Abstract: Longevity and weight gain of brachypterous females of Nilaparvata lugens that had emerged within 24 hours were investigated on two rice varieties,a susceptible variety TN1 and a resistant variety Mudgo, at 28±1℃. A positive correlation was detected between mean weight gain and longevity of females on Mudgo. Individuals that lived less than 7 days lost weight over their lifetime, while those living more than 7 days significantly increased in weight.Furthermore, the longevity, fecundity and weight gain over 1-4 days of newly emerged macropterous females were studied on TN1 and Mudgo, and positive correlation was also found to exist between these three parameters in females on Mudgo. Among them longevity and weight gain better reflected the virulent capability of individual females, and thus could provide an index of Individual virulence. Based on these results, it was proposed that newly emerged females on Mudgo that could survive over 7 days, or had a weight gain of more than 0.1mg within the initial 4 days should be defined as virulent.

Key words: Nilaparvata lugens, rice, Mudgo, individual virulence index