While this is just an occasional pest of Magnolia, the Serpentine Leafminer causes damage that is highly visible (I kind of like it). All Magnolias species are susceptible to this pest, but I rarely see it in nurseries or landscapes. Scale and Twig Borers are much more common and damaging to Magnolias in the South.
Injury is caused by the larvae of a specific moth, Phyllocnistis magnoliella, which burrows through the upper leaf epidermis in a meandering fashion.
The life cycle has not been really been studied, so there is not much information about when to time pesticides to control this pest.
Luckily, this Leafminer is seldom a serious pest of Magnolia (and it does not infest any other genera of plants). The best way to manage the Magnolia Leafminer is to remove infested leaves as the trails appear.
Begin looking for injury in late summer- Damage will begin to appear in early August if the moth has laid eggs on the leaves.