Larva in my dining chairs

The fixing holes in my wooden dining chairs are exactly the right size for an unidentified insect to hide its larva, swaddled in green leaves.

In all, six holes across three chairs have tell-tale little green leaves peeking out of them, and when I introduce the end of a sharp stick, each hole reveals a chrysalis or larva, orange-brown, gooey in death.

The chairs are in the conservatory, which is regularly exposed to the outside, for reasons of thermal ventilation.

More mysterious is why one chair was spared from the infestation/colonisation.