chestnut gall wasp

Description:

Large gall on chestnut leaf, red on the upper side, green & reddish on the underside. The infected leaf is stunted.

Habitat:

I spotted this gall in the woods behind our house; many of of the trees here are chestnut trees (Castanea sativa).

Notes:

If the ID is right, this is bad news, because it means that the worst chestnut pest has now reached the vicinity of Paris, and is thus spreading North.