Participants in the “Diagnostic Challenge” held during the 2026 Ohio Forest Health in Columbus last week observed tiny Elm Leafminer Sawfly adults (Fenusa ulmi (= Kaliofenusa ulmi), family Tenthredinidae) bagged in Southwest Ohio. The tiny, shiny-black adults look like gnats on steroids, buzzing around the leaves of their namesake host.
Of course, gnats are mini-flies belonging to the order Diptera. Sawflies belong to the wasp and bee order, Hymenoptera. The “fly” in their common name refers to their body shape. Although they are related to wasps, they lack the narrow "waists" associated with wasps, which makes them look like flies. The “saw” in their common name references their ovipositors, which look and function like tiny saws.
Life in a Leaf Sandwich
Elm leafminer sawflies spend most of the summer and the entire winter as pupae in the soil. Adults emerge around the same time Carolina silverbell (Halesia carolina) is in full bloom.
The adults mate, and the females use saw-like ovipositors to slice into leaves where they deposit their eggs. Most egg sites are located near a leaf vein; however, some may occur away from the veins.
Once the eggs hatch, the resulting larvae live and feed between the upper and lower leaf surfaces, where they mine the leaf parenchyma to produce blister-like, "blotch" mines. It’s analogous to consuming the contents of a sandwich without touching the bread.
The mines are usually bounded by the leaf veins as they extend towards the leaf margins. However, the blotch mines may blur the lines near leaf edges, and large numbers of mines may merge. The larvae develop through 5 instar stages before dropping from their mines to pupate in the soil.
Impact
Larval development is rapid, with the associated leaf mines quickly blossoming. Fortunately, the sawfly only has one generation per season. Heavily defoliated trees have plenty of time this season to produce replacement foliage without the new leaves becoming fodder for another generation.
The elm sawfly leafminer is reported to be a native of Europe that was introduced into North America in the late 1800s. Although the sawfly has a wide host range, certain elms are particularly susceptible. American elm (Ulmus americana) is listed as a host; however, it’s rare for the sawfly to produce noticeable damage on this native elm. On the other hand, slippery elm (U. rubra) appears to have an “eat here” sign hanging around its truck.
Likewise, only certain European elms are attractive to the sawfly. European field elm (U. carpinifolia) usually shows negligible damage, while Camperdown elm (U. glabra 'Camperdownii') and other Wych elms consistently provide an annual sawfly brown-out. Among the Asian elms and associated hybrids, only Manchurian (cut-leaf elm) (U. lanciniata) appears to attract the sawfly; however, the damage seldom rises beyond a moderate level.
Research has shown a strong correlation between host plant phenology and susceptibility to the ravages of the elm sawfly leafminer. The most susceptible elms leaf out at around the same time as the peak emergence of adult sawflies. Elms that were less susceptible leaf out after the peak emergence had passed.
Management
Although the leafmines may appear unsightly, the damage seldom causes significant harm to the overall health of established elms. As noted above, heavily damaged trees have enough time this season to produce new leaves and accumulate the required volume of stored carbohydrates to support leaf expansion next year.
However, damage to elms in nursery production is less tolerated. Newly planted trees in landscapes may experience added stress from heavy early-season defoliation, making them more susceptible to other issues.
Topically applied contact insecticides, such as pyrethroids, targeting the newly emerged adults can be effective. However, once the adults have emerged and commenced egg laying, systemic insecticides are more effective in preventing or halting leafmining damage. These include acephate (e.g., Orthene), various neonicotinoids that can rapidly reach the leaves, such as dinotefuran (e.g., Safari, Transect, etc.), and systemic anthranilic diamide insecticides, such as chlorantraniliprole (e.g., Acelepryn) and cyantraniliprole (e.g., Mainspring GNL). Of course, it’s important to read and follow label directions, paying particular attention to the application timing to make sure the insecticide reaches the larvae before they produce heavy leafmining damage.
Selected Resources
Miller, F., and G. Ware. 2014. Seasonal Activity, Within-Tree Distribution, and Ovipositional and Feeding Preference of the Elm Leafminer, Fenusa ulmi (Hymenoptera: Tenthredinidae), Journal of Environmental Horticulture 32 (1): 39–48.
Smith, D.R. 1995. The Elm Leafminer, Kaliofenusa ulmi (Sundevall) (Hymenoptera: Tenthredinidae) in Virginia, and Summary of Host Records, Virginia Natural History Society, Banisteria, Number 5, 39-41
