I had a similar problem with wild rocket under enviromesh at the end of August. I had put this on specifically to exclude flea beatle from the pristine plants! Unfortunately it was badly affected by flea beetle (the most likely candidate given the damage), whereas wild rocket in the neighbouring bed was unaffected. I suspect that the flea beetle was already on the soil and was trapped under the mesh. But the neighbouring bed got more water which the flea beetle didn’t like. I removed the mesh and the rocket is now looking good.
However I am starting to suspect another pest as the cause of “flea beetle-like” damage on brassicas in September. Perhaps Charles or someone else may be able to shed some light on it. They are tiny grey/brown weevils, the same size as flea beetles, which nestle at the base of brassica salads, such as wild rocket and mizuna. I deliberately waited til September to plant out the mizuna to avoid flea beetle. Again the plants were pristine on planting out, but soon got peppered with tiny holes (along with some slug damage). I’ve checked on the internet and they look like brassica stem weevils or seed weevils. They’ve been troublesome in the past and first appeared when three neighbouring fields were all planted with oil seed rape. According to what i’ve seen on the internet they’re not supposed to be a problem on the leaves. Has anyone else had a problem with them?