Locusts eat each other

"Actually, locusts don't like each other very much"

These days, East Africa is being hit by the largest swarms of locusts in decades. In some regions of Kenya, Somalia and Ethiopia, the insects have almost completely destroyed the harvest.

Iain Couzin researches animal swarms at the Max Planck Institute for Animal Behavior and at the University of Konstanz. Years ago he almost died of thirst while looking for swarming desert locusts in Africa.

Have you ever seen such a crush yourself?

Iain Couzin: Years ago I studied the swarming behavior of migratory locusts in Africa. I also got caught in swarms of migrating young animals. These don't fly, they all run in the same direction. As far as the eye can see, just these wandering young grasshoppers - that was almost surreal!

Why do the locusts do this?

It has long been believed that swarms are a form of cooperative behavior. But our experiments have shown that it is rather the opposite that drives the locusts: namely, the fear of cannibalism.

The animals don't like each other because the greatest enemy of a migratory locust is another migratory locust. A swarm of locusts is more like a great escape in which everyone chases the other.

The fear of being eaten then makes the animals all run in the same direction. You can compare it to a motorway: the best way not to collide with others is to stay in the direction of travel.

Migratory locusts can be solitary animals. What is the trigger for them to form swarms?

The triggering factor is the food supply. When it rains a lot in East Africa, like this autumn, the locusts have so much vegetation to eat that they can reproduce strongly. Actually, they are by nature loners who avoid each other. But when they are very many, they can no longer avoid each other in search of food.

Just the smell and the sight of conspecifics transforms the locusts: Within a few hours, cautious, hidden loners become daredevils full of the urge to move.

If the swarm-triggering stimuli disappear, the animals transform themselves back again. But that takes a lot longer. In this way, individuals who remain behind can become completely inconspicuous, harmless grasshoppers again after a while.

All of this happens even though the genetic make-up remains the same. So the same genome produces two completely different behaviors. They are really fascinating animals!

How does the swarm know in which direction to migrate?

There is a lot of chance involved. We know that the locusts orient themselves towards vertical structures because this is how they recognize vegetation. And they don't like flying over open water.
On their way, the insects constantly meet fellow species and transform them into swarms. A swarm is literally contagious, similar to an infection. So it keeps growing. This can turn a small bush fire into a wildfire.

The affected countries are now considering spraying insecticides from the aircraft. In Europe, such drugs have a bad reputation. Is there no alternative to that?

Of course, from an environmental perspective, insecticides are not a good thing. But at the moment there is probably no alternative. The damage caused by the swarms is really enormous. Around ten percent of humanity is repeatedly affected by it.

Don't the swarms dissolve on their own at some point?

As long as the insects can find enough to eat, they will move on. They can develop from solitary animals to swarming animals within hours. In the opposite direction, however, it takes much longer. Therefore the swarms are stable for a very long time. So just waiting doesn't help.

Could there be alternatives to insecticides in the foreseeable future?

Not to my knowledge. There are experiments with fungi that can attack the locusts. However, it is very difficult to apply such agents to larger areas without losing their effectiveness. I therefore fear that we will still have to rely on insecticides in the next few years.

Prof. Dr. Iain D. Couzin
Max Planck Institute for Animal Behavior, Radolfzell / Konstanz
+49 7531 88-4928
[email protected]

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