Effects Of Environmental Factors On Diapause Formation And Variety Of Biological Characteristics In Post-Diapasue Adult Of Beet Webworm, Loxostege Sticticalis (Lepidoptera: Pyralidae)

The beet webworm, Loxostege sticticalis (Lepidoptera:Pyralidae), which overwinters as a diapausing mature larva, is a serious economic pest of crops and fodder within a belt zone from 36°N to 55°N in China. Unlike many other insects, the beet webworm employs both options of diapause and migration, which constitutes considerable life-history flexibility in distributing its offspring over space and time. There have been 3 times outbreak since 1949, resulted in great yield damage and econimic loss during the outbreak years. To clarify the theoretical foundation of managing overwintering populations and improving forecasts of the population dynamics of this pest, the paper mainly focused on the factors of diapause induction (eg. temperature, food plant and population density), the effects of larval diapause on the post-diapause adult reproduction and migration, the major results obtained were as follows:The effects of environmental factors on diapause induction of L. sticticalis were investigated under L14.5:D9.5 photoperiod at 22±1℃and 70%～8% RH. The temperature played an important role in the mature larvae diapause induction. Development duration shortened significantly and diapause rate reduced gradually with the temperature increasing. The effect of food plant on the formation of diapause in L. sticticalis larvae was expressed most distinctly under critical day length. The survival and diapause rate of caterpillars feeding on lambsquarters were higher than from feeding on lucerne, and developmental duration of the former shortened. Together, the change of food plant also affected the diapause induction of beet webworm, when the caterpillars were transferred from weed plant (lambsquarters) to cultivated plant (lucerne) in the 3rd instar. The diapause rate of L. sticticalis larvae feeding on the lambsquarters was the highest, followed by the caterpillars transferred from lucerne to lambsquarters, and lucerne. But, in the critical photoperiod, only 0%～1% of caterpillars diapaused, leading to diapauseless development after transfer from lambsquarters to lucerne. Besides, this study indicated that the diapause formation of L. sticticalis was affected by the phenological condition of plants. A more distinct effect was manifested under critical day length:11.54% of caterpillars feeding on young leaves of lambsquarters and 9.01% feeding on the young leaves of lucerne diapaused; 59.76% and 0% of caterpillars feeding on older leaves of the two plants diapaused. The results of a laboratory study conducted at four conditions of feeding (lucerne, lambsquarters, from lucerne to lambsquarters, from lambsquarters to lucerne) suggested that the proportion of larvae entering diapause markedly increased and survival rate decreased with an increase in the population density of L. sticticalis. In an interactive experiment of larval density and food plant, L. sticticalis fed on lambsquarters, and lambsquarters to lucerne, displayed the highest and lowest diapause rate at a density of 20 larvae per jar, respectively, relative to the other interactive treatments. Under the same density, the diapasue rate of caterpillars reared from lucerne to lambsquarters was significantly higher than those from lambsquarters to lucerne, showing that 3rd～5rd is the most sensitive stage of diapause formation in food plants. Finally, the diapause rate of L. sticticalis larvae reared at different food plants and population densities, was analysed under interactive treatments based on repeated analysis of variance with double factors, suggesting that effects of environmental factors on diapause induction:food plant> the interactive of food plant and larval density> larval density.To determine the role of diapause plays in the population dynamics of L. sticticalis, the reproductive and flight potentials of adults originating from diapause and non-diapause larvae were investigated under controlled laboratory conditions. Pre-oviposition period, lifetime fecundity and daily egg production of females originating from diapause larvae were not significantly different from those originating from non-diapause larvae, showing that diapause has no significant effect on reproductive capacity when adults are provided with an adequate carbohydrate source. However, females that developed from diapause larvae lived significantly longer than those from non-diapause larvae. Flight capacity, including flight duration, distance and velocity of 3-day-old adults were all significantly greater in adults originating from diapause larvae than those from non-diapause larvae. Furthermore, A large proportion of adults originating from diapause larvae (n=45) engaged in long-duration flight, with 73.3% and 40%, making a continuous flight> 3 h and> 5 h, respectively, during the 12 h test period. In contrast, only 40% and 7.5% of adults originating from non-diapause larvae (n=40) flew> 3 h and> 5 h, respectively. Only 6.7% of adults originating from diapause larvae did not make a long-duration flight> 1 h, whereas 20% of adults originating from non-diapause larvae flew≤1 h. Similarly, L. sticticalis adults developing from diapause larvae tended to have more extreme values of furthest single flight distance than those from non-diapause larvae, with 51.1% vs.27.5%, respectively, flying> 16 km. Together, these results suggest that long-distance flight potential of L. sticticalis is greater after larval diapause than after direct development to adulthood. However, There were no significant differences between sexes within the two categories of moths in terms of total flight duration, total flight distance, flight velocity and longest flight duration.