‘De Groene Vlieg’ (the Green Fly) is an independent company offering various services concerning integrated pest management in open field agriculture in the Netherlands. One such service is the supervised control of carrot fly (Psila rosae). From 1993 to 2008 the company has monitored the carrot fly population on a total of nearly 65 000 hectares resulting in an extensive dataset. The data was not collected according to proper experimental methods, hence it is titled dirty, but the size of the dataset does allow valuable insights to be gained. De Groene Vlieg monitors the carrot fly population on each field separately. Sticky traps are collected weekly and flies are counted in the laboratory. Growers receive messages containing advice regarding when to apply insecticides and they are responsible for doing so. Overall, over a third of the fields require no spraying at all and for 75% of the fields growers are advised to spray three times or less. One insight from the dataset is that small carrots require more insecticide treatments than large carrots (‘winter carrots’). Reasons for this difference are probably both regional and cultural. Additionally, focusing on a small and uniform area reveals that carrot fly population dynamics are highly variable between years. Another insight is that sowing date has a large effect on the number of insecticide sprayings that are required. Fields sown before the first flight have more flies in the second flight than fields sown later. Comparison of biological and conventional carrot fields shows that carrot fly populations are similar for the first flight. However, during the second flight the number of flies is much higher on organic fields than on conventional ones. Finally, insecticide seed treatment with chlorfenvinphos to control the reproduction of the first flight appears slightly more efficient than supervised control. However, supervised control seems as efficient as seed treatment with clothianidin and beta cyfluthrin.