Hi Howard:

Thank you for your message and for contacting Iowa State University.  There are no easy answers for the various beetles that feed inside wooden furniture.  It's also not always known which beetle is involved.  If this were a brand new twig chair, I would suspect bark beetles, but not in a 10-year old chair.  Another possibility is false powderpost beetles that are in the wood because of a high moisture content.

Reducing the moisture content in outdoor furniture might not be practical, but it comes up as a suggestion.  It might be possible to heat-treat the furniture to kill beetles inside the wood.  I suggest wrapping the chair in black plastic and place it in the sunshine.  A temperature of 140 degrees is lethal to all stages of insects.

Spraying is not a practical solution, though a pest control professional could treat bare (unvarnished, unpainted) wood with borate compounds.

It turns out I wrote an article about this 14 years ago.  Bless the internet!  It is still available at https://www.extension.iastate.edu/news/2007/nov/070201.htm

Orkin's online article might also be useful.  https://www.orkin.com/other/beetles/furniture-beetle

If you can capture an insect emerging from the tiny holes in the wood we could identify it and give a slightly better answer.  They chances of catching a powderpost beetle are very small.

Thanks.

Donald Lewis