One of the biggest pests I encounter growing Citrus is the Citrus Leaf Miner (CLM). This little rascal is really more of a nuisance cosmetic pest on larger trees.
As you can see by these pictures, they can interrupt the photosynthesis.
On a large tree this usually isn't a problem because there are plenty of leaves on the tree. On a small tree the leaf count is much smaller. Even when infestations of Citrus Leaf Miner are heavy on young trees however, trees are unlikely to die. CLM can cause damage in new plantings because the growth of young trees is retarded by leaf miner infestations. Citrus Leaf Miner damage weakens the leaves, making them more susceptible to wind damage and other pests. Studies are underway to determine if yield can also be affected.
The bug that is doing the mining is very small and worm like.
That's the little guy, at the end of the tunnel. This one is getting ready to pupate and turn into the adult. It will emerge as a moth, like this one.
Control is the hardest part. A systemic (insecticide the plant takes up from its roots) is not a good idea, because you do want to eat the fruit right? The larvae are protected by the leaves epidermis as it tunnels through, so contact insecticide is temporary at best. It is effective for only 2 to 3 weeks because Citrus Leaf Miner adults lay eggs on the new flush of growth that was not present at the time of treatment. Horticultural oils have shown to work as a deterrent, the adult female will not lay her eggs on the oily surface, but should be used with care to avoid phytotoxicity (becoming toxic to the plant). There is also a chemical called spinosadtm, which is an all natural control, it is very expensive, if you can find it.
Citrus Leaf Miner moths are attracted to the new flush of growth. Citrus tend to have multiple flushes of growth through out the year. Once the leaves harden however, the pest will not be able to mine the leaves.
There are traps baited with a pheromone (insect sex attractant) that are available for Citrus Leaf Miner and are a useful tool for determining when moths are flying and depositing eggs. For the homeowner who only owns one or two trees, this is probably not economically feasible.
Like I said at the top, they are mostly a cosmetic damage pest. One thing to remember, do not prune off leaves damaged by CLM because undamaged areas of the leaves continue to produce food for the tree.
I hope you never have to see a "Leafminers Daughter", it really does make for an ugly tree sometimes.