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Locust swarms in Africa. Part I: Causes

Locust swarms in Africa. Part I: Causes

World media is abuzz with incoming news of the severe plague of locusts that has befallen Africa. The massive increase in numbers began in Somalia and Ethiopia, with swarms of about 150 million insects moving to Kenya, Uganda and Tanzania. We asked JU Institute of Zoology and Biomedical Research entomologist Prof. Stanisław Knutelski to tell us more about the phenomenon’s root causes.


Currently, the main swarms are moving towards South Sudan. Lesser outbreaks have been observed in Djibouti, Eritrea and Sudan. Aside from East Africa, locusts are also swarming on the Arabian Peninsula as well as India, Iran and Pakistan, the latter of which has declared a state of emergency. Areas stricken by the plague of locusts are left completely devoid of any foliage, with hundreds of millions of insects descending on crops, pastures, meadows and forests, devouring everything in their path: grass, seeds and plants, leaving behind only naked trees and barren lands.

How large is the scale of destruction? It’s estimated that the area currently affected by the plague of locusts (approximately 2.4 thousand square kilometres) may contain from 100 to as many as 200 billion insects. Each locust can eat as much as it weighs, i.e. 2 grams, per day. This means that every day the swarms eat from 200 to 400 thousand metric tons of plant matter, a potential food source for other animals, including people.

The problem may escalate even further, with new swarms appearing in the spring in other parts of Africa as well as Western Asia. Currently, the biggest trouble spot seems to be Ethiopia, since the swarms there are close to the Great Rift Valley, an area that provides the most food for the 100 million people living in the country. In the past, it was customary for the locals to actually celebrate the arrival of locusts, since they heralded the advent of wet season, which usually meant an abundance of food for every living creature. However, the present situation far exceeds the norm, and Ethiopians are fully aware that if the insects survive until after the rains come, their numbers may grow even further and bring about a period of mass famine.

For some parts of Africa, an emergency of this magnitude has not been observed in generations (though similar events happened in Somalia and Ethiopia 25 years ago and in Kenya 70 years ago). The current plague of locusts is most often compared to the one described in the Bible: . . . and when it was morning, the east wind brought the locusts. And the locust went up over all the land of Egypt, and rested in all the coasts of Egypt: very grievous were they; before them there were no such locusts as they, neither after them shall be such. For they covered the face of the whole earth, so that the land was darkened; and they did eat every herb of the land, and all the fruit of the trees which the hail had left: and there remained not any green thing in the trees, or in the herbs of the field, through all the land of Egypt (Exodus 10:13-15, King James Bible).

To understand the causes and consequences of this phenomenon, let’s first look at our anti-hero: the locust.

A closer look at locusts

The term ‘locust’ is most commonly applied to insects (Insecta) from the order Orthoptera, suborder Caelifera, infraorder Acrididae, which spans many different types and species commonly known as locusts or grasshoppers.

Some examples of insects that form swarms include the Moroccan locust (Dociostaurus maroccanus), Italian locust (Calliptanus italicus), Australian plague locust (Chortoicetes terminifera), brown locust (Locustana pardalina), red locust (Nomadacris septemfasciata) and Senegalese grasshopper (Oedaleus senegalensis), just to name a few. However, the two most infamous ones that cause humanity the most trouble are the migratory locust (Locusta migratoria) and desert locust (Schistocerca gregaria). Both of those, particularly the last one, are behind the current crisis in Africa.

Locusts usually feast on meadows, pastures or crops. But to pose a significant threat, they first need to reach sufficient numbers. For instance, in 2012, Australia was plagued by a swarm of locusts that was 6 kilometres long and covered the distance of 100 kilometres in less than five hours. It’s estimated that all species of locust consume a total of one fifth of world crops per year. It’s worth to note at this point that as some species are edible, they are part of local culinary customs in some parts of the world, though they also frequently cause allergic reactions.

