I'm still quite a novice veg grower. Tried some lettuce in the polytunnel this year, a friend sowed it while she was sitting for us and did it very thickly. When I went to thin it I noticed a lot of black flies. I was quite ruthless in my thinning but a week later the whole veg bed is teeming with these little flies, mostly crawling rather than flying. I have lettuce, peas, carrots, onion and garlic in that bed. I have lettuce in other beds in the polytunnel but they are unaffected, they are also different soils though.
Any ideas what they are? I can try and take a picture if it helps but they are quite small. They don't seem to be causing damage to the plants but I have noticed white round specks on some of the leaves from lettuce from this bed.
Dans