The insects you describe are probably cicada killers. They look like giant wasps
or bees and make holes, which lead to dens where they lay their eggs and insert
a cicada for the larvae to feed on till it is morphed into a flying cicada
killer.

V


On Mon, 27 Oct 2003 04:50:33 GMT, Elliot Richmond
opined:

Could be some sort of wasp. In our yard we get some honking big wasps
that dig around in the fern beds by the front door. But the holes are
a little bigger than you describe, maybe 15 mm. And they are long gone
by now.

Earthworms leave castings (worm poop) around their tunnels like you
describe but the holes would be much smaller. Perhaps 2 or 3 mm.
(Maybe 1/4 inch?)

Could be crayfish as Victor suggested, but they leave their diggings
in a little levee around the hole. You would not have to scrape
anything away to see it. And they dig down to the water table, so if
the holes are crawdad holes you have some subsurface irrigation. Lucky
you.

Cut worms (the larvae of June bugs, aka May beetles) don't leave
mounds and the adults are long gone anyway. The May beetle larvae will
not hatch until next spring.

So, this is not much help except to say that my guess is the holes are
most likely some sort of burrowing arthropod or annelid and absolutely
harmless to your lawn.

Elliot Richmond
Freelance Science Writer and Editor