Abstract 1 Two new concepts describing the origin of insects caught in an attractive trap are presented. 2 Male European pine sawflies Neodiprion sertifer Geoffroy (Hymenoptera: Diprionidae) were marked and released from 50, 100, 200, 400 and 800 m in the four cardinal directions around a centrally placed pheromone trap. 3 Based on linear regression of transformed data, we calculated the seasonal sampling range (rs) as 1040 m. 4 We estimated the previously defined ‘effective sampling area’ (α) at 4.9 ha, assuming that the insects are evenly distributed around the trap and that they are attracted from a circular area around it. This is the area from which all insects originate if the trap is 100% effective within the area but captures nothing outside of it. The effective sampling area reveals nothing about the origin of the insects caught. We defined the Cumulative Proportional Catch (CPC) that gives the proportion of the trap catch that originates from an area within a distance r from the trap. At r = rs CPC = 1, and in our study 50% of the captured insects originated up to 450 m from the trap. Thus, for the trap used in this study, a relatively large proportion of the catch originates some distance from the trap. 5 We also defined the Catch Concentration (CC), which is the ratio of the radius of the effective sampling area (rα) to rs. For our data, CC = 0.12, which is intermediate to high compared to the few other studies that we have extracted information from. If rα is considerably lower than rs, then only a small proportion of the insects caught originate from close proximity to the trap. When rα is close to rs, the catch adequately mirrors the population within most of its sampling range. 6 By using these two new concepts, we will better understand why monitoring traps mirror the local population in some cases but not in others. This will help in designing more reliable monitoring programmes.