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Ants in the Lawn

Is it just me, or are there more ants around than there used to be?

I frequently get questions from anxious garden-owners, fretting about the large bumps ruining their lawns: and certainly I would say that over the last few years, I have seen more and more of them myself.

So what's going on? What are they doing? Well, they are building homes for themselves by creating a complicated system of galleries, which are tunnels and chambers, underneath the ground. To do this, they excavate the earth and take it up to the surface. As the surface rises, the bare ground is able to absorb heat from the sun, and the whole thing becomes a sort of organic storage heater. Inside the heap, the worker ants move the eggs, larvae and pupae around as the sun moves, in order to keep them in the warmest part.

Unfortunately, in doing so, they ruin our lawns.

So what can we do about them? To encourage them to go and nest elsewhere, keep the grass cut short, and keep running about on it: the galleries are fragile in the early days, and a lack of disturbance is vital - the mounds cannot form in frequently mown areas. They are also vulnerable to damage by erosion and compaction by trampling, so get out there and dance under the moonlight!

If you prefer chemical warfare, get out the ant powder and dust the tops of the hills lightly whenever you find them. Use a hand tool to fluff up the soil, and if you find ants, dust them. If not, spread out the loose soil and dust it anyway, then work it down to ground level by gently brushing the grass to and fro. The idea is not to smother the ants, but to produce a well-dusted zone that they can't avoid walking through. Each ant picks up a light coating of the ant powder, then walks it back inside the nest - ants never wipe their feet

If it's wet, or if you have big lumps, use a trowel to make two slits about 4" long at right angles to each other, then lever up this "flap" to reveal the horrors below. If it's teeming with ants, apply powder, put the flap down and stomp it flat.

As for the lawn lumps, have a midnight chanting, stomping party! As soon as they start to appear, do the slit-and-powder, and stomp them flat. The longer you leave them, the worse they will get so it's worth flattening them as soon as you spot them. The more activity you have on the lawn, the less chance the lumps will have to grow out of control: firstly because you will spot them and powder them, and secondly because you will be physically flattening them.

So, make stomping round your lawn part of your daily routine!

Comments (2)

  1. Grower

    Andy J

    Hi Rachel, ant mounds on a lovingly-tended lawn are indeed problematic.

    In the UK the only ant which builds significant and persistent soil mounds is Lasius flavus (aka the mound ant or yellow meadow ant). It happens to be one of our most common ants, although it's almost totally subterranean,so it's rarely spotted above ground.

    Once a queen has founded a colony she can't relocate. Only a tiny proportion of queens ever get to establish a colony because most suitable habitat is already occupied, and to wander into another colony's territory is certain death. No amount of overhead stomping by large mammals will induce a queen to move, so you have to live with the colony, or destroy it.

    Destruction may seem like the easiest option, but remember that what you are doing is just opening up a rare piece of prime real estate for a new queen to colonize later this summer. Before using proprietary ant powders on the lawn, check the small print, as they are formulated for use in and around the house, and may be toxic to plants.

    In the best habitats, Lasius flavus mounds are evenly spaced around 3-4 metres apart, suggesting that each colony needs (and defends against all comers) a territory of 10+ square metres. This means that if there is already a mound present, you won't get lots of new ones popping up like molehills. The mound isn't needed by the colony, its just a waste tip of mined soil that grows, typically to about 30cm high, as the colony gets older and larger. In warm dry habitats Lasius flavus is more likely to be found living under stones or patio slabs, so doesn't produce a mound at all. The colony will persist for the lifetime of the queen, which could be 20 years.

    Mowing mounds or stomping them won't redistribute the soil much, so over time a raised area will result, which can shaved bald if the grass is cut too short. It would probably be better to remove excavated soil (as with molehills) every week or two during the months the ants are active, so a level mowing surface is maintained.

    The ants don't eat grass or roots, so whether colonies do any real harm to lawns (other than dumping unwanted soil) is unclear. It is generally assumed that Lasius flavus farms root-feeding aphids as a source of honeydew, and this might not be good for lawns. However, these aphids are not always found when nests are carefully excavated, so it's still unsure if aphid farming is actually going on under our lawns.

    On the plus side, the ants do improve lawn aeration and drainage, and they do add fertiliser. Like most ants, Lasius flavus is omnivorous, and major dietary components include soft mites and wireworms (beetle larvae), which do serious damage to lawns by chomping roots.

    Although Lasius flavus has been studied quite a bit in natural habitats very little seems to be known about its effect on lawns, and it would be interesting to hear of other GPS members experiences; - does grass seem to grow faster/slower, greener/yellower, or with more weeds/less weeds around mounds? Or is there no noticeable effect? Lasius flavus does not readily colonise cultivated or disturbed ground e.g. boarders, so how soon after seeding/turfing can lawns get colonised?

  2. Grower

    Rachel the Gardener

    Andy, a very interesting and detailed comment: I would have to say, from experience, that anthills in lawns do indeed pop up like molehills, I've found them much closer together than 3-4m/yards (darn, wish I'd taken photos!) and I have found black ants inside anthills far more often than any other colour.

    I'm interested in your comment that the above-ground mound is not "used" by the colony, because if it is only loose un-used soil then it should be easy to remove it as it occurs, thus avoiding the build-up of the nasty brown "molehill" effect which ruins the lawn. In my experience, if you break open one of these mounds (if it's been allowed to grow to ankle height or more) then it is found to be teeming with ants, so it seems that the ants in Oxfordshire haven't read the manual!

    I shall pay more attention to ant mounds in future!


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