Something attached to our linden tree leaves
Growths on these leaves appear to be tissue galls. These are caused by feeding injury of an Eriophyid mite inside the leaf, which saliva stimulates the abnormal cell development in spring. Eventually, the mite exits the gall, leaving the tissue.
The mites cause little harm other than aesthetic, and are usually not controlled.
For further information, I refer to the University of Minnesota fact sheet located at http://www.extension.umn.edu/garden/insects/find/insect-and-mite-galls/docs/DG1009.pdf .
from this fact sheet written by
Robert P. Wawrzynski, Jeffrey D. Hahn, and Mark E. Ascerno of
Department of Entomology, University of Minnesota Extension Service
"Most galls do not adversely affect plant health. Therefore, management is generally not suggested to protect plant vitality. Chemical applications are an option, but are often ineffective since the precise timing of sprays is critical. To be effective, sprays must be timed to coincide with initial insect/mite activity before gall formation begins. Once galls start to form, they conceal the causal organism and it is too late for treatment. For insects/mites that overwinter on the host plant, horticultural oil applications can be made before insect/mite activity begins in the spring."