Answer: From your description it sounds as though the leaves are showing leaf miner damage. Leaf miners are larvae of a sawfly. The adult lays her egg between the upper and lower surfaces of the leaf; when the egg hatches into a little worm-like critter it feeds on the tissues between the upper and lower surfaces of the leaf. This tunneling leaves what looks like little white tracks within the leaf. Insecticides won't affect these pests because they are within the leaf where nothing can reach them. After the larvae are finished feeding the exit from the leaf and spin a web down to the soil. They pupate in the soil until they're ready to emerge as an adult. You can discourage them by placing an inorganic mulch beneath your mandarin tree - one such as gravel or decorative rocks which will be inhospitable to the larvae.
Best wishes with your landscape!
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