Swarms
An ancient plague emerges from the civil war in Yemen (8 minute read)
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In October 2019, long before rumours of a new virus emerged from Wuhan, the United Nations Food and Agriculture Organisation (FAO) warned the world that a plague was coming. The warning generated a few news reports, but, even without a pandemic hogging the headlines, it didn’t hold the attention of our media. The plague was nothing that would infect humans, nothing new, and nothing which sounded even remotely alarming to anyone who can rely on food from a well-stocked supermarket.
This imminent plague was not a human disease but a crop pest, the desert locust. It’s one of the world’s most devastating pests and occurs in dozens of countries in Africa and Asia. But this particular outbreak, already affecting twelve countries by the time of the FAO warning, started in one specific place, unseen and uncontrolled due to a bitter civil war which had already killed tens of thousands.
Most of the time, the desert locust behaves just like any other grasshopper, a solitary creature which hops about and munches on a few plants without troubling anyone. It’s an inconspicuous brown or green and spends much of the day camouflaged among the sparse desert vegetation, flying only at night. But every now and again, it changes from its solitary life and becomes a very different type of insect. It’s not just a case of the population building up from time to time and causing a more noticeable problem, as is seen with many other pests. The desert locust changes everything – it becomes gregarious, it travels in large groups which fly during the day, it changes colour to a bold yellow or pink.
The changes in the locust’s behaviour and appearance are triggered when environmental conditions force large numbers of the locusts together – at first they try to avoid each other, but after a while they begin to move together. In the laboratory, it takes as little as an hour of forcing the juvenile locusts into close contact for them to begin to aggregate. Small groups of locusts form, then the groups join together, forming larger and larger bands which can eventually contain millions, or even billions, of locusts.
Once they have changed to their swarming form, the locusts move out from the desert areas where they spend most of the time. Solitary desert locusts occur in arid areas from West Africa to western India, but swarming locusts move as far south as Uganda, as far east as Bangladesh and as far north as France and Russia. The juveniles – known as hoppers – don’t fly, so they walk and hop, moving up to 1.5 kilometres in a day. But it’s the adults which really travel – a swarm can cover 100 kilometres in a day.
It’s easy to see how early farmers saw locusts as a plague sent by God – they must have appeared to come from nowhere. The solitary desert locust and the swarming desert locust are effectively completely different insects. They differ in appearance, in behaviour and in where they live and breed. And yet they have the same set of genes. This phenomenon – known as polyphenism – is not uncommon in insects. Honeybees, for example, have the fertile queens and sterile workers, and aphids have both winged and wingless forms. But polyphenism is at its most dramatic, and devastating, in the swarming locusts.
Understanding when and where the switch from solitary to gregarious happens is central to managing locust swarms. Since the 1970s, affected countries have been monitoring their locusts for the first sign that they are forming gregarious populations and reporting the results to the FAO, which issues monthly desert locust forecasts. These forecasts consider weather conditions and locust sightings, and signal when and where control may be needed. As a result, locust plagues have been reducing in both frequency and duration, but they are still a serious threat to food security for millions of people.
To subsistence farmers, dependent on the crops they grow to feed themselves and their families, locusts are devastating. Most crop pests are selective, which means that farmers can protect themselves by growing a range of different crops. But locusts eat almost every crop plant and wild species as well – from millet and maize to date palms and pine trees. The swarms can be huge and dense, and can destroy an entire crop in hours. Once a swarm of locusts has moved through, the farmers are left with nothing. It must be utterly soul-destroying to watch.
The outbreak that the FAO warned of in October 2019 had already been building for more than a year. The first indications of trouble came in May 2018, when two tropical cyclones formed in the Arabian Sea, resulting in heavy rainfall in the horn of Africa and parts of the Arabian Peninsula. That month’s Desert Locust Bulletin warned that regular monitoring was needed in these areas to detect locust breeding. In some places, like Sudan, that did happen. But the areas involved were vast and in some place monitoring just wasn’t possible. In the months following the cyclone, the ongoing, destructive civil war prevented any monitoring in Yemen, one of the main areas which had received heavy rainfall.
By December 2018, Sudan and Eritrea were reporting the first signs of trouble. Along the Red Sea coast of both countries, swarms had been seen and controlled over several thousand hectares. They reported that the breeding had been going on since October, but had not been detected until December, allowing a whole generation of gregarious locusts to escape control. The FAO raised the locust alert level in the region from green (calm) to orange (threat), skipping the yellow (caution) completely. But Yemen was still unable to undertake any monitoring.
In January 2019, the swarms in Yemen that had been undetected for two generations began to move. Saudi Arabia, the United Arab Emirates and Iran all reported swarms originating from that area, and Yemen itself was finally able to do some monitoring, and reported swarms too. In the meantime, the swarms from Sudan and Eritrea had moved north into Egypt and had crossed the Red Sea to the coast of Saudi Arabia. Control programmes were now being conducted over tens of thousands of hectares.
Over the next few months, the FAO reported a largely improving picture, with the control operations and drying weather conditions bringing the swarms that originated in Sudan and Eritrea to an end. While there were continued reports of swarms from Yemen and its neighbours to the north and east, extensive areas were now under control in Saudi Arabia and Iran. But control in Yemen remained impossible.
By June 2019, the situation had turned again, with swarms re-invading Sudan and Eritrea from the Arabian Peninsula, and reports of swarms now being received from further afield – Pakistan in the east and Somalia to the south. The locusts that had been breeding while Yemen had been gripped by war were now spreading in more than ten neighbouring countries. By October, the swarms had reached India, and by December the FAO were reporting the worst locust swarms in 25 years.
This isn’t the first time that a locust plague has emerged out of an area of conflict. In 1986, a locust plague began in Ethiopia, Eritrea and Sudan, but couldn’t be controlled due to wars in those areas. As a result, massive swarms moved westwards across the Sahel, invading Chad and Niger and eventually reaching Africa’s west coast. It took three years to bring the swarms under control. In the 1990s, there were three separate outbreaks – two of these arising, at least in part, in Sudan during its civil war. Many of the countries most often affected by locusts, from Mali in the west to Pakistan in the east, also have to contend with ongoing instability and conflict.
As Covid-19 emerged and then spread around the world, the locusts were spreading too. They reached Kenya – with the worst swarms that country had seen for 70 years – and Uganda. Crops were destroyed and almost five million people face famine. Now, there are warnings that it may also spread westwards from Sudan, to Chad, Nigeria, Niger, Mali and Mauritania, which have so far been spared in the current outbreak. They have a long history of locust plagues, and there is no lack of knowledge and expertise in locust control, but they’re also affected by a worsening conflict in Mali, and the fight against Boko Haram in parts of Nigeria and Chad.
Locusts are a plague which have been with us for thousands of years, but more than a century of research has largely solved the technological challenges of tracking and controlling outbreaks. We can see this with the success of Sudan and Eritrea, which were able to bring the swarms that formed on their Red Sea coasts under control within months. Just as with Covid-19, swift action saves lives. But the current crisis also contains a powerful warning. A seemingly localised conflict can have far-reaching impacts.
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