This can be a cosmetic problem but rarely kills the tree. It can look very bad. For a thorough discussion of the elm leaf miner, see the reference below. Within the link, there is a separate treatment option link  and  under "chewing insects" you will see the sawfly compound list. The organic compound that would work the best, though probably not at this stage, is spinosad . There is a systemic non-organic compound called imidacloprid that can be drenched and moved from the roots into the leaves. These populations ebb and flow from year to year, mostly due to weather conditions and predation. Hope this helps. Feel free to call me directly if you have more questions. Chip Bubl OSU Extension Agent/Columbia County<personal data hidden> cell <personal data hidden>

https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/elm-ulmus-elm-leafminer