Pest description and crop damage Small legless sawfly larva feed between the layers of leaf epidermis, resulting in large brown blotches. The elm leafminer, Fenusa ulmi, has been in the Northwest for a few years but has been noticeable in its expansion to new areas in Washington and Oregon recently. The sawfly attacks Scotch and Camperdown elms, Ulmus glabra), and Engish elm, U. procera.
Biology and life history The adult sawflies emerge coinciding with the breaking of the leaf buds of most elms, although after leaf expansion of American and European white elms (Scannell, 2000). The timing of emergence ranged from mid-March through mid-April depending on temperatures during the course of several years of study. The adults are all female and begin to lay eggs immediately after emergence. The eggs are usually laid near leaf veins initially but later oviposition is without regard to location of the veins. There are five instars of the larvae. The larvae were found about 10 days after the first emergence of the adult sawflies. The larvae eventually drop to the ground where they are reported to pupate through the summer, fall, and winter. There is one generation per year.
Remove infested leaves and destroy. American elm may have some resistance.
Spray when blotch mines first appear or before leaves fall. Do not use acephate on American elm.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 186).
PNW Nursery IPM: Elm leafminer (http://oregonstate.edu/dept/nurspest/elm_leafminer.htm)