Hi, I have a small pomelo tree at my yard and recently I noticed white lines on leaves (photo attached). Do you know what this is? And should I pick those leaves? Thank you.

This is leaf miner, a tiny grub burrowing through the leaves. Very, very common on fruit trees. You need to use a pesticide to get rid of them. Neem or wood vinegar can do the trick or if you don't mind chemical warfare there's a plethora of products available from your local farm shop. Take a leaf to show them.

 

I kind of live with it most of the time.

Thank you for your reply with the information. I'll look for a pesticide.