What’s going on?

Last week, the UN issued a warning about the numbers of desert locusts currently swarming through much of East Africa; they have now reached Uganda and Tanzania. The outbreak is the largest for decades and has already devastated crops in Somalia, Ethiopia and Kenya, countries with fragile food security. The problem is not confined to Africa, with swarms also occurring in Saudi Arabia and Pakistan.

How big are the swarms?

In the current outbreak, one swarm in Kenya measured 40km by 60km and as many as 150m locusts can gather in a square kilometre. It is estimated that even a small swarm can devour enough food for 35,000 people daily.

How do swarms arise?

When a single desert locust – actually a type of short-horned grasshopper in the family Acrididae – lives alone it is light brown in colour and does relatively little damage. However, when the environment is favourable (often after heavy rain and cyclones) and lots of locusts come together in the same place – they change colour to pink (immature) and then yellow (mature) and form swarms, a process known as gregarisation.

What can be done?

There is no reliable way to tackle a swarm of locusts. Their occurrence and movements are hard to predict and they move very fast over large, often very remote areas. They can be sprayed with pesticide from planes and drones. The best tactic is to spray the locusts before the gregarisation process begins; Mauritania has been praised for taking this pre-emptive approach.

What about the future?

Heavy rains help create perfect breeding conditions for locusts and these have been caused recently by a powerful cyclone off Somalia. Some forecasters suggest that an increase in the frequency of Indian Ocean cyclones could be due to global heating, which may lead to more regular swarming in Africa.