Abstract: The flight capacity and fecundity of adult Mythimna seperata Walker as affected by larval rearing density under 5 regimes (1, 10, 20, 30 and 40 larvae/850 ml-jar) were investigated in the laboratory. The flight capacity, based on 12 hour tethered-flight test of the unfed newly emerged moths from different larval rearing conditions, was the highest at a density of 10 larvae/jar, on the average of 225.4 minutes in duration and 17.5 kilometers in distance and significantly greater than those from other densities. Incidence of long-fliers in this rearing condition was also the highest whether based on flight distance or duration. In other rearing conditions, either flight capacity or proportion of long-fliers was decreased as larval density increased and vice versa, regardless of being significant or not. Neither pupal weight nor wing-loading was significantly correlated to the moth flight capacity orincidence of longfliers. However, there was a relationship between the long-fliers and pupal weight. Incidence of long-fliers exceeded 50% when pupal weight was in the range of 300-400 mg., but decreased beyond this range, and there was no long fliers when the pupal weight less than 225 mg. Fecundity was also significantly affected by and negatively related to larval density. Correlation between, pupal weight and fecundity was insignificant within any given larval density but was significant when data from all densities are analysed together (r=0.78, P < 0.01). Results obtained from this study suggest that larval density is one of the major environmental factors for Mythimna seperata migration, and migration begins when its popula tion reach to certain density threshold in order to avoid its detrimental effect of further increase.

Key words: Mythimna seperata Walker, larval density, flight capacity, fecundity migration