The present investigation was carried out at Kafr El-Sheikh region during 2006/07 and 2007/08 seasons to evaluate the seasonal activity of the main mortality factors causing mortality among the hibernation larvae of Sesamia cretica Led. and Ostrinia nubilalis Hub. The ectoparasitoid, Bracon hebetor  Say was mainly observed associated with corn borers larvae. Percentages of parasitism against larvae of S. cretica  varied from 2.56 – 6.82 % during 2006/07 and from 4.51–5.24 % during 2007/08, respectively. While percentage of parasitism of O. nubilalis larvae varied from 1.31-1.97 % and from 3.44-8.70 % during winter months. In spring (March–May) percentage of parasitism on S. cretica increased from 6.25 to 50.0 % and from 11.50 % to 33.30 %. It ranged from 2.50 to 45.36 % and from 6.70 to 42.71 % on O. nubilalis in 2006/07 and 2007/08, respectively. The pathogenic bacteria Serratia marcescens Bizo caused disease in larvae.Percentage of diseased larvae of S. cretica ranged from 2.56 to 14.28 %, during a period extended from December to April in 2006/07, and between 2.94 to 13.18 % in 2007/08, respectively. On O. nubilalis it varied from 0.08 to 4.04 % and from 0.76 to 9.50, in the two seasons, respectively. The predators Monomorium pharaonis and Labidura riparia encountered in the course of the field work. Biology of B. hebetor  on hibernating O. nubilalis larvae was studied under constant condition of 20 + 2 ^oC and 70 + 5 % RH. Developmental durations were 2.6, 9.2, 2.3 and 9.7 days for egg, larva, prepupa and pupa, respectively. Longevity was 34.2 and 12.0 days, while total life-cycle amounted to 58.0 and 35.8 days for female and male, respectively. Fecundity averaged 210.2 and eggs hatchability was estimated as 93.8 %. Number of hosts destroyed per female was 39.8 through oviposition and 90.4 through paralysation. This finding is important from the biological point of view it may increase the value of the parasitoid. The peak period of egg-laying and destruction of hosts was that of 8 – 9 days of age. Pre-ovipostion and post-oviposition periods were 4.4 and 3.6 days. Sex-ratio was 2 ♂ : 1 ♀.