exit holes

Adult female borer beetles lay eggs within susceptible timbers; books and stacked paper may also provide a suitable habitat. The eggs hatch into larvae, which feed on the starch content of the wood and create networks of tunnels. When fully grown, the larvae pupates to become an adult beetle, finally emerging through a hole made in the surface of the wood to continue the life cycle. The presence of exit holes and frass indicates the presence of borers. Exit holes are perfectly round; an elliptical hole usually means the hole existed before the wood was shaped or cut. 

A beetle emerging from a painting. Photo courtesy of Queensland Art Gallery.

Exit holes in a comic book. Photo courtesy of the State Library of Victoria.

Exit holes on a gilded wooded frame - not all of the holes are perfectly round, suggesting that some infestation was present prior to the construction of the frame. Photo courtesy of Alice Cannon.