Holy tree damage! Insects take a bite out of oaks, elms

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This past week, I have had several calls about Oak and Elm trees that look like they were used for target practice.

The leaves had holes like they were shot with a shotgun. No, it is not varmint season; rather, the holes are being left by insects.

On Oak trees (Quercus spp.) the holes are being left by the Oak Shothole Leafminer (Japanagromyza viridula, syn. Agromyza viridula). The Oak Shothole Leafminer is a native North American fly that’s grouped together with several species native to Asia. Of course, they may all share a common ancestor somewhere in the genetic woodpile.

The Leafminer produces four sequential symptoms: Small pinprick-like holes, larger holes, dark brown “blotch mines,” and ragged-looking leaves with missing pieces. Although the damage occurs in the spring, the symptoms become more noticeable as the season progresses.

The symptoms start with female flies using their sharp ovipositors (egg laying device) to pierce the leaf epidermis of newly expanding oak leaves to release nutrient-rich sap which they then ingest using their lapping mouthparts. The damage produces small pinpricks with tiny spots of necrotic or dead tissue in the center. Numerous piercings can cause leaves to become distorted. If the females skewer newly expanding leaves or new 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 the characteristic “Swiss cheese” or shot gun appearance.

Elm trees are being damaged by the Elm Flea Weevils (Orchestes steppensis). Weevils produce three types of damage on their namesake host: holes in leaves, distorted leaves, and blotch-type leafmines. The weevils most commonly focus their holey work on American elm (Ulmus americana), lacebark elm (U. parvifolia), and Siberian elm (U. pumila).

Weevils are beetles with a snout (rostrum) and their chewing mouthparts are located at the tip of their snout. They are also known as flea weevils as they have hind femurs that are thickened to hold powerful muscles allowing them to flee by jumping like a flea.

Damage begins as overwintering elm flea adult weevils’ chew small holes in nascent leaves or new leaves furled in the bud in the spring. As the leaves expand, the holes get larger. Females also chew small notches in the mid-veins and major lateral veins of the leaves into which they lay eggs. Damage caused by oviposition may be made more noticeable if leaves fail to fully expand beyond the wounded leaf vein, and the affected area becomes distorted.

Once the eggs hatch, leaf mining larvae feed by tunneling through the leaf tissue toward the margins to produce “blotch” type mines. The leafmining activity usually occurs over about three weeks; then the larvae pupate inside their leaf mines. The necrotic tissue of old leafmines commonly drops from the leaf. The new adults that emerge from the mines produce the second round of seasonal leaf damage. These adults feed heavily for about a month, adding substantially to the number of holes in the leaf that were produced by the spring adults.

Most of these adults eventually drop from the trees around mid-summer and appear to become dormant for much of the summer. However, a few adults may continue to be found in the tree canopy until leaves drop in the fall. It’s this second round of adults that overwinter to get the ball rolling again next spring.

A few early season holes on oaks and elms 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 oak and elm trees. 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.

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