Description and crop damage Adults are tiny, black, fly-like sawflies which can be seen on new foliage on sunny days. Small, legless leafminer larva feed between the layers of leaf epidermis, resulting in large brown blotches that gradually coalesce into one large mine consuming the entire leaf. Large trees may be completely defoliated; however, as new growth continues in May, the trees begin to refoliate out at the branch tips and fill in with little apparent effect. On Camperdown elm, however, the mines don't coalesce and the dark blotches are visible on the leaves that remain on the tree.
Biology and life history These sawflies overwinter as pupae in the soil. As trees begin to leaf out, adults emerge from the soil and can be seen as black specks on the leaves. They begin to lay eggs, at first, in the angles where the secondary vein meets the midrib. As the larvae feed and grow, the mines appear to coalesce. Mature larvae drop to the soil to pupate. There is only one generation per year.
Pest monitoring Look for the small black adults as leaves unfold in the spring. They are most active when it is sunny within the tree canopy. The little black adults are easily visible on the upper leaf surface. Sticky cards can be placed on the sunny side of the plant in early April to provide early warning that monitoring should begin. Degree day models are available.
As leaves drop larvae are found on the soil and underlying pavement. Use of a shredding leaf blower in reverse, may remove a number of larvae, and dispatch them while making useful mulch.
See "Leafminer" in:
For more information
Rosetta, R. 2004. Elm leafminer. PNW Nursery IPM (http://oregonstate.edu/dept/nurspest/elm_leafminer.htm)
Scannell, C.M. 2000. The Biology and Seasonal Life History of the Elm Leaf Miner, Fenusa ulmi (Sundevall), in the Pacific Northwest (Hymenoptera: Tenthredinidae). Master's Thesis, University of Washington.