"Actually, locusts don't really like each other"

Iain Couzin from the Max Planck Institute for Animal Research on the causes of the locust plague in East Africa

East Africa is currently being plagued 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 investigates animal swarms at the Max Planck Institute for Animal Behavior and the University of Constance.

Have you ever experienced such a swarm yourself?

Years ago, I studied the swarming behaviour of migratory locusts in Africa. I also came across swarms of migratory young animals. These do not fly, but all run in the same direction. As far as the eye can see, everything is covered with migrating young locusts - that was surreal!

Why do the locusts do this?

It was long believed that swarms are a form of cooperative behaviour. But our experiments have shown that it is rather the opposite that drives the locusts: the fear of cannibalism. The animals do not like each other, because the biggest enemy of a migratory locust is another migratory locust. So, a swarm of locusts is more like a great escape, where each one hunts the other.

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

Migratory locusts can be loners. When do they form swarms?

The triggering factor is food supply. When it rains a lot, as it did this autumn in East Africa, the locusts have so much vegetation to eat that they can reproduce strongly. They are naturally solitary animals that avoid each other. But if the rains stop, and if they are populous, they are forced to come together to exploit the remaining available food.

The sight and smell, out the contact of others, causes these insects to transform: Within hours, a cautious, secretive loner turns into a go-getter full of movement. If the swarming stimuli disappear, the animals transform back again, but that takes much longer. After a long time, individuals that remain isolated do become completely inconspicuous, harmless grasshoppers again.

All this happens although the genetic make-up remains the same. So, the same genome produces two completely different types of behaviour. They are really fascinating animals!

How does the swarm know which direction to go?

There is a lot of coincidence involved. We know that locusts orient themselves towards vertical structures because that is how they recognize vegetation. And they avoid flying over open water - although they sometimes do cross it if necessary. On their way, the insects constantly encounter conspecifics and also transform them into swarm animals. So, a swarm is literally contagious, similar to an infection. By this, it keeps on growing. A small bush fire can then turn into a wildfire.

The affected countries are now considering spraying insecticides from aircraft. Here in Europe, such products now have a very bad reputation? Is there no alternative?

From an ecological point of view, insecticides are of course 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 affected.

Won't the swarms eventually dissolve on their own?

As long as the insects find enough to eat, they'll keep moving. Although they can develop from loners to swarms within hours, it takes much longer in the opposite direction. That's why swarms are stable for a very long time. Therefore, simply waiting won’t help.

Could there be alternatives to insecticides in the foreseeable future?

Not to my knowledge. There have been experiments with fungi that can infest the locusts. But it is very difficult to apply such products to larger areas without losing their effectiveness. I therefore fear that we will depend on insecticides in the next few years.

Interview: Harald Rösch