Abstract: In the perspective of implementing agroecological farming practices, conservation biological control may be one of the levers allowing for a satisfactory pest control in low chemical input systems. The precocity of natural enemy activity as compared to pest population dynamics could be of major significance to limit the amount of pest developed per plant and subsequent damage to the crop. In the present study, we compared the mortality rates of the cabbage root fly in situations where ground dwelling predators were excluded thanks to barriers and pitfall traps during different periods of time or not excluded at all. We also monitored carabid emergence in the spring broccoli field and in the surrounding winter cereals. We found that adult carabids active before the egg laying peak of the cabbage root fly can be fundamental to obtain high levels of pest mortality. Two carabid species that probably contributed much to early pest regulation emerged significantly inside the broccoli field. Simplifying soil tillage before crop establishment in the spring may thus be an interesting lever for conservation biological control.