Tuesday 31 January 2012

A few things you (probably) didn't know about weaver ants

 Ants aren't usually the first things people look at when on safari, but they are fascinating beasts when looked at up close. We briefly featured siafu here once before, but that's not enough for a really important group of invertebrates, and it's time to rectify that. Finding I had some nice pictures of Weaver Ants Oecophylla longinoda (right) I thought they might make a good start as they're not only fairly common in some areas (particularly near the coast), but they're pretty interesting too. In fact, on starting a bit of research I discovered they're even more interesting than I first thought! There are actually two species in this genus, the African species, and a closely related species that occurs across Asia and into Australia. There being (I suspect) rather more myrmecologists in Australia than Africa, a lot of the relevant research comes from there, but it seems highly likely 'our' species do the same, so here are a few things you might not have known about weaver ants before.

Firstly, weaver ants are are the first recorded organism ever to be used in 'biocontrol', their use being recorded back in 304AD by Chinese farmers to protect their fruit orchards. The descendants of these farmers, and other farmers across Asia and Africa still use the ants in exactly the same way today. Studies have found that these predatory, arboreal ants are much better at keeping fruit trees clean of important pests than other ground nesting ant species, and that farmers who look after their ant colonies (by leaving the undergrowth rough to discourage the ground-nesting species that are dominant in the shorter-canopied orchards, and by preventing pruning of nesting colonies) currently use on average half the chemical control needed by their neighbours who don't do this, and a significant proportion (20%) don't need any additional chemical control at all. So they're rather handy things to have around.

Weaver ant nest, Ushongo, July 2011
Next on the list is the remarkable way they go about building their nests. The nests consist of living leaves, joined together in a ball using specially produced silk. But how they sew the leaves together is a remarkable feat of cooperation. Typically, some tens of ants will need to form a chain to first bridge a gap between two leaves, then pull them together so another team can hold them in position whilst yet more sew the gap together with silk. But adult ants can't make silk, so they have to use larvae to do it, picking the larvae up and using them like little pots of glue to spin a mat of silk between the two leaves. This is a pretty remarkable piece of evolution on its own, but even more so when you consider how hundreds of ants can cooperate to achieve this, a far more complicated procedure than simply digging holes. Recent research has suggested that there really is no central control to this process. If given two exactly identical gaps to close, there's no central decision made over which to fill first, but still the nest 'decides' to do one first. How? The observations and models suggest simply by each ant doing his own thing with a very simple set of rules, most important of which is something like 'join the biggest chain of ants you see'. That simple rule alone means that a initial chance difference in the number of ants seeking to fill each gap is rapidly re-inforced until there's only one gap being worked on. Remarkable how such simple rules can result in apparent group decisions very quickly indeed.

There's money in that nest!
And finally (though there are lots of other fascinating things I could have thought of, but three points is alwaysa  good number to remember!), I was amazed to learn that there's a major market for these ants for human consumption. Yes, it's true! On some Asian markets the price per kilo is twice that of beef! Must be very tasty... Apparently you can also sell them to Europeans as pet food, or in China and India for traditional medicine, but most of the markets across Asia and also in Cameoon and Congo are for human consumption. So valuable are these markets, and so easy to keep are the ants, that farmers who feed the ants on invertebrates atracted to kitchen scraps and the like stand to make over 4.5 times their costs on each brood sold. Now that's a good return rate - I'm thinking of changing job...

So, there you go, three pretty remarkable things about weaver ants. If you've not seen them before but have been in East Africa, believe me, it's only because you've not been looking hard enough. Now you know how interesting they are, hopefully you won't overlook them next time! And that's before we start talking about how important to the ecology of many ecosystems are ants in general. Stories for a future post, I think...

Main references:

ResearchBlogging.orgVan Mele, P. (2007). A historical review of research on the weaver ant Oecophylla in biological control Agricultural and Forest Entomology DOI: 10.1111/j.1461-9563.2007.00350.x

Lioni, A., & Deneubourg, J. (2004). Collective decision through self-assembling Naturwissenschaften, 91 (5), 237-241 DOI: 10.1007/s00114-004-0519-7

Offenberg, J. (2011). Oecophylla smaragdina food conversion efficiency: prospects for ant farming Journal of Applied Entomology, 135 (8), 575-581 DOI: 10.1111/j.1439-0418.2010.01588.x


  1. Nice post, Colin! How interesting to know of weaver ants.

  2. Hi Júlio, nice to see you here! I find the more I know about anything, the more fascinating it becomes...

  3. Indeed, I feel just the same. I have come across some weaver ants in South India and learned quite a bit while preparing a blog post around my photos. See http://maria-fremlin.blogspot.com/2012/02/close-encounter-with-weaver-ants.html
    And I mention you in it! Thanks for your interesting post.

  4. Hi Maria, thanks for your link - amazing photos you managed there! Nice blog too - looks like you had a lot of fun!