Abstract: In some parts of China, the diapause ( Loxostcgc sticticalis Linnaeus) occurring in beet webworm is one of the important factors that affects the distribution of the over wintering webworm and the variation of its species group. The results observed from 1982 to 1984 showed that the light periodicity is a determinant to beet webworm diapause. The critical photoperiod is 14-15 hr., the shortest period is 13 hr.,the longest is 16 hr..Meanwhile, the temperature is another important factor affecting the diapause quantity from year to year.We suppose that according to light periodicity, beet webworm diapause may be divided into two regions: diapause region arid non-diapause region. It provides a scientific basis for learning the place where the over wintering beet webworms come from. The diapause quantity detected in the light and temperature variation at diapause region is pracatically significant to forecast the n-atural accumulation and decline of the species group (the pattern of species group fluctuation).

Key words: Beet webworm, Diapause