SummaryTraps placed within brassica crops to monitor changes in cabbage root fly activity could be made more selective by painting black the inner wall of the standard fluorescent yellow water‐traps. Traps could also be made more selective by covering them with cylinders of Netlon mesh, although this procedure reduces the numbers of cabbage root fly caught by about 70%. Of the single‐coloured traps tested, those painted “marigold” yellow were the most selective in capturing female cabbage root flies. Although white and certain blue traps were as effective as the best yellow traps at capturing cabbage root flies, such traps should be avoided, as they catch 4–5 times as many of the closely‐related bean seed fly. The presence of bean seed flies makes cabbage root fly identification more difficult and adds considerably to the time required to sort t