Damage by the Hawthorn Leafminer Sawfly (Profenusa canadensis, family Tenthredinidae) on its namesake host is becoming evident in southwest Ohio. Several small hawthorn trees I saw yesterday in a local park had noticeable damage on a substantial number of their leaves.
Adults emerge in the spring when the accumulated Growing Degree Days (GDD) reach 260. This is around the same time that Japanese flowering crabapple (Malus floribunda) and coral burst crabapple (M. coralcole) are in full bloom. Once mated, the females use their saw-like ovipositors (= sawfly) to insert eggs into newly expanding leaves.
The resulting larvae feed singly between the upper and lower leaf surfaces mining the leaf parenchyma to produce large, blister-like, reddish brown "blotch" mines. The leafmines are usually confined to leaf tips although they may also occur along the edges, or occasionally cover entire leaves.
Holding the mined leaves up to a sunny sky provides “x-ray vision” exposing what’s going on inside of the leafmines. This “sky-lighting” technique is also useful for other leafminers.
Of course, you can also see what’s going on first-hand by inserting a knife tip between the delaminated upper and lower leaf surfaces and carefully peeling back the upper epidermis. I underlined “carefully” not because of knife safety, but because a defense strategy practiced by this, and other sawfly larvae, is to fiercely wriggle back and forth presumably to thwart predators and parasitoids. Several larvae thwarted my photographic efforts.
Opening several leafmines yesterday revealed that the sawfly larvae are well-developed, perhaps nearing pupation. The leafmining sawfly has one generation per season, so this means that the vast majority of the damage has already occurred this season. However, the damage will remain evident for several more weeks.
Although the leafmines may appear unsightly, control measures are seldom warranted. The damage is mostly aesthetic with the leafmining larvae seldom causing enough damage to significantly harm the overall tree health. Additionally, the continued growth of new foliage usually hides the leafmines and there’s plenty of time for the tree to acquire and store enough carbohydrates to fuel regrowth next spring.