In order to describe seasonal flight activity of the cabbage maggot Delia radicum (L.) adults in relation to Julian days (JD), degree-day accumulations (DDA) and precipitation, flight dynamics were followed weekly with the use of yellow sticky traps (YST). Climatic data were collected and DDA were calculated using the lower developmental threshold of 4.3 °C. The efficacy of four insecticides applied either as standard foliar treatment or through dipping the seedlings before transplanting was determined. Seasonal flight activity during the cultivation season of a mid-early variety of white cabbage was correlated with DDA and JD and was characterized by having two peaks. The first peak occurred between 119 ± 7.5 JD and 125.5 ± 8 JD when DDA was 471.35 ± 74.97 °C. The second occurred between 172.8 ± 6.1 JD and 179.3 ± 6.7 JD when DDA was 1,217.28 ± 96.12 °C. The DDA, cumulative capture of flies and JD are suitable for predicting the timing of insecticide application. Spraying with insecticides should be applied when the cumulative capture of flies reaches 100 flies/YST and when DDA reaches 400 °C. If only one parameter reaches the threshold, additional visual surveys should be employed to establish the level of infestation. Insecticides were able to ensure only partial control. In the future, alternative control tactics which employ seed treatments and nonpesticide measures should be investigated in Croatia.