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Well, on second thought. There are bees of the genus Anthidium and they make such nest of pine resin.
http://www.wildbienen.info/Fotos/small/anthidium_strigatum_03_277.jpg
http://www.google.nl/imgres?imgurl=http://www.natuurfotoalbum.eu/map/watermark.php%3Ffile%3D7413&imgrefurl=http://www.natuurfotoalbum.eu/map/showphoto.php?photo%3D7413%26title%3Dkleine-harsbijanthidium-strigatum%26cat%3D663&h=480&w=640&sz=137&tbnid=6l1uoZ1WV4tPIM:&tbnh=90&tbnw=120&zoom=1&usg=__R8LapHNJ_WxsBozfSUyFgyUOsYg=&docid=JtY3F9GaVtKXiM&sa=X&ei=j3CQUsO7L8GatQb85IGYDw&ved=0CDkQ9QEwAg
Those are nests of A. strigatum, the smallest member of the genus in The Netherlands. But there are two bigger species who produce similar but bigger nests...

Greetings,

Ectemnius