A global community of nature enthusiasts
photographing and learning about wildlife
These galls are caused by the gall midge (Mikiola fagi), which lays its eggs inside the leaf's tissue. When they hatch, the larvae secrete chemicals that cause the leaf's tissue to form a protective 'bubble' around them.
The eggs of Mikiola fagi affect the leaves of Fagus sylvatica, which is its hostplant.
Spotted near Gerritsflesch, Veluwe, Holland. (sources: see reference)