| The beet armyworm, Spodoptera exigua (Hubner), is a polyphagous pest with worldwide distribution mainly in tropical and subtropical regions. In recent years, the beet armyworm has risen to the status of an economically important pest of many kinds of crops in China, especially in the northern part of the country. The long distance migration of the beet armyworm has been reported based on some indirect evidences. Our understandings on the genetic, physiological and ecological factors affecting flight capacity of the moths are still very poor.. In this paper, the ecological mechanisms of beet armyworm migration were explored, including population status, ecological fitness, cold hardiness and overwintering, reproduction and flight syndrome, re-emigration capacity, genetics of flight capacity and activities of enzymes in energy metabolism. The results were summarized as follows.Life tables of laboratory population at five different temperatures were constructed. The results showed that the developmental rate increased with temperature from 20# to 32#, and the relationship could be simulated by a logistic model. The developmental temperature thresholds and sum of degree-day accumulation required for egg, larva, pupa, preoviposition stages and generation were 13.73#, 15.68#, 15.09#, 12.83#, 15.06#, and 37.9, 126.3, 73.8, 29.2, 265.6 degree day respectively. Survival rate of immature stage, mating rate of adults and population trend index were the highest at 26#. Mean egg productions at 26# and 29# were 604.7 and 611.4 respectively. Adult longevity, preoviposition and ovipositon periods declined at high temperature. Relationship between generation survival rate (5), population trend index (I) and temperature (t) could be descried by the equations: S= -0.5056 t2+ 27.652 t - 326.02, I= -3.2532 t2+ 178.45t - 2270.40.Supercooling points and freezing points at different developmental stages of the beet armyworm for different geographical populations were measured with a thermocouple. Supercooling points of pupae were the lowest and its supercooling phenomenon was also the most obviously, so pupa is the most possible stage for overwintering. Cold hardiness of the population from Hengyang, Hunan Province was the most powerful, followed by Longrao population (Hebei Province) and Nanjing population (Jiangsu Province).Overwintering survival rate estimated by field trials with pupae burying suggests that the beet armyworm could not possibly overwinter in Nanjing.Ovarian development of beet armyworm was classified by ovarian dissection, including transparent stage (grade I, before 5-days old pupae stage), vitellogenesis stage (grade II, late pupae and newly emerged moths), matured stage (grade III, the 2nd and 3rd day after emergence), egg-laying stage (grade IV, the 4th and 5th day after emergence), late egg-laying stage (grade V, since the 6th day after emergence). There was obvious correlation between ovarian development and the amount of egg-laid. The ovarian development of beet armyworm is characteristic of a quite short preoviposition stage (2.40 +1.06d). The second grade would be seen in the late stage of pupae and 80% of the individuals emerged in the second day after emergence would be with mature eggs (grade III and IV), among which 15% of the moths with the 4th grade ovary had laid their eggs.Flight capacity of the virgin moths at different ages was significantly different. The maximum flight activity were performed by 3 and 4 days old moths, although the newly emerged moths could also flight a while, Ovarian development of virgin females was slowly and no eggs were laid until the 5th and 6th day after emergence when a few eggs were laid. But mated moths showed quick ovarian development that more than 50% of the females had laid eggs at 3 days old, meanwhile, the strongest flight capacity appeared on the 2nd and 3rd day after emergence. The tethered tests demonstrated that flight ability of virgin and copulated moths at the same ages showed no difference, and that there were no significant correlations b...
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