| The virulence indexes (honeydew excretion and weight gain of female adult in 48h )of brown planhopper (BPH), Nilaparvata lugens (Stal) were evaluated by Bayes' theory. To explore mechanisms of virulence of BPH, variations of virulence in the laboratory populations, relationships between inheritance of virulence in the laboratory populations and sex and virulence of BPH in different generations in the field populations were studied. The main results were as follows:1) The indexes of virulence of individual BPH were evaluated and deduced by Bayes' theory. The virulence of BPH in TN1 population, which were reared more than 80 generations on TN1 variety (carrying no resistant gene) to Mudgo (carrying Bphl gene) were evaluated comprehensively by multiple methods, such as standard seedbox screening technique (SSST), fecundity and oviposition rate tests, and observation of appearance of female adult's abdomen. After that, we calculated P(v), the percentage of individuals in TN1 population virulent to Mudgo variety, finally tow indexes of virulence, honeydew excretion and weight gain were evaluated and deduced by Bayes' probability. According to honeydew excretion of female adult in 48h, Bayes' probability, the probability that individual excreted more than 10mg honeydew belongs to virulent individuals is 79.57%, and the results suggested that the experimental standard is reasonable, but the standard will be more suitable if the critical value is 12mg in our testing condition. Our study also indicated that weight gain with a critical value of O.lmg is less reliable, and with the help of Bayes' probability and unpublished data, we deduced that the critical value of weight gain is 0.7mg.2) Variations of virulence in laboratory populations of BPH were studied with honeydew excretion of female adult in 48h. Honeydew excreted by BPH in avirulent population (TN1), in which most individuals are avirulent, or virulent populations (Mudgo or ASD7), in which most individuals are virulent on susceptible and resistant varieties were weighted, and the results showed that the susceptibility and resistance of host plants interact with the virulence of BPH. The difference of virulence between avirulent and virulent populations on susceptible varieties is narrow, but it becomes notable while on resistant variety, so BPH with different virulence should be reared on resistant varieties to observe evident difference. Crosses were conducted between avirulent and virulent populations to investigate mean honeydew excretion and variance of honeydew distribution in F1 populations compared with those in parent populations. The data indicated that mean virulence in F1 populations is an intermediate value and near to that in virulentparent, and the variance of honeydew distribution in F1 populations is obviously bigger than that in avirulent parent and near to that in virulent parent. It seemed that virulence is dominant to avirulence. Finally, differences of mean honeydew excretion of BPH and variances of honeydew distribution on resistant varieties between self-cross F2 populations and hybrid F1 populations were studied. When compared with those in F1 populations, the mean virulence in F2 populations performs little change in the treatment TxM, and is significantly bigger than that in the treatment AxT. The variance in F2 populations is significantly smaller or bigger but not significantly than that in F1 populations. Based on the above results, we can conclude that there are no new genetic segregations in F2 populations compared with F1 populations.3) According to honeydew excretion and weight gain of female adults, we analysed the virulence in reciprocal cross populations (F1 population) to study the relationships between virulence inheritance and sex. If using honeydew excretion as index, the mean and variance are significantly different in reciprocal cross populations in both treatments (T population and M population, T population and A population), which indicated that variations in reciprocal cross populations...
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