Beet leaf miner

This year my chard and beetroot are plagued by beet leaf miner. It's not a problem I've suffered from before. I understand the flies look like small house flies and the lay eggs on the underside of the leaf, the grubs eat through the leaf and then drop to the ground and overwinter as pupae.  I'll not be sowing chard or beet in this spot next year. Anyone else had this problem?  I could cover the plants with mesh but I always thought chard was relatively pest free.

Yes, I had this problem on beetroot last year and this. Both times in ground that hadn't had such plants in before, so I guess they flew in.
The leaves were heavily damaged, but eventually the insects stop, the plant grows new leaves and recovers somewhat.