Shipworm (Toredo navalis)

Not insect damage. Damage is caused by a mollusc when the wood is left in sea water. These creatures can grow up to 1 metre long!

Can be mistaken for House Longhorn damage because of the large holes. Note: the holes are round. Some superficially appear oval because they have been cut on an angle.

Also note the 'white calcareous lining' in the holes - this is the give-away. Obviously no frass.

Not common since the attack occurs in the sea water and very few such timbers would be incorporated into a house.

Identification:
There are a number of insects that 'attack' wood in one way or another. Some utilise the wood for food whilst others use it to make pupal chambers to rear their offspring. In most cases the wood is well rotted.