Abstract: On the basis of Watt's mathematical model of the index of population t rend (I), an approximate variance estimator of I was developed with delt a method in this paper. Taking the natural population life tables of rice leaf roller Cnaphalocrocis medinalis (Lepidoptera:  Pyralidae) published by Wu et al. (1986) as an example, it was applied in evaluating the control effectiveness of biologica lagent (Trichogramma japonicum Ashmead) and chemical insecticide (powder, 1.5% mevinphos + 3% alpha-hexachloro cyclohexane). By Z-test criterion, biologically and statistically sound conclusions were drawn that at the generation level, the suppression effectiveness of trichogramma wasp against rice leaf roller was better than the blank control with P =0.0111 and smaller I value (0.0390 versus 0.1768), and it was also statistically better than the insecticides with P =0.0036 and smaller I value (0.0390 versus 0.3035). The insecticides was not statistic ally worse than the blank control with greater I value (0.3035 versus 0.1768), and P =0.2236. Although these conclusions are close to that drawn by Wu et al. (1986) only based on the index of population trend point estimates, they are supported by an accuracy measurement P-value.

Key words: Approximate variance estimator, index of population trend, life t able, rice leaf roller (Cnaphalocrocis medinalis), delta-method