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Abstract: Behavioral and physiological characteristics of the emigrant and immigrant populations of the oriental armyworm, Mythimna separata(Walker) moths trapped in field in Ganyu County, Jiangsu Province and Shenyang suburb, Liaoning Province were examined. The results indicated that moths of the emigrant population exhibited stronger flight potential than that of the immigrant population. The flight distance, total flight duration and average flight speed in he emigrant population were 13.6 km, 153.1 min and 5.1 km/h, while in the immigrant population were 6.0 km, 64.3 min and 4.9 km/h, respectively. All of the flight potential variables except flight speed in the immigrant population were significantly less than those in the emigrant population. Dry weight of the thoracic dorsallongitudinal muscle in female in the emigrant population was 6.53 mg, significantly higher than the value of 5.92 mg in the immigrant population. Most females in the emigrant population exhibited little or no ovarian development and were seldom mated, and mean basal ovariole width, length, wet weight, mating rate and frequency were 0.21 mm, 4.01 cm, 16.34 mg, 27.5% and 0.32 respectively, which were significantly lower than those of 0.51 mm, 5.43 cm, 56.7 mg, 98% and 1.44 in the immigrant population. In addition, the ovarian development and mating rate in the immigrant females early trapped were lower than those later trapped and increased as trapping time prolonged. Juvenile hormone titer (JHⅡ) in the emigrant population was 4.63ng/mg, also significantly lower than that of 8.69 in the immigrant population. Comparison of all of these variables between the emigrant and immigrant populations suggests that there are obvious tradeoff between flight and reproduction. The migrants have strong flight potential, and reproduction activities are depressed before migration, while mating and oviposition begin rapidly after they arrive at new habitats. This model fits the “oogenesisflight syndrome" hypothesis well. The juvenile hormone plays a significant role in the tradeoff process between flight and reproduction in the female

Key words: Mythimna separata, emigrant population, immigrant population, oogenesisflight, syndrome, juvenile hormone titer