The migratory locust (Locusta migratoria) varies in colouration, though in most cases it is greenish brown, at least during the solitary phase (more on that later). Its length varies between 35 and 55 millimetres. It’s widely distributed on the Eastern hemisphere, particularly the deserts and semi-deserts of Eurasia, Africa and Australia. It’s been known to appear in Poland, but very rarely, with first encounters dating back to around 11th century, and the last one registered in 1967. The migratory locust is an oligophage subsisting mainly on grass. Upon transitioning from solitary to gregarious phase, it creates swarms and travels north – in favourable conditions, these swarms can reach as far as Stockholm, Sweden and Perm, Russia.

The desert locust (Schistocerca gregaria) is a little larger than the migratory locust, measuring up to 60 millimetres. In the solitary phase, it’s also largely greenish in colour, while in the gregarious phase it changes its colouration to brown, yellow and black. Its habitat ranges from Western Africa to India, but it’s also been spotted on some of the Japanese Islands, including Hokkaido. It mostly inhabits steppes, savannahs and agricultural lands. It’s one of the most studied insects in the world, with 2,798 papers written about it in 1900–2020 listed by the Web of Science.

The lifecycle of desert locusts is representative of other hemimetabolous Acrididae, i.e. those that have no pupal stage, and consists of three phases: egg, nymph and imago (winged adult). After mating, females choose a spot in wet and soft sandy soil and use their relatively long ovipositors to deposit the oothecae (protective egg packets). Nymphs take between two to four weeks to fully develop when weather is favourable and food plentiful, but the process can take up to six months if the conditions are suboptimal. The desert locust is polyphagous, meaning it can eat most cultivated and wild plants. Adults are very capable fliers and migrate in swarms that normally consist of up to 50 billion individuals on distances up to 200 kilometres.

Dr Jekyll and Mr Hyde

Like other similar species, the desert locust is polyphenic and has two phases: the solitary phase and the gregarious phase. Both of these phases are characterised by differences in colouration, size and behaviour. Before the wet season, when the locust population is relatively low, nymphs develop into rather docile nocturnal adults. In this form, locusts are not a threat to crops and are not particularly noteworthy.

So what transforms this innocuous insect into a voracious devourer of plants? This is mainly caused by the increased population density of locusts brought on by changes in the environment. When the wet season comes, rains cause lush growth of plants, increasing the amount of available food. Since this greatly increases the survivability of young locusts, they proliferate and quickly clear the surrounding fields of any vegetation. Dwindling supplies of food draw these usually solitary insects together, causing them to touch one another and release pheromones that induce their transition into the gregarious phase.

As a side note, these pheromones are responsible for increased production of serotonin, a hormone that influences cognition, reward, learning, memory and numerous other functions in living organisms. When tested, gregarious locusts exhibited a concentration of serotonin that was three times higher than in solitary locusts. Thanks to the hormone, the locusts change colour and become increasingly hungry; however, chemically suppressing it prevents the insects from entering the gregarious phase, even if they are exposed to pheromones. Conversely, administering substances similar to serotonin hastens the transition. This may mean that in the future we will be able to prevent locust swarms from forming by devising a way to suppress their serotonin production.

Upon entering the gregarious phase, locusts gradually start to change. Nymphs develop into slightly smaller yellow-and-black diurnal adults. Even though they are the same species, they’re so different from each other that up until 1921 they were considered to be two different insects. The most apparent difference is their behaviour. In the solitary phase, locusts tend to steer clear from each other and are active mostly during the night. However, as more and more insects begin to enter their gregarious phase, they begin to crowd together and start functioning during the day. In favourable conditions, desert locusts create vast migratory swarms, turning them into one of the most dangerous threats to agriculture.

