We have a traditional well water system. Pump outside, tank in the basement, water softener and filter. No ants in the basement. We change the water filter every 6 weeks or so (more in the summer, less in the winter). Today, the water filter was filled with small black ants (dead) and we had little water pressure before changing.
Where could they be coming from?

The ants could be entering the well head. I've seen that happen when a colony installed itself right next to the well.

This has been quite a while, so how did you fix the problem?

The ants could be entering the well head. I've seen that happen when a colony installed itself right next to the well.

This has been quite a while, so how did you fix the problem?

Quite a while is an understatement. This is a good answer, however.

Azw agrees :
Thanks for replying, Castmine. I've got a similar issue and am thinking of spreading some Diatomaceous Earth (DE) around the well head, along. Is there a better solution?

I'll reopen the question since you're having the problem. Diatom dust becomes ineffective with repeated wettings, it clumps. Try baiting instead. I normally recommend against baiting but this isn't indoors so attracting them won't increase your problem. Commercial baits do fine (Terro, Maxforce) or you can make your own by mixing Boric Acid powder with peanut butter (as much as will mix) and stuffing soda straws full. The straw will keep the bait in the middle from spoiling until the ants can get to it.

Wow, that's an interesting way to deliver the bait! Thanks, Art

Is there a good method for stuffing the straw with the peanut butter mixture?

Is there a good method for stuffing the straw with the peanut butter mixture?

Not really. Cut the straw into 1 inch lengths(2cm). The first few are easy enough, just push the straw in all the way. When you get down to the last of the mixture you have to trowel it in. I use a butter knife. When you're done you have enough to last a year.

Ah, the trick is cutting the straw into 1" lengths! That makes sense. Thanks, again!