I put nugget bark mulch (1 to 2 inch chips) in my yard in Seattle and
have found carpenter ants nesting in two spots, both under large flower
pots which sit in more or less permanent locations on top of the mulch
layer. I don't see them nesting in mulch that is in the open. I prefer
the large nuggets to fine grind for several reasons and intend to keep
using it in spite of the ant appearances because they're actually easy
to deal with when you know the exact location of their nest. However, I
never saw ant nests in all the many years I used fine grind bark. I
think it's because it compresses to densely for them to maneuver in.