A brief history of locust swarms

Crises brought about by the desert locust have been known to occur since antiquity. Throughout history, there have been many examples of plagues of locusts in various time periods. One of the more recent examples are the swarms that fell upon the historical region of Levant (present-day Palestine and Lebanon, to be specific) in 1865, 1892, 1899, 1904 and 1915. The last one was particularly devastating, as it reached Alexandria, Egypt and Khartoum, Sudan. Locust nymphs and adults devoured everything in their path: wild plants, crops, and even bees and their hives. Trees were stripped of leaves in a matter of minutes. Insects invaded homes and littered the streets, making everyday life much more difficult. In Beirut, water supply was disrupted for two days because maintenance services needed to remove dead locusts from the tanks. Witnesses claimed it was unlike anything they’ve ever seen in their lives and that it was ‘a disaster of biblical proportions’. According to some memoirs, it took two hours for the locusts to fly over towns, which darkened the sky and induced feelings of helplessness in their inhabitants. Scores of wingless nymphs ‘flowed on the ground like streams’, causing motion sickness in some people. That year went down in history as the ‘Year of the Locust’.

Despite the efforts of the authorities, the plague greatly affected food availability in the region. Although the cereals were already largely harvested, the swarm destroyed most of fruit and vegetable crops, like apricots, watermelons, melons, cucumbers, tomatoes, figs, maize and olives). Very few farms and orchards managed to survive the onslaught. Olive oil was scarce, wine became a luxury, and a lot of basic foodstuffs were rationed out. Due to the ongoing French-British naval blockade against Germany and Austria-Hungary, all prices skyrocketed even further. Even in the early stages of the plague, there were some cases of deaths from starvation. There were even some reports of cannibalism. To make matter worse, Jerusalem experienced an outbreak of cholera. All these events led to famine in Greater Syria in the years 1915–1918.

Africans remember well the year 1954, when Ethiopia was ravaged by a plague of locusts not unlike the present one. Insects destroyed nearly every edible plant, with the rest falling victim to drought. The country was crippled by lack of food for the following year. Another memorable plague took place in 1987–1988. It spread from the region of Sahel to the countries of West and North Africa, the Middle East, South-East Asia and even the Caribbean. People living in these regions live in fear of history repeating itself, and do whatever they can to drive locusts off of their fields. Sadly, their attempts are mostly ineffectual, as there are simply too many insects.

Why are there so many locusts?

Latest research on locust population surges seem to corroborate the hypothesis that the largest swarms are likely to originate from fairly small groups of insects. Currently, it’s much harder to fight against new locust swarms due to restrictions on insecticides, such as dieldrin, which leads to technical, logistic, and financial issues. In addition, said insecticides are only effective in the early morning, when locusts are still on the ground. However, because of the wet season, it is most often fruitless to spray them, as rains would simply wash them off. Furthermore, as locusts can fly as high as one kilometre above ground, they pose a threat to planes, which is in important issue, since Ethiopia and Kenya possess only three and five agricultural aircraft, respectively.

The situation is particularly though in South Sudan and Somalia. The former has just barely dealt with internal conflict and starvation, and the ongoing plague threatens to plunge it back into it again. The latter is one of the world’s poorest and most dangerous countries, with large parts ruled by jihadist fundamentalist group Ash-Shabab, which might lead to problems in combating the locust swarm.

One of the main reasons for the sudden surge of locust population we’re experiencing are the weather conditions in the last two years. It started in 2018, when the tropical cyclone Mekunu hit Yemen, Oman and Saudi Arabia. Large amounts of rain, a rarity in these parts, caused on overgrowth of plants. Together with warm and wet weather, it provided the perfect conditions for locusts to breed. They would’ve surely been eliminated by the next drought, but instead, another cyclone made food even more abundant than before.

In the past, cyclones have been observed in the region once every few years, perhaps even a decade. However, due to climate changes, the waters of the Indian Ocean are getting warmer, leading to unprecedented rainfall. Over the period of six months, the population of locusts has increased eight thousand times. The insects spread and devour plants at an alarming rate, and it is now beyond the power of local and national governments to stop them. The Food and Agriculture Organisation of the United Nations (FAO) estimates that if no insecticide is sprayed before the next wet season, i.e. early March 2020, the locust population might increase by as much as 500 times. It appealed to UN members to provide financial support in order to fund humanitarian aid and aerial spraying.

If we fail to address the issue of locust swarms in Africa, the continent may experience a disaster that will leave tens of millions of people starving and cause a plethora of problems that will further exacerbate the situation.

Part II: Consequences

Original text: www.nauka.uj.edu.pl

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