Possibly Harmandiola tremulae, one of the many gall midges in Cecidomyidae, on one of the "corner" Aspens by the first junction beyond the dog bin. 

Two of these species form an ID pair, on the upperside and not projecting much below the lamina, H. tremulae the bigger, more globular with thicker walls, shiny red when mature and H. globuli, smaller, thicker necked sometime with a collar, and duller when mature. The larva of H. tremulae is a somewhat redder orange.

Note: sphericality, w.r.t. H. globuli

There seem to be more reports of this species by the smaller and less spectacular Harmandiola globuli. Perhaps those are just less often spotted. 

I get the impression that H. tremulae is more commonly found towards the base of the leaf, nearer the junction with the petiole. Perhaps this is less true of H. globuli.

About 3 - 4 mm across, a shiny red when mature, then blackening.