What's making holes in newly expanding oak leaves in Ohio? The common name of the oak shothole leafminer (Japanagromyza viridula, syn. Agromyza viridula) clearly describes both the culprit and the damage they do to oaks. This small fly belongs to the family Agromyzidae; the leaf miner flies.
The name of the genus may imply its non-native. However, the oak shothole leafminer is a native fly that is grouped in a genus that also includes several Asian species. Of course, it's likely they all share a common ancestor somewhere in the genetic woodpile.
The oak shothole leafminer produces two types of leaf damage: holes and dark brown "blotch mines." The females use their sharp ovipositors (ovi = egg) to pierce the leaf epidermis releasing nutrient-rich sap which they then ingest using their lapping mouthparts.
They may skewer newly expanding leaves or nascent leaves furled in the bud. If the flies spear leaf tissue in the bud, the resulting holes on one half of the leaf will match holes on the other half. Although the feeding holes are very small at first, they expand as the leaves expand to eventually give the leaves a characteristic “Swiss cheese” appearance.
The leafmining larvae (maggots) produce “blotch mines" by consuming interior leaf tissue between the upper and lower epidermis. This causes the upper and lower leaf surfaces to delaminate; a tell-tale symptom of leafmining activity.
Active blotch mines are usually most evident in early to mid-May in Ohio. Once larvae complete their development, they leave their mines and drop to the soil where they pupate and spend the rest of the summer and the winter. There is one generation per season, so there will be no more flies to produce additional damage this season.
The leafmines are light green to tan but turn dark brown to blackish-brown once they're abandoned. The damaged tissue eventually drops from the leafmines to produce large, sometimes ragged-edged holes. Like the female feeding holes, the leafmining damage will remain evident throughout the rest of the growing season.
The blotch mines may be mistaken for oak anthracnose and vice versa. Of course, the fungal infections don't cause the upper and lower leaf surfaces to delaminate. Also, the anthracnose symptoms are usually centered on leaf veins and early infections cause the leaves to curl. However, separating the two types of damage becomes more challenging once the necrotic tissue produced by leafmining and leaf infects starts falling apart.
Oak shothole leafminer populations appear to be very high this spring throughout much of Ohio. As the season progresses, the leafmining damage may combine with the old female feeding holes to produce tattered leaves. Although the shothole / leafmining damage can detract from the aesthetics of heavily affected trees, the leaf injury appears to cause little to no harm to the overall health of the oak hosts. Of course, there's nothing that can be done about it once symptoms are evident.
A Tattered Tale
A disorder called "oak tatters" was reported in the early 1980s in a number of Midwestern states including Ohio and recurrence of the condition continues to be reported. Oak tatters has been described as leaves on affected trees losing the majority of their interveinal leaf tissue resulting in "leaf skeletons".
No clear cause has ever been determined. However, possible candidates have included early-season herbicide damage and freeze damage to the buds causing cells in the nascent leaf tissue to die producing missing leaf parts on expanded leaves.
Unfortunately, images of "oak tatters" posted on the web often show clear evidence of heavy damage caused by the oak shothole leafminer as well as oak anthracnose, or a combination of both. I'm not suggesting that leafminer and/or anthracnose symptoms are the true cause of oak tatters. However, I'm cautioning that we must separate these known causes of tattered oak leaves from the possible unknown cause(s) behind oak tatters.