The rape stem weevil (Ceutorhynchus napi Gyllenhal, 1837) and the cabbage stem weevil (Ceutorhynchus pallidactylus Marsham, 1802) are becoming increasingly important pests in crops of oilseed rape (Brassica napus L. ssp. oleifera). Biology and ecology of rape stem weevil (C. napi) and cabbage stem weevil (C. pallidactylus) are very similar, so they often referred to as complex pests, but their life cycles are very different which causes different strategy in pest control. The aim of this study is to determine sex ratio of rape stem weevil and cabbage stem weevil and determine the fertility of females of both species, and results will give us the point at which females are carrying mature eggs and that will help us to get the optimal time for the application of insecticides. The research was conducted in 2016 on the site of Šašinovec in the period form beginning of February to the end of May. Population dynamics of adult forms was followed using yellow traps, and samples were taken twice a week on average. Adults form of rape stem weevil occurred in significantly higher numbers than adults form of cabbage stem weevil, and the first appearance of females in both species was observed three weeks after first appearance of mail. The first females with mature eggs in the abdominal, in both species occur in early April, when the oilseed rape was in the beginning of flowering the stage of development (BBCH 60 62) . The percentage of males, in both species, was higher than females, but with no significant difference. The population of both species was very low, there was no need for the application because the threshold is not exceeded.