During the 2011–2012 the research on optimal treatment time to control cabbage root fly on cabbage crops was carried out. The study focused on determining the suitability of allylisothiocyanate-baited traps to control Delia radicum L. on cabbage crops. Treatment time was determined based on a number of captured females. During cabbage harvesting time an analysis of a root damaged caused by cabbage root fly larva was done. The results show a high efficacy of used monitoring system in control of D. radicum on cabbage crops. Flight monitoring method could be useful for control cabbage root fly in Integrated Pest Management.