What would attract ants into the attic?

Totally non professional opinion.

If it were happening to me and there were really no problems in the main house I’d just go up there and spray all around the perimeter of the attic where they could be possibly getting in and monitor it. Most sprays say they last a couple of weeks if not more so you should be fine. If you really dont want to go up there opt for just spraying around the top of your main living area.

You know, when ants travel, they lay down a trail of formic acid that other ants can follow. I suppose it is possible that an ant randomly made their way into the attic, and a bunch of other ants are following even though there’s nothing of interest to them in there. You might try washing down the area where the ants are entering and see if they come back or not. You may have to wash it down a few times until all the ants in there have left.

The worse news might be that they found a good place for a home in there. So they’ve got a queen and are settling in. But then I’d expect to see more and more ants following that trail.

There’s the possibility that some other critter got in there and died and the ants have found themselves a lovely attic buffet. Or, I’ve known ants to come into a house (and maybe your attic) when it’s super hot outside, looking for water, or when it’s pouring rain outside and they’re looking for some place drier. Is there a possibility that there’s a leak in a pipe up there, or that because of the heat, there’s some type of pool of condensation that looks like a ant’s watering hole?

Some ants turn into termites, I would probably call an exterminator just to be sure.

Maybe there is something you stored up there, like Christmas tree decorations with a stray candy cane? Ants can find all kinds of things.
I’d spray their trail(s) and respray until they disappear. You may find what is at the end of the trail so you can remove it.

Ant traps work better than spray.
Maybe a mouse died up there?

An aside, there were the most amazing ants in Belize IMO. They were nicknamed cleaner ants. They would come out of no where and completely take over your house in the millions to the point that everything was black because its covered in ants. While they’re in your house they will devour absolutely everything they come across. I heard stories where people had tons of dirty dishes and everything was spotless when they left four hours later. Really wish I could have witnessed that event actually.

Water in and on wood is quite attractive to some ants.
Look for water leaks in the roof.

To wipe out the pests you can try 2 approaches. 1)Baited traps – The ants pick up the bait and take it back to the nest killing the colony
2) Spray/ poisons : you kill what you see.

Never do both at the same time. You are wasting money. I would suggest a staged approach. Put out baited traps for 2 days and leave them alone. Use a few of each type of trap: sweet and grease. On the third day spray. On the fourth day, when everything is dry, dust with a mixture of diatomaceous earth and boric acid.
On the fifth day, you rest.

I am grateful for all the comments and advice.

To clarify two points:

• The ant trail is quite thin and scraggly, leading us to believe that there is no feast (such as a small deceased rodent) in the attic. We would expect to see a major trail if something big and yummy were attracting them.

• The location is really quite inaccessible. There is noplace to lay a trap. There’s just the outside wall of the house, flush except for street-level windows and that little vent up near the peak of the roof. If we could get into the attic without a huge upheaval, we could see if there’s a nest and a food supply and deal with it.

The attic is nothing but an insulation space. There is nothing stored up there. No one has even opened the trap door at the top of the steps in probably 15 years, and it would take some doing because it’s inside a storage closet.

The question is: is there enough evidence to say we have to do that? How likely is it that this is a situation that will get out of hand? Can we afford to ignore it?

I would wait a few weeks and see. You could always staple or tack an ant trap to the wall for a week or two.

Seriously. Just spray the ant trail area with water and see what happens. Do it a few times. It won’t hurt anything, and it might help.

What kind of ants?
Could they be carpenter ants?

Well, I don’t see any little teeny hammers and saws…

They’re just the normal little ¼” small black ants that we see around here all the time. I guess I think of them as generic, even though of course they must be of some particular kind. I’m going to try the water spraying solution as a first resort.

^^ You’d recognize a carpenter ant by his size. When I was younger and more naive, I had “Terminex” here to check out some piles of sawdust. While we were outsize inspecting the house, a large ant wandered by with a small splinter of wood in his hands grasping appendages.

I signed a year’s contract; then my bro-in-law said that the Terminex guy probably had the ant in a match box in his pocket ready to be released when I was looking at something else.

I then spent a lot of energy to cancel the contract.

a small flat dish of honey mixed with boric acid will kill them, if that is your goal. I vote for peaceful coexistence, myself.

(It turns out that the sawdust was generated by the carpenter aka borer bees, who were themselves fodder for the flickers, who gouged out long strips of my cedar siding in order to eat the bee larvae, who made rustling noises. I’m hoping that I will outlast the siding.