That's leaf miner damage. Citrus leaf miner

Thanks!

Lots of good information on CLM here; no cure once you have it; only prevention.

I find it interesting that CLM shows up even though one doesn't live in typical citrus growing areas.

Nobody told the CLM

It's just like those giant swallowtail butterflies showing up in Brian's greenhouse way up in Canada! Citrus is their main host plant.

I do live in a citrus growing area, lots of citrus in Southern California. The site suggested "Imidacloprid" , I had some Bayer Tree and Shrub Protect and Feed on hand so I used it on the two trees that have the CLM. I used it on other plants/trees against aphids and scale and it seems to work well. Since they are far from fruit bearing age, I am not worried about the systemic pesticide. The CLMs are nasty little things, I can't believe they found my little lemon trees as they are surrounded by structure on all four sides. I have fully grown mandarin and grapefruit trees for decades and have never seen these racetracks on the leaves before.