The holy handiwork of the Elm Flea Weevil (Orchestes steppensis) is evident on native, non-native, and hybrid elms in southwest Ohio. Holes in elm leaves result from the adult “snout beetles” feeding on the leaves as well as the larvae tunneling between the upper and lower leaf surfaces as leafminers.
Fortunately, all of the leaf damage produced by this weevil has already occurred for this season. Unfortunately, the symptoms will remain evident throughout the rest of the season.
This weevil was incorrectly identified as the European Elm Flea Weevil (O. alni) for many years. That's because O. steppensis was a species that had not yet been identified in its native range of Eurasia when damage began to appear on elms in the U.S. It was determined that the damage was being caused by a non-native weevil. However, the only known weevil species that “matched” was O. alni which is almost identical to O. steppensis in terms of size, color, life cycle, and lifestyle (= leaf damage).
Eventually, the weevils causing damage to elms in the U.S. were found to be morphologically identical to an unnamed weevil species found in Eurasia. This lead to the eventual description of the species as O. steppensis and the recognition that this was the true U.S. invader, not the European elm flea weevil which has not yet been found in North America.
On the upside, all of the observations and research reports published between 2003 and 2016 regarding the flea weevil affecting elms in the U.S. remain valid even if the information applies to O. steppensis and not to O. alni. On the downside, O. steppensis has not yet been given an approved common name through the Entomology Society of America (ESA).
In the meantime, I'm going to use "Elm Flea Weevil" for O. steppensis even though this common name has not been recognized by the ESA. This was the common name used by James N. Radl in his 2018 Ohio State University M.S. thesis. Recognition that the weevil wreaking havoc on elms in the U.S. was an unknown species was the result of sleuthing by Radl and his major advisor, Dave Shetlar (“Bug Doc,” Professor Emeritus, OSU Entomology).
Weevils are beetles with a snout (rostrum) and their chewing mouthparts are located at the tip of their snout. So-called “flea weevils” have hind femurs that are thickened to hold powerful muscles allowing them to flee by jumping like a flea.
The elm flea weevil has one generation per year; however, adults cause damage at two different times during the growing season. The weevil spends the winter as adults in protected locations such as beneath bark plates of their elm hosts.
They emerge very early in the spring to feed and frolic (mate). The weevils use their mouthparts at the end of their snouts to create small pit-like holes on the undersides of newly emerging leaves. The holes expand as the leaves expand to produce a characteristic "shothole" effect on elm leaves.
Females also use their chewing mouthparts to 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, larvae feed as leafminers 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 leaves to produce large holes as well as leaves with missing areas, particularly at the tips.
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.
The majority of these adults eventually drop from the trees around mid-summer and appear to become dormant (aestivate) for much of the summer. However, a few adults may continue to be found in the canopy until leaves drop in the fall. It's this second round of adults that overwinter to get the ball rolling next spring.
Although the leaf damage produced by the adults is very noticeable, it has not been observed to be severe enough to cause harm to the overall health of landscape trees. Thus, insecticide applications are not warranted. In fact, topical insecticide applications could make things worse by killing bio-allies. According to various reports in the U.S., parasitic wasps may be capable of keeping populations below acceptable levels.
Of course, trees grown in nurseries are a different matter. IR-4 Project insecticide trials conducted in 2013 measured efficacy on American elm (Ulmus americana 'Patriot') in three categories: leaf area affected; percent canopy affected; and presence of leafmining activity. The trials revealed that soil drenches of the systemic neonicotinoids imidacloprid (e.g. Merit, Xytect) and dinotefuran (e.g. Safari, Transect, Zylam) provided adequate control with imidacloprid being the most effective. While nothing can be done to undo the damage being observed this season, nurseries with heavy damage should schedule applications early next spring to target both the overwintered adults as well as leafmining larvae.