The parasitic Lasius: queens that take over someone else's colony

Every August somebody finds a big gingery ant on the patio and assumes she is a large Lasius niger, drops her in a tube with some cotton wool, and waits. Nothing happens, because nothing was ever going to.

Lasius umbratus, L. mixtus and L. sabularum are the parasitic yellow Lasius, and they fly late in the British season, from mid August into October depending on the species. Their queens are large, gingery and hairier than niger's dark brown, and they turn up on paving and paths at exactly the point in the year when the easy black garden ant queens have mostly stopped appearing. The two are close enough in size, 8 to 9 mm (about a third of an inch) either way, that colour is the only quick tell.

The identification isn't really what earns them a post of their own. What happens after you catch one is.

A niger queen founds by sealing herself into a chamber and living off her own reserves until the first workers hatch. A parasitic Lasius queen has no such option. None at all, really. She has lost the fat and the flight muscle that a claustral founder needs, and her whole strategy depends on somebody else's labour from day one.

After mating she has to find and enter an established nest of the right host species, get past or kill its queen, and be accepted by workers that have no reason to accept her. L. umbratus targets L. niger, L. platythorax and L. brunneus. L. mixtus goes for L. flavus almost exclusively. If she manages it, her eggs are raised by the host's own workforce, and the colony gradually becomes hers as the host workers die off over the following year or two and are replaced entirely by her own brood.

Getting in is the hard part, and it produces one of the odder things you can watch on a British lawn. L. umbratus queens have been recorded walking over the surface of a target niger nest carrying a dead niger worker in her jaws, which, if you squint, looks a bit like a queen wearing the enemy's uniform to get past the door.

If you already have a colony of the right host species established, in principle you could try introducing a parasitic queen to it, but this is a genuinely difficult, failure-prone project even for experienced keepers, not a beginner weekend job, and it can just as easily end with the host workers killing her.

If you don't have a host colony, there is no honest version of this that ends with a founded colony in your kitchen. The kindest thing, and probably the one that gives her the best odds, is to release her back outside near ant activity and let her take her chances the way she was built to. Not a satisfying answer. The true one, though.

L. sabularum has the longest window of the three, flying from July right through to October, and some of its alates skip autumn entirely and overwinter to fly the following May. L. mixtus is mostly August and September.

Lasius fuliginosus, the shiny jet-black ant that runs long foraging trails along hedge banks and fence lines, takes the whole arrangement one step further. Its queens found new colonies by entering nests of the umbratus group, which had themselves been founded the same way inside somebody else's nest a generation earlier. Occasionally you find a mixed nest with black fuliginosus workers and their yellow or brown umbratus hosts still running around together, a snapshot of a takeover in progress by a species doing to umbratus exactly what umbratus did to niger.

Nobody keeps fuliginosus by trying to replicate this at home. It is here because once you know the umbratus story, the fuliginosus one is the natural next question, and the answer is that yes, it goes two layers deep.

The second wave covers the rest of what's flying alongside these in August and September, and fully claustral or not is the wider explainer on founding strategies, of which social parasitism is the awkward third case. If what you actually caught turns out to be an ordinary niger queen after all, you caught a queen this week, now what covers the first few days properly.

Dealate's forecast covers the species that are genuinely in season where you are, day by day, but it can't tell you which queen is in your hand. That part is still down to a hand lens and a colour.

British flight periods and host relationships are from the species accounts of the Bees, Wasps and Ants Recording Society, the national recording scheme for British ants. Karl Hölldobler's 1953 paper on Lasius umbratus founding behaviour, and Cover & Sanwald's 1988 account of the related North American Acanthomyops parasites, are cited in full on fully claustral or not.