Leafminers are only around 3mm long, so you’re more likely to notice leaf damage than see the bugs themselves. When leaves and buds are just emerging in early spring the female leafminers feed on the leaves and create microscopic holes with dead leaf tissue around the outer ring of the hole. These holes are often not noticeable at first but grow in size along with the leaf – so by the time damage from leafminers is visible, the damage is already done. The flies also lay eggs inside of the leaves and the larvae feed on the leaves – ‘mining’ them – before maturing and falling to the ground.
An additional problem created by the Shothole Leafminer is the proliferation of pathogens that lead to anthracnose, a fungal disease that results in leaves becoming discolored, curling, and eventually falling off prematurely. The pathogens enter the oak leaves through holes left by the leafminer then spread through wind and rain. You can read more about anthracnose here. While anthracnoses can result in a defoliated tree and aesthetic concerns, it is rarely fatal or even significantly harmful to trees.
Unfortunately, there is currently no good control for leafminers other than their natural predators, although during outbreaks these predators are less effective. Pesticides are generally ineffective for this pest as by the time damage is noticeable, the leafminers are no longer on the tree. Since the leafminers cause relatively little long-term damage, not much research has been done on their life cycles and ways to control outbreaks when they do occur. The NJ DEP is currently asking residents to help track the Shothole Leafminer and learn more about them. If you suspect damage to oak trees may be caused by the leafminer, take a picture of the damaged leaves and send it along with the county where the tree is located to the forest health program at foresthealth@dep.nj.gov. With your help, hopefully this pest can be tracked and more ways to control it can be discovered!