I'm a big fan of elms; particularly American elm (Ulmus americana) and its impressive comeback. There is an outstanding specimen in Spring Grove Cemetery and Arboretum that can be enjoyed by travelers on Spring Grove Avenue. I suspect it may be resistant to the Dutch elm disease fungi, Ophiostoma ulmi, and the more virulent O. novo-ulmi.
However, the leaves of native elms, non-natives, and hybrids can look a bit bedraggled at this time of the year owing to the adult pit feeding activity and larval leafmining activity of the elm flea weevil (Orchestes steppensis). Fortunately, leafmining by the weevil has drawn to a close, so the damage you see now will be the most damage that you'll see this season. Unfortunately, round two of the adult weevil damage is already underway.
A Wobbly Weevil Identification
The true identity of the non-native flea weevil damaging elms in the U.S. was cloaked by understandable confusion for many years. That's because the non-native flea weevil that was first identified to be the culprit in the early 2000s was almost identical in size, color, life cycle, and lifestyle to the European elm flea weevil (O. alni). Adding to the challenge, the true culprit, O. steppensis, was an unknown species that had not yet been identified in its native range in Eastern European Russia and Central Asia.
However, in a backward non-native discovery, it was eventually found that the weevils in the U.S. were 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.
On the bright side, all of the observations and research reports published between 2003 and 2016 regarding the flea weevil in the U.S. remain valid even if the information applies to O. steppensis and not the European elm flea weevil, O. alni. Equally important, Dave Shetlar and his M.S. graduate student James Radl began studying the life cycle and host preference of O. steppensis in Ohio and James' 2018 M.S. thesis provides a better understanding of this non-native flea weevil.
On the downside, O. steppensis has yet to acquire 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.
Weevils are beetles with a snout (rostrum) and their chewing mouthparts are located at the tip of their snout. If you use a hand lens, you clearly see the snouts on elm flea weevils. If you're careful, you will also see that their hind femurs are thickened to hold powerful muscles allowing them to jump like a flea. If you're not careful, the flea weevils will flee.
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.
The weevils emerge very early in the spring to feed and frolic (mate). They 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 mid-veins and major lateral veins of the leaves into which they lay eggs. Damage caused by oviposition may be noticeable with leaves failing to fully expand beyond the vein wounding and the affected area turning yellow and curling downwards.
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 a three week period, then the larvae pupate inside their leaf mines.
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 leaf-holes 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. Ultimately, 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 well below acceptable levels.
Of course, trees grown in nurseries are different matter particularly if they are scheduled for sale this fall. IR-4 Project funded 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), as well as foliar applications of the topical pyrazole insecticide tolfenpyrad (e.g. Hachi-Hachi), all provided adequate control with imidacloprid being the most effective.