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The large nests of the wood ant are hard to miss! Photo: Pdeva1509, Wikimedia Commons

If you see a large ant nest in your lawn or garden and medium-sized black or black and red ants coming and going, it’s likely the work of wood ants (Formica spp.), also called mound ants or field ants. They are much larger than the little brown ants (Lasius spp., with L. neoniger being very common in Eastern North America) that make small but numerous craterlike nests in lawns and on patios and smaller than carpenter ants (Camponotus spp. ) that build their nests in wood of some sort (tree trunk, stumps, barns, houses, etc.).

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Wood ants can be entirely black or read and black. Photo: Wikipédia

There are many species of wood ants found throughout the Northern Hemisphere. In North America, black ones may be F. glacialis among others, while in Europe the most common black ant is F. niger. The red wood ant (F. rufa and similar species) is found on both continents. It is bicolored red and brownish-black and tends to build its mound of grass, twigs or conifer needles.

A Humungous Nest

Little brown ants may annoy lawn lovers with their numerous small nests, but wood ants make humungous ones and do much more damage. Their large nests, with several to many entrances, can measure up to 3 feet (1 m) in diameter, although 1 foot (30 cm) is more common. It’s made up of sand and soil particles the worker ants removed in building the nest. Their constant tunnelling and digging disturb that plants that originally grew there and most die or barely hang on to life. And don’t think the problem will go away on its own: the colony can live 7 to 10 years, sometimes longer!

So, what are you to do about them?

The Queen Must Die

The key to success in controlling all types of ants is to eliminate the queen. You can kill all the worker ants you want; the queen will simply produce more. What complicates the control of wood ants is that they are often polygynous … that is, the colony may have several queens… and you have to kill all of them to get rid of the nest.

Because you need to kill the queens, who live safely out of sight underground, the insecticides you might use to kill other harmful insects (diatomaceous earth, insecticidal soap, pyrethrum, etc.) will not be effective against wood ants. They act too quickly and kill the workers without reaching the queens. To get to the latter, you need a slow-acting insecticide that won’t harm the workers that carry it, but that they can bring into the nest to feed to the queens. Thus the colony’s mothers be poisoned bit by bit … and when there are no queens left, the colony will collapse.

Commercial Ant Pesticides

Always read the label when you buy any insecticide to control ants. Many are designed to keep ants from entering the house, not to kill nests, and wood ants almost never venture indoors.

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Commercial ant traps are often effective… but do give them time to act.

For wood ant control, you’ll need an ant bait (sometimes called an ant trap, although in fact ants are never trapped inside them.) They consist of some sort of bait mixed with a mild pesticide, both placed inside a container with small openings that only ants can enter. (This is done to protect children, pets and wild animals from getting to the bait). Ants then pick up bits of poisoned bait, carry it to the queens and they end up slowly dying.

These commercial traps do work, but you have to be patient: they can take up to 3 weeks to kill an ant colony.

Homemade Ant Bait

You can also make your own ant bait trap.

20170701DPrepare a solution of one half cup (125 ml) hot water, 2 tablespoons (30 ml of borax) (found in hardware and drug stores) and ¼ cup (60 ml) of sugar, adding the solids gradually to the water, stirring so they dissolve. (Keep any surplus in the fridge in a closed container for future use.) Pour two spoonfuls into an empty soda can: with its small opening, ants can enter and exit, but pets and children won’t be able to touch the liquid which is, after all, slightly toxic.

Place the can on its side near the nest. Boron (of which the natural mineral borax is composed) is slightly toxic and therefore the solution will also be slightly poisonous. The workers, attracted by the sugary compound’s sweet taste, will bring drops of the solution to the queens. After being fed small amounts of boron day after day, the queens will become poisoned and die. And without queens, the colony will soon collapse. But patience! It may take up to 3 weeks to see results.

Or Leave Them Be

Personally, I’m not a lawn aficionado and really don’t care much if a wood ant colony sets up shop in my lawn and will even, more grudgingly, let them build their nest in a corner of a flower bed. They are, after all, somewhat beneficial (wood ants are great scavengers and fierce predators of several harmful insects, although they can also annoy gardeners by maintaining aphid colonies from which they harvest honeydew.) Also, they are highly territorial, so any nests will be spaced widely apart and you’re not likely to have more than a colony or two on the average lot.

Live and let live: a good motto for laidback gardeners!

3 comments on “How to Control Wood Ants

  1. Milena Frieden

    Thank you for posting an answer that finally makes sense . . . killing the queen/s. I’ve had a colony in one of my hugelbeds for a couple years now. They were never much of a bother, but, unfortunately, this year, they’re starting get out of hand. After a couple of nasty bites, I will, now try your suggestion.

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