The oak shothole leafminer (Japanagromyza viridula, syn. Agromyza viridula) is a small fly belonging 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 North American fly that’s grouped in a genus that also includes several species native to Asia. Of course, they may all share a common ancestor somewhere in the genetic woodpile.
The leafminer produces four progressive symptoms: small pinprick-like holes, larger holes, dark brown "blotch mines," and ragged-looking leaves with missing pieces. The symptoms start with female flies using their sharp ovipositors (ovi = egg) to pierce the leaf epidermis of newly expanding leaves to release nutrient-rich sap which they then ingest using their lapping mouthparts.
The method of feeding and subsequent leaf damage is the same as with other agromyzid flies such as the native holly leafminer, Phytomyza ilicicola. However, ovipositor punctures in holly leaves never develop into holes.
The female oak shothole leafminer ovipositor damage produces small pinpricks with tiny spots of necrotic tissue in the center. Numerous piercings can cause leaves to cup downward.
If the females skewer newly expanding leaves or nascent leaves furled 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 southern Ohio. Once larvae complete their development, they drop from their leafmines unto the soil where they pupate and spend the rest of the summer and the winter.
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.
There is generally one generation per year, so there are typically no more flies to produce additional damage this season. However, there are reports in the literature of a second crop of oak shothole leafminers occasionally appearing to attack leaves produced in a second flush of oak foliage or on leaves sprouting from epicormic growth.
The blotch mines may be mistaken for oak anthracnose and vice versa. However, a close examination will reveal several tell-tale differences which is particularly important if both maladies occur on the same leaves.
First, the fungal infections don't cause the upper and lower leaf surfaces to delaminate. Second, anthracnose symptoms are usually centered on leaf veins and early infections may cause the leaves to curl. Third, the fungal infections are typically confined to the earliest leaves with later leaves being unaffected.
Many of us once considered the shotholes to be nothing more than an oddity and would search oak foliage for leaves with matching holes across the midveins to photograph or to use in diagnostic workshops. However, this has changed in recent years with populations continuing to climb year after year.
Oak shothole leafminer populations were very high in 2019 in Ohio and seemed even higher last season. Likewise, Tawny Simisky, Extension Entomologist, UMass Extension Landscape, Nursery, & Urban Forestry Program, and Nicholas Brazee, Extension Plant Pathologist, UMass Extension Landscape, Nursery, & Urban Forestry Program, reported high populations in Massachusetts in 2019 and noted that outbreaks were also being observed in coastal Rhode Island, southern New Hampshire, eastern New York and eastern Pennsylvania [see More Information below].
A few early season holes are a minor issue. However, if there are numerous holes and leafmines, the damage becomes amplified as the leaves expand to produce tattered leaves. Although the cumulative leaf damage may look dramatic, it appears to cause little to no harm to the overall health of the oak hosts. However, the damage can detract from the aesthetics of heavily affected trees. 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 several Midwestern states including Ohio and recurrence of the condition continues to be reported. Oak tatters has been described as leaves on affected trees losing most 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.