Stopping swarming locusts

Why do locusts swarm?
18 August 2020

Interview with 

Steven Rogers, University of Cambridge


Grain growing in a field


Locusts are famous worldwide for forming giant swarms that devastate crops. Even in the Bible it tells how Moses unleashed them on the Egyptians, plunging the country into chaos. But the question is, where do these swarms come from, and what transforms what’s normally a solitary creature into a gregarious beast that migrates en masse to find and devour food? Scientists think that - for at least some types of locust - the answer is a chemical pheromone. The animals find it attractive, and when they’re exposed to it, they make more of it, triggering a positive feedback loop that culminates in a feeding frenzy. Eva Higginbotham asked Steven Rogers, who works on locusts but wasn’t involved in the present study, to take her through the findings...

Steven - Locusts are notorious for the amount of vegetation they eat, which commonly includes farm crops of all descriptions. The solitary locusts typically live in regions where there aren't many people, and there's not a lot of agriculture when they go into their gregarious phase and they start swarming, they leave these regions and go into areas where there's often quite substantial agriculture.

Eva - They can sort of, strip away all of the farmers hard work, and then continue moving on to another farm.

Steven - Absolutely. And it's one of the cruelties of locust swarms. The farmers in these adjacent regions to where the solitary locusts live are often farming environments, which are very marginal and difficult to work. And they may often go for themselves many years without a bumper harvest. With the same conditions, which promote locust swarm formation, are also those which are actually very good for farmers. Sometimes it's the case because they think, Oh, this is a great year, lots of crops. And then a locust swarm will sweep in and eat the lot. And what this paper does is identify a pheromone, which is a chemical signal, which acts as a stimulus to other locusts and draws them towards each other.

Eva - How did they find it?

Steven - So the first question they asked is: How do solitary and gregarious locusts differ in the sort of bouquet of smells they produce. They homed in on those substances where to gregarious produce more of these particular chemicals than solitary ones do. And they homed in on about maybe half a dozen different substances. And they then looked systematically at how each one of these chemicals affects the locusts in the laboratory and in the behavioral arena. So they had a box where half of which has got air which has got the chemical of interest being piped through to it, and the other end of the box has just got clean air being pumped through, and what they look to see is, well, where do locusts spend their time?

If a particular substance is attractive, then you would expect them to spend time in the area of this arena, where this smell is being produced. And doing that, they're able to identify a particular substance, which was attractive to locusts called 4-Vinylanisole, is produced by gregarious locusts only. It's not produced by solitude locusts at all. So a particularly interesting feature of this substance produced by gregarious locusts, the more of them there are in a group, the more each individual locust within it produces. So it ends up as being a snowball effect. If you like. And one of the difficulties we've had in the past, of trying to identify substances, which might be acting as these aggregating pheromones, is that normally, if you're a locust and you're producing a smell, why don't you just smell yourself? Having a situation where groups of locusts produce much more and are therefore smellier than anything you may produce on your own is actually critical to how this mechanism would work.

Eva - So now that we know what the substance is, what can we do with that information?

Steven - They could use it as an attractant and things like lures and traps. A key part of locust control is identifying where locusts are, where populations are building up, and at the moment, that means traipsing around looking in very remote places, trying to find these individual locusts. But if you've got a pheromone, this chemical, you can possibly draw the locusts towards you. Makes a sampling of locusts, and working out where they are and how many they are in a given region. It potentially makes it much easier to do.


Add a comment