The only veg I am growing this year are beans. I wasn't going to grow anything except flowers, and replaced my veg bed with dahlias, which are in full flower and doing very well. However, my grandchildren had other ideas and collected all the bean seeds that were on the earth after the winter, dug a hole in the most awkward place, planted them, and I now have a wigwam of runner/borlotti beans!
Yesterday I filled a yellow bucket with water and popped it next to the beans, and I it has already attracted qúite a few blackfly. For a non -chemical solution, it really does trap them - must be a yellow bucket though.