Inhibiting Effects Of Zhongshengmycin On The Symbionts Of Brown Planthopper (BPH),Nilaparvata Lugens St(?)l And Its Control Efficacy On BPH Populations Mixed With Pymetrozine

Brown planthopper（BPH）,also known as Nilaparvata lugens（St（?）l）,is one of the leading monophagous rice pests in China and is mainly controlled by chemical pesticides methods and resistant varieties.However,BPH has become increasingly resistant to application of chemical agents for a long-term,and large-scale planting of resistant varieties led to new virulence of BPH,which hindered the effective control of BPH.It has been shown in some studies that BPH has a close symbiotic relationship with its symbiotic bacteria.Endosymbionts play critical roles in host growth,development and reproduction,such as providing nutrients,detoxifying and metabolizing toxic substances and performing immune regulation.For this reason,bacteriostatic-pest control has become a preferred new strategy to control BPH.For the purpose of clarifying the effects of different antibacterial substances on the BPH population and the number of endosymbionts in the body.In this study,susceptible strains of BPH were taken as the research objects,which continuously cultivated for more than dozens of generations without insecticide and fungicide stress.To study the optimal ratio with obvious inhibiting effect of antimicrobial zhongshengmycin and the compatibility with chemical insecticides pymetrozine against endosymbionts and host population of BPH.The results provided a basis for the development of a new insecticide for BPH.The specific results are as follows:1.The median lethal concentration(LC₅₀)of zhongshengmycin and pymetrozine against the third instar nymphs of BPH was determined by concentration gradient detection;The susceptibility of the isolated YLS to different concentrations of zhongshengmycin was examined;The ratio of the combination of both with obvious synergistic effect was screened out by interactive measurement.The median lethal concentration of the third instar nymphs of BPH treated with the mixture of both was determined and the co-toxicity coefficient was calculated based on the co-toxicity coefficient method.The results showed that the LC₅₀value was 152.61mg/L after 72 h of zhongshengmycin treatment and 3.80 mg/L after 120 h of pymetrozine treatment.When the volume ratio of pymetrozine to zhongshengmycin was 80:20 and 50:50,the toxic effect ratio were 1.28 and 1.29,respectively,both of which were greater than 1.25,compatibility of zhongshengmycin and pymetrozine targeting third instar nymphs of BPH showed synergistic effect.In the indoor toxicity determination,when the effective component quality ratio of pymetrozine to zhongshengmycin was 1:10,the LC₅₀value of third instar nymphs of BPH was 15.10mg/L（with 95%confidence interval of 8.67-26.31 mg/L）,and the co-toxicity coefficient was 221.63;when the effective component quality ratio of pymetrozine to zhongshengmycin was 1:40,the LC₅₀value was 11.60 mg/L（with 95%confidence interval of 6.63-20.31 mg/L）,and the co-toxicity coefficient was 672.87.Co-toxicity coefficients of the two combinations with different effective component quality ratios were all greater than 120,thereby indicating the significant synergistic effect.2.Based on the theory of age-stage two-sex life table,the effects of zhongshengmycin,pymetrozine and compatibility of zhongshengmycin and pymetrozine（effective component quality ratio of pymetrozine to zhongshengmycin of 1:40）of both on BPH populations were analyzed.The results showed that,compared with the negative control group（water treatment）,the whole nymph period of the three chemical agents treatment groups was prolonged.Among which,the whole nymph period of the combination treatment group was 16.70±0.26d,which was significantly longer by 2d and 1d compared to 14.12±0.10d in the negative control group,15.20±0.16d in the zhongshengmycin treatment group and15.58±0.17d in the pymetrozine treatment group.Compared with the negative control group（water treatment）,the longevity of the adult in the three treatment groups was shortened by 6-9d for female adults and 9-11d for male adults.For the number of egg,the difference between the negative control group and treatment group was the most significant.Among them,that of the negative control group was the largest with up to603.93,that of the pymetrozine treatment group was 204.38,while that of the combination treatment group was only 74.00,being the least.Life table population parameters showed that compared with other groups,the combination treatment group possessed the shortest mean generation time（T）being 20.05,the lowest innate rate of increase（r_m）being 0.095,the lowest net reproduction rate（R_o）being 6.68 and the smallest finite rate of increase（λ）being 1.100/d,which denoted that it was the combination treatment group that possessed the shortest time to complete a generation period for BPH populations,the slowest population growth trend,the smallest multiplication factor after a generation period,and the least average number of offspring that each individual can produce under the condition that the population is not limited by resources.To put it in a nutshell,BPH populations treated with the compatibility of zhongshengmycin and pymetrozine had a more effective inhibitory effect on its reproduction and survival than those treated with zhongshengmycin or pymetrozine alone.3.Quantitative real-time PCR was used to detect the changes of symbiont numbers in the gut and fat body of BPH before and after treatment with zhongshengmycin,pymetrozine and compatibility of zhongshengmycin and pymetrozine（the effective component quality ratio of pymetrozine to zhongshengmycin was 1:40）of both.After being treated with three kinds of compounds,the total number of symbionts in the gut and fat body of BPH decreased significantly,but the degree was different extent.In the gut of BPH,the total number of symbionts decreased after 3d and 5d of treatment with three kinds of compounds,and the inhibitory effect was more significant with the extension of treatment time.Among them,zhongshengmycin had a poor inhibitory effect on fungi in the gut of BPH,and had a great inhibitory effect on fungi in the fat boody of BPH.The number of symbiotic bacteria and fungi in the combination was significantly reduced,showing an excellent inhibitory effect in the gut of BPH.In the fat body of BPH,the number of symbiotic bacteria decreased was significantly reduced in the combination treatment group on the 3rd day.On the 5th day,significant differences were observed among the three compounds.There was significant inhibition effect of symbiotic bacteria between the zhongshengmycin treatment group and combination treatment group.The inhibition of fungi showed that the combination treatment group had the most obvious inhibition effect of the three compound treatment groups.