From the Biblical plagues of Egypt to a major infestation in Madacasgar two years ago, locust swarms have caused chaos throughout history. Just one swarm can cover 20% of the land surface of the Earth, affecting the livelihood of 10% of the world’s population by consuming up to 200 tonnes of vegetation per day. Understanding how swarms form and what can break them apart, then, is of great importance.

But how many locusts does it take to make a swarm? It sounds like the start of a bad joke, but understanding and controlling locust plagues is something we’ve been striving to do for thousands of years. If we can understand what makes a swarm, hopefully we can understand how to stop it. This is exactly what our international research team tried to do – and the results are in. The answer is just three.

Cannibal creatures

Our study, published in Physical Review E, used mathematical modelling to tackle the problem. We managed to show that a locust in a swarm must be able to interact with at least two neighbours simultaneously. Without those two neighbours, the insects can’t reproduce the striking spontaneous changes in direction we are used to seeing in flocks of starlings, schools of fish – and swarms of locusts.

The finding adds to our previous observation of how locust swarms can be so stable. By watching locusts move around a ring, we identified some interesting behaviour. Small numbers of locusts move around the ring at random, but larger groups start to march together in the same direction. These groups will then spontaneously switch to march in the other direction. The more locusts in a group the longer the time interval between switches and the more stable the swarm.

We were able to reproduce this behaviour in a mathematical model – and discovered that locusts move more randomly when they don’t have near neighbours.

So, forming a swarm is all about neighbours – but that doesn’t make it “neighbourly”. The reason locusts pay so much attention to their neighbours is because locusts are cannibals.