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I've got a smallish black plastic compost bin in my garden and I recently noticed that an ant colony has taken up residence. This has happened a couple of times in the past, but they tend to move on when I turn the compost (not surprising, given I'm ruining their home).

However, is their presence actually good for the compost? Do they speed up the composting process by aerating it and dining on the 'food' therein? And if so, should I leave them be for a while?

The ants themselves don't bother me.

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my personal preference (for what it is worth). I wouldn't want an ant colony living in my actual compost pile. Yes ants & lots of offer insects, bugs, organisms (hopefully mainly beneficial) will be in there working their magic... Personally I want as many worms as possible in my compost pile, the more worms I see, the more I think (& believe) my compost pile is in a healthy state. – Mike Perry Jul 12 '11 at 2:44
you may wish to take a listen to this, Gardeners' Question Time, 22 Jul 11: RHS Show Tatton Park, Cheshire or direct link to MP3, & starting listening @ 10mins:45secs in. – Mike Perry Jul 24 '11 at 23:42
Gah! That contradicts the (seemingly well researched) advice given in bstpierre's answer below. :-/ Well spotted, though. – Mal Ross Jul 25 '11 at 8:52
I agree with Mike. I prefer a worm bin over an Ant Farm. I'm from the south and Fire Ants tend to take over an outside worm bin and feed on the scraps and kill/eat the worms which slows down the composting process. – Charles Byrne May 6 at 18:26
up vote 18 down vote accepted

The composting process depends on a complex chain of organisms -- which can include ants -- to consume and thus break down the food in your compost pile. See this page from the University of Illinois extension service, which comments on ants in compost:

Ants - Ants feed on a variety of materials including fungi, seeds, sweets and other insects. They help the composting process by bringing fungi and other organisms into their nests. Ants can make compost richer in phosphorus and potassium by moving minerals around as they work.

Since the ants don't bother you, letting them work on your compost a bit would have some benefit -- you're enhancing the food web by allowing them to stay. The downside is that, by not turning the bin when the internal temperature drops, you may not achieve high enough temps for long enough to kill off weeds and harmful bacteria. But if your mix wasn't going to achieve this anyway, then it's not an issue.

See also the following diagram of the compost food web, taken from this chapter on composting:

Compost food web diagram. NRC (National Research Council), 1981a Food, fuel and fertilizer from organic wastes. National Academy Press, Washington, DC., 132 p.

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Ants in compost heaps usually means the heap's too dry. The absence of obvious brandlings and other worms should confirm that. Add water and continue turning it.

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Turning the compost buries them and a good portion become compost for something else, so go with a little more moisture and let nature readjust the food chain that is a compost heap. Unless you're dealing with Fire ants. – Fiasco Labs May 19 '13 at 3:31

Agreeing with @bstpierre but as I find they are usually a pest (bites,etc) I put ant powder on them. There's always a tinge of regret and I try to use a minimal amount of powder because that stuff also kills all the other bugs and insects that live in compost. I need a more selective alternative but I suspect there isn't one. luckily we haven't had ants in our barrel for a while.

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I wouldn't put any such powders on my compost pile (that's just me). If I wanted to eradicate the ants, I would get an ant bait (trap), but not one that kills them immediately. Instead one that the ants take the bait out of and carry the food (bait) back to the nest. Thus killing the queen and the nest. – Mike Perry Jul 11 '11 at 16:45
That is actually how the ant powder works - it is slowish acting so that it is carried back to the nest. – winwaed Jul 11 '11 at 17:45
Are you actually applying the powder directly to the compost pile? (That was my initial understanding, please forgive if I got that wrong). If yes, that is why I suggested an ant bait (trap), self enclosed ie The bait isn't spread out over the compost pile. – Mike Perry Jul 11 '11 at 17:58
Yes I have done, but as I say "regret" as it probably kills everything else. I'll have to look for "bait traps" at Lowes/etc - I assume they're similar to roach traps. There is however, a potential in our problem as we do have a dog - he's been known to try and chew roach traps (!), so perhaps it won't work for us after all. – winwaed Jul 11 '11 at 18:05
@win - A one-shot method to destroy a colony is to pour boiling water over it. It will kill beneficials that get boiled, but since it's just water it does not have any carry-on effects. – bstpierre Jul 12 '11 at 17:52

Putting poison in your compost is crazy. If you feel really strongly about not having ants, then just cut off their access to your compost: put the compost bin on feet that rest in containers of water. Then just have patience and nature will take care of the rest. Eventually they'll go away; either another insect will eat them or they'll run out of food, but at some point Pachamama always takes care of things.

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What possible harm could the presence of ants bring to your dirt? I am thrilled when I see bugs in my compost, it means that it is a healthy ecosystem and bodes well for the plants.

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