Looks like leafminers to me... my parents' lemon tree is infested too :(

You have citrus leaf miner. They can be controlled with spinosad. The new growth will need to be sprayed each time it flushes out. Bayer also has a very good citrus spray.

I was going to post a sympathy response also with my own pics, but it's too depressing. I got hit BAD this Fall, despite my luck with very little damage in prior years. Perhaps it's moving northward in CA even stronger now since the introduction (I live in central CA San Joaquin Valley).

Nearly ALL my young growth on more than half my container trees was damaged like the above picture or worse! I would estimate only about 10% of the young shoots/foliage were spared, and every warm day during the past couple weeks I see a few more bite the dust.

Citrus in this area is (was) generally pest and disease free. hmmpphh..