Observation notes: Aspen Serpentine Leafminer. Have not seen evidence of them in the leaves yet but very commonly do later on in the summer.
Very tiny. I noticed one fly to freshly opened willow catkin. It sat there (presumably) feeding for as long as I continued to photograph it. Primarily aspen forest (Balsam Poplar) with some birch and a few willow in the draws. Temp 55 F.