I found Elm Leafminer Sawfly (Fenusa ulmi) adults flying around their namesake host yesterday in southwest Ohio. The emergence of this leafmining sawfly was predicted in my part of the state by accumulated GDD (219) and phenological indicators such as the full bloom of common chokecherry. This also means there is little doubt that Birch Leafminer Sawfly (F. pusilla) and Hawthorn Leafminer Sawfly (Profenusa canadensis) adults are on the wing.
Sawflies are so-named because of their appearance and the shape of their ovipositors. Although sawflies are related to wasps, they lack the narrow "waists" associated with wasps which makes them look like flies. Their ovipositors look like tiny saws. The three species presented here use their ovipositors to slice into leaf edges where they deposit their eggs.
Once eggs hatch, the resulting larvae mine the leaf parenchyma producing large, blister‑like, "blotch" mines. The mines usually extend from the leaf margin toward the midvein. Although the leafmines may appear unsightly, these sawflies seldom cause enough damage to significantly harm the overall health of established host trees. However, severe leafmining damage may produce stress on newly planted trees.
The hawthorn and elm leafminers have one generation per year while the birch leafminer has three generations. For most insect pests, the occurrence of multiple generations usually means upwardly spiraling populations and ever increasing damage as the season progresses. However, the opposite is true for birch leafminer because larvae can only mine new leaves. Most damage occurs in early spring when the first generation larvae mine the newly expanding leaves. After they finish feeding for the season, around 80% of the first generation larvae drop to the ground and remain as pre‑pupae until next spring. Control efforts should target the first generation since the second and third generations cause little damage, unless the tree is re‑foliating after leaves were stripped by some other problem such as a general defoliating caterpillar.
If control of these sawflies is deemed necessary, a soil drench application of dinotefuran (e.g. Safari) made now will prevent larval leafmining activity. Imidacloprid (e.g. Merit, Xytect, etc.) will also suppress larval leafmining; however, it is probably too late in southwest Ohio to prevent all damage since it takes around 30 days for the insecticide to move into the tree in sufficient concentrations to provide control. The best time to make soil drench applications of imidacloprid to prevent larval leafmining damage is in October or November.
It's important to note that it will be too late to halt damage caused by these leafminers this season once leafmines become obvious. Home gardeners may find products with the aforementioned active ingredients in their local garden centers; look for products that list these leafmining sawflies on the label and always follow label directions.