout tropical,subtropical and temperate regions.At present,the control of brown planthopper?BPH?predominantly relies on the use of synthetic insecticides.However,due to the excessive use and abuse of these insecticides,BPH has developed resistance to many of the major classes of insecticides.After application in the field and subsequent environmental changes,the toxicity of insecticide gradually decreases over time,resulting in sublethal doses.Exposure to sublethal doses may cause changes in many aspects of insects,including biological characteristics?growth and development,fecundity?,insecticide resistance development,and ecological behavior of the surviving individuals and offspring.These are called sublethal effects of insecticides.In this study,changes in biological characteristics,insecticide tolerance or resistance development and the expression of resistant related genes and fecundity related genes were investigated in both a susceptible strain and a filed strain of BPH after exposure to the corresponding sublethal concentration of nitenpyram for six generations.Our findings help in predicting population dynamics of insects after long-term exposure to sublethal nitenpyram.The results were summarized as follows:1.The sublethal treated-insecticide susceptible strain?SL?was initiated by treating the insecticide susceptible strain?SS?with the LC₂₀ concentration of nitenpyram for successive six generations;the sublethal treated-field strain?FL?was initiated by treating the field strain?FS?with LC₂₀ concentration of nitenpyram for six generations.After six generations of continuous treatment of the LC₂₀ nitenpyram,the resistance ratio of the FL strain BPH to the nitenpyram increased from low level resistance?5.65-fold?to moderate level resistance?21.65-fold?.However,there was no significant difference in the susceptibility between the SL strain and the SS strain to nitenpyram?resistance ratio 1.26?.After six generations of continuous breeding of brown planthopper with LC₂₀ of the nitenpyram,the sublethal treated strains?SL and FL?and corresponding control strains were further treated with LC₂₀ nitenpyram.LC₂₀ treatment significantly reduced parental emergence rates by 26.11% and 11.32% respectively in SS and FS two populations;LC₂₀ treatment significantly reduced female longevity in SS strain,while LC₂₀ treatment increased the fecundity of FS strain,demonstrating that the effects of LC₂₀ treatment on the development and reproduction of F0 generation were significant.The number of eggs per female and mating success rate of offspring of SS strain was higher than those of control group after LC₂₀ treatment;the female and male longevity of offspring were significantly increased by 3.1 days and 3.0 days respectively compared to control group.While the nymph period in F1 generation of FS-treatment group was shorter 0.6 days than that of the control group,the number of eggs produced per female in F1 generation showed no significant difference between the treatment and control group,but the female longevity was significantly shortened by 3.3 days than that of the control group.These results proved that the sublethal effect on the offspring after parental exposure to LC₂₀ was different from that on the parental generation.The adverse effects were reduced in offspring generation,and changes in effects also differed between SS and FS strains.There was no significant difference in adult emergence rate,developmental period and female longevity between the treatments and their corresponding control groups in F0 generation after re-treatment of sub-lethal LC₂₀ nitenpyram in SL and FL strains,respectively.However,the proportion of females in the offspring of FL strain increased significantly by 23.85% and the longevity of females increased significantly by 4.2 days in the LC₂₀ treatment group.These findings indicated that after successive exposure to LC₂₀ concentration of nitenpyram for 6 generations,BPH exhibit a better survival capacity to nitenpyram than no exposed individuals when encountering LC₂₀ nitenpyram stress again.This phenomenon was especially obvious in FL strain.Moreover,after six generations of continuous treatment of LC₂₀ of nitenpyram,the longevity of both female and male BPH was significantly prolonged in two sublethal treated strains?SL-CK;FL-CK?without insecticide stress compared to their corresponding control stains?SS-CK;FS-CK?,respectively;Additionally,the nymphs development period of offspring was significantly shortened,adult emergence rate was increased,the number of eggs produced per female was significantly increased in both two sublethal treated strains.Moreover,the FL parental development period was significantly shortened,the proportion of female and male longevity of offspring was higher than that of FS population;and the male and female longevity of SL strain was also increased compared to SS strain.These results suggest that long-term exposure of BPH to low doses of nitenpyram may stimulate its growth and reproduction.2.The obtained FL strain showed a significant decrease in sensitivity to nitenpyram by 3.85-fold,while also exhibiting low-level of interaction resistance to cycloxaprid and imidacloprid.The cycloxaprid resistance level and imidacloprid resistance level of FL strain increased from 29.31 to 99.34-fold and 14.15 to 22.90-fold compared to SS strain,respectively.There is no significantly difference in the susceptibility to sulfoxaflor or 10% triflumezopyrim suspension between FL and FS strain.These trends were also observed in SL strain.For the SS?SL?FS?FL four strains,10% triflumezopyrim suspension and nitenpyram had the best insecticidal activity,followed by sulfoxaflor and cycloxaprid,and imidacloprid had the worst effect.Therefore,in the field control of BPH,in order to delay the development of insecticide resistance,the triflumezopyrim and sulfoxaflor can be used interchangeably with nitenpyram,with interval use of cycloxaprid.3.The expression profiles of insecticide resistance related P450 genes and reproduction related gene?Vitellogenin,Nlvg?were compared between sublethal-treated strains and corresponding control strains.The induction expression of these genes was also analyzed in these strains after exposure to LC₂₀ concentration of nitenpyram.Results show that the expression of CYP6ER1 was significantly up-regulated in both sublethal-treated strains?SL and FL?.This finding suggested that the constitutive over-expression of CYP6ER1 in both SL and FL strains obtained by exposure to LC₂₀ nitenpyram for six generations,possibly contributed to the development of resistance to nitenpyram and interaction resistance to cycloxaprid and imidacloprid in these two strains.The significant induction in expression of CYP6CW1 gene was observed in both SL and FL strains after BPH 3th nymphs were treated with LC₂₀ nitenpyram.,indicating that it may also be involved in the development of resistance to these insecticides.The expression level of Nlvg was significantly down-regulated in the SS-LC₂₀ treatment group but not in FS-LC₂₀ treatment groups,and up-regulated in the SL-LC₂₀ and the FL-LC₂₀ treatment groups with no significant difference,compared to their corresponding control group,respectively at 96 h after LC₂₀ treatment.Also,the expression level of Nlvg in the SL and FL strains was constitutively over-expressed by 5.02 and 4.74 times compared to their corresponding control strains,respectively,consistent with a trend in the number of eggs produced per female being significantly higher in SL and FL than that in the SS and FS strains,respectively.This finding indicates that both for insecticide susceptible and field collected strains,the fecundity of BPH was enhanced by increasing the expression level of Nlvg gene after multi-generation sublethal dose treatment of nitenpyram.In conclusion: after six generations of continuous treatment on the field population of BPH by LC₂₀ of the nitenpyram,the growth and reproduction of BPH was stimulated greatly,The increased reproduction might be attributed to the up-regulated expression of Nlvg gene;the increased resistance level to nitenpyram,cycloxaprid and imidacloprid might be attributed to the constitutive over-expression of CYP6ER1 after BPH were exposed to LC₂₀ nitenpyram for six generations.Overall,our findings indicate that after long-term exposure to sublethal nitenpyram,BPH population resurgence might occur in field conditions through increased reproduction and development of insecticide resistance.