very likely yellow jackets. check online for pictures. You can usually get rid of them with a garden hose if they are really causing problems. Digging them up would not be a good idea until after they are dead.

Thanks, tonybeeguy, for the reply. I also posted my question to the California gardening forum, and the two people who replied there also thought these were probably yellowjackets rather than bees. I know I've also had paper wasps building small honeycomb-looking nests in my house's eaves, but these are definitely smaller insects than the paper wasps I've seen flying around. If they are definitely not bees (the good guys), I'll want to get rid of them. Do you know if the colony will die off in a mild climate in the fall or winter? marilano

a hard freeze kills the colony. That may be no help to you

I've had problems with yellow jackets on the hillside when mowing the grass, got stung up on both legs before, then fully dressed with work boots and they didn't get me, but this day I wore a thin long sleeve shirt and they stung right through it. Got Stung all over my torso, front, back, sides and both arms. These yellow jackets here are really bad when provoked, they hung on my clothes and even followed me. Also don't know how a garden hose can get rid of them since we had been having heavy rains most of this summer and the yellow jackets are still here! I did try Hornet Killer, but that did very little if anything to these yellow jackets.

My neighbors pour gasoline in yellow jacket ground nests. (They don't set it on fire: just pour it in.)

I have had mixed luck with Spectracide's hornet spray on a yellow jacket nest (though it always works well on bald-faced hornets' paper nests).