I’ve had similar problems with various ant species at our home and cottage and done considerable research and now fancy myself an experienced ant killer. So, my apologies in advance for the long answer but since this is the Straight Dope…
My learnings:
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If an ants are in your house in the winter, it means they have a colony against or inside your exterior wall. Normally colonies go dormant when it’s freezing out and then get active in the spring when they wake up and start to forage. If they’re inside now, they’re still active and foraging, which means they’re warm.
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Most poisons are low dose, the workers bring the poisoned food back to the colony for the queen to eat. Eventually the queen dies and the colony dies.
The “food” you’re offering must be attractive enough to compete with whatever else they have available so they keep eating it. To be most effective, you need to identify (google) what species of ant you have and research what they eat in your area and then tweak the poison mixture for max. appeal (see below)
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In general, the store “ant traps” & poisons rarely work. They are broad-species targeted and although they may have enough of the poisoning agent in them, if the species doesn’t like the flavouring agent, they don’t eat enough and it won’t work. Don’t waste your money.
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My best results are with a homemade mixture or a sweetener + boric acid + water. Boric acid (not Borax) is available in most drug stores, just ask.
The general starting ratio is: 100 parts water: 10 parts sugar: 1 part boric acid. So if you want to make 1 litre of solution you use: 1000ml water: 100ml sugar: 10ml boric acid.
For “sweetener”, sugar is usually used, but at our cottage I didn’t find it very effective (same results as you had). I was told that since we had a lot of maple trees in the area the ants likely preferred maple flavour. So I added a bit of maple syrup to the mix and it worked amazing.
Next thing you need to do is tweak the ratios, this is critical. You need to tailor the strength to your species: the amount of poison to kill a large carpenter ant is different than a tiny “sugar” ant.
Start with the basic recipe and then strengthen or dilute it until it works. Add a small measured amounts of boric acid to strengthen or measured amounts of water to dilute.
If the colony isn’t gone in a week or so, the poison ratio is too low, strengthen it. If it’s too strong, meaning the boric acid level is too high, it won’t work. It kills the workers before they return to the colony and feed the queen. If you see dead workers, there’s too much Boric acid, dilute it.
Also, if it’s too strong and kills off too many workers some ant species (like the red ants I had) consider the colony to be “under attack”. The queen then spits out new queen eggs which hatch and make new colonies. You now have a much bigger problem.
- For the “feeding stations” the best option I’ve found is to use the cheap lidded plastic “tupperware” tubs from the dollar store. Drill or punch holes in the sides about a 1/4 up. and fill them with cotton balls then the mixture, then add the lids. You can’t have enough of these, I lay down 20+. Ants forage in many directions, you want to cover them all.
I usually make a litre or two of the poison and then keep topping it up as it evaporates or gets eaten.
- Last two tips: I’ve found this 100% effective, but as you already know, there is no quick homemade solution. Be patient.
Write down the tweaking you do, so you know exactly the ratios that work for you. The ant colony will return at some point, (other ants in your area are always looking for good sites to expand to). This will save you re-doing the entire process.
Good luck.