The population fluctuations and effect of certain weather  factors on the flight activity of three leaf worms moths, Spodoptera littoralis Boisd. ; Spodoptera exigua Hb. and Spodoptera latebrosa Led. and their predators were studied by a light trap at Mansoura district. The obtained data showed that the peaks of S. littoralis according to biweekly catch occurred in the fourth week of April; the first week of June; the first week of July; the end of July; the second week of September; the end of October and the third week of November; represinting seven peaks of moths per year during the two years of study. The results showed that, S. exigua moths had five peaks per year during the two years of study. These peaks were occured in the first week of May; the first week of July; the end of August; the end of September and the fourth week of October, respectively. In addition, S. latebrosa moths had five peaks per year during the period of study. These peaks were occured in the first week of June; the first week of July; the end of July; the second week of September and fourth week of October, respectively. The statistical analysis showed that there was a highly significant positive correlation between the temperature parameters and the number of trapped moths of the three leaf worm species during the two years of study. The maximum relative humidity had a highly positive significant effect on populations of the three leaf worm moths in the first year and a slight negative effect in the second year of investigation. The other relative humidity parameters exerted a slight negative effect during the two years of investigation. The obtained results showed that there were four predators caught by a light trap during this study namely: Coccinella undecimpunctata L. ; Cydonia vicina isis Cr.; Paederus alfierii Koch. and Labidura riparia Pall.. The data cleared that C. undecimpunctata had four peaks and C. vicina isis had five peaks per year while that was two and three peaks for P. alfierii and L. riparia, respectively.     The statistical analysis showed that there was a highly significant positive correlation between the temperature parameters and the number of trapped predators during the two years of study. The maximum relative humidity had a highly positive significant correlation on the population density of L. riparia and C. vicina isis in the first year of study, while the minimum relative humidity had a negative significant effect on the population density of L. riparia in the first year, while minimum relaive humidity had a possitive significant effect on the population density of P. alfierii in the second year of investigation.