Elm Flea Weevils (Orchestes steppensis) produce three types of damage on their namesake host: holes in leaves, distorted leaves, and blotch-type leafmines. The three symptoms are currently on display in southern Ohio.
The weevils most commonly focus their holey work on American elm (Ulmus americana), lacebark elm (U. parvifolia), and Siberian elm (U. pumila). These elms are least preferred by the Elm Leafminer Sawfly (Fenusa ulmi), so the two pests are seldom found on the same trees.
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.
Overwintered elm flea weevils chew small holes in nascent leaves 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, 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.
The True Weevil Revealed
For years, we incorrectly identified this weevil as the European Elm Flea Weevil (O. alni). 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 led 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.
Recognition that the weevil wreaking havoc on elms in the U.S. was an unknown species was the result of sleuthing by James N. Radl and his graduate program advisor, Dave Shetlar (“Bug Doc,” Professor Emeritus, OSU Entomology). Radl used the common name “Elm Flea Weevil” for O. steppensis in his 2018 Ohio State University M.S. thesis.
It's important to note that the name change does not affect the validity of previous research on this weevil in the U.S. All of the observations and research reports published between 2003 and 2018 regarding the flea weevil affecting elms in the U.S. remain legitimate even if the information applies to O. steppensis and not to O. alni.
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.