Citrus Leaf Miner

This looks like citrus leaf miner, a very common insect pest of citrus. A moth lays eggs under the leaf surface and the larvae tunnel through the leaf. It makes the trees look terrible but only threatens young trees. The insects can be difficult to control. You’ll want to follow good cultural care practices on the trees to keep them optimally healthy and continue to grow and produce fruit. (Specifically, do not prune and/or fertilize at inappropriate times of year as these
insects feed on the newest growth). These two web links have the best info:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74137.html

http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/citrus.html
Those websites will give you control options, organic and synthetic. Timing is very important with organics, as are repeat sprays.