Any insect which lays its eggs in the spongy layer between the upper and lower surfaces of leaves is known as a leaf miner. Larvae develop between the leaf surfaces and tunnel or ‘mine’ out the spongy middle layer as they grow, giving leaves a spotty and brownish appearance. The four stages of its development are egg, larva (leaf miner), pupa, and adult (a small fly).
Although the mines may be considered to be unsightly, this pest can be tolerated as it has a little real impact on the health and vigor of a holly. Leaves with mines may turn yellow and drop in early summer but this is the natural shedding of older leaves and not due to the pest.
Pinch the leaves of small trees to kill the leaf miner.
Insecticides are unlikely to be effective as the thick glossy surface of holly leaves means that sprays run off the foliage and do not penetrate to where the grubs are feeding. On small specimen plants, it is feasible to remove mined leaves but not if this would result in significant defoliation.
When the leaves are fully formed in late April or early May, this is your first opportunity to use insecticides. Managing leaf miners at this time can significantly reduce the chance of a problem later in the season. In June, if populations are severe, time your insecticide application to coincide with the second period of adult flight. Once you’ve noticed that the larvae have left the leaf, start to look for adults emerging two to three weeks later. Apply insecticides when most of the adults have emerged. Using insecticides to manage late season generations is generally not worth it. If late season problems are severe, consider an insecticide application next spring.