In article , geoff
writes

"Mark Allison" wrote in message
...
I've got about 20 of them trying to make a nest in my cavity wall. I've
had
to fill any breathing holes in the wall with blu tack, to stop them
building.

There's a species of bees that will dig a hole through mortar and build
their nest in the wall.

More than one species - called Mason or Mortar Bees. They are solitary
bees, not colonial like honey bees, because each hole only contains one
female and her eggs or larvae, but sometimes large numbers live in holes
bored very close to each other and they can occasionally cause
considerable damage to house walls.

See http://www.nhm.ac.uk/entomology/insident/masonbee.html

--
Malcolm