On Sun, 26 Oct 2003 15:07:38 -0600, Brian Anderson
wrote:

A whole village of little black dirt hills appeared in my yard
seemingly overnight. There must be 20-30 of them in one 10x12
area, most about three inches in diameter. There are similar
but smaller villages elsewhere in the yard.

I scraped some of the dirt hills away and found a small hole,
maybe 3/8" in diameter, under each one. It looks like the
hole of a burrowing snake or something.

What have I got? Is this thing harmful to the lawn, or does
its tunneling help aerate the ground? The areas it has chosen
to tunnel in are only sparsely covered with grass, so it
really can't do much damage unless it spreads to the healthier
areas.

Hmm

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