As part of the JIRCAS research project on “Development of technologies for the control of migratory plant pests and transboundary diseases”, we are elucidating the ecology and behavior of desert locust in the field in order to develop effective control methods. This article provides basic information on desert locust based on previous studies and recent research results to explain about the recent outbreaks in East Africa extending to Southwest Asia.

The desert locust is a migratory pest that causes serious agricultural damage. It is widely distributed from Mauritania in West Africa to the Middle East, and extending to India in Southeast Asia. About 60 countries suffer from extensive agricultural damage due to desert locust outbreaks, affecting 20% of the earth’s land area and about 10% of the world population.

Desert locusts are usually small in number and hard to find. However, when various environmental conditions overlap, a large outbreak occurs forming huge swarms, and they are transformed into pests that can cause enormous damage to crops. One of the causes of such dramatic changes is the special ability of this locust species to undergo “phase polyphenism” which changes its behavioral, morphological, and physiological characteristics in response to changes in population density. Locusts that grow under normal low density are in the so-called “solitarious phase” and behave as individuals while those that grow under high density at the time of an outbreak are in “gregarious phase” and behave as a collective mass that form a swarm. Individual locusts in the solitarious phase avoid each other, but when they cluster into dense groups, they are attracted to each other and move as swarms. Morphological changes take from days to weeks, but behavioral transformation occurs in hours, and although they are morphologically solitary, they behave as part of a group and become gregarious when they experience congestion. In 1921, Russian entomologist Sir Ubarov reported that the migratory locust (Japanese name: Tonosamabatta) exhibited phase polyphenism, and the same phenomenon was subsequently reported in locusts in found in other continents. It is considered that locusts that transform into swarms contribute to the explosive increase in populations because their developmental and reproductive abilities are improved compared to solitary condition. It is no exaggeration to say that the history of locusts research began in 1921, because the locust invasions caused agricultural damage. Up to now, FAO has been a leader and at the center in combating the desert locusts for many years.

• Q1    How do desert locust outbreaks occur?
• Q2　How does an outbreak end?
• Q3　Is desert locust different from an ordinary grasshopper?
• Q4　What are the characteristics of desert locust movement?
• Q5　Why is desert locust not found all over the world?
• Q6　Where do desert locusts lay eggs?
• Q7　What do desert locusts eat?
• Q8　What kind of damage may occur in case an otbreak occurs locally?
• Q9　Desert locust swarms have been reported in India. Is it possible that desert locust swarms from East Africa will also reach China and Japan?
• Q10  Is it possible for desert locusts swarms to cross Mt. Everest?
• Q11  Is it possible for desert locusts to fly to China and Japan?
• Q12  What happens if desert locust swarms fly to China?
• Q13  What happens if desert locust swarms fly to Japan?
• Q14  What is the future scenarion in terms of expansion and distribution of desert locusts?
• Q15  What is the current control method for desert locusts?
• Q16  Does the control method using insecticides induce resistance?
• Q17  Is desert locust edible?
• Q18  Why is it difficult to solve the desert locust problem and what should be done to improve?
• Q19  How can the damage caused by desert locust be reduced?
• Q20  What kind of support does Japan provide to help in controlling desert locust?
• Q21  What research is JIRCAS pursuing to address the desert locust problem?
• Q22  Why does JIRCAS pursue research on overseas locust desert problem overseas?

Q1　How do desert locust outbreaks occur?

A: The process leading to an outbreak is complicated and still unknown, but it is closely related to drought, heavy rain, wind, plants, soil, and season. An outbreak does not always occur in large number, and its occurrence is characterized as occasional and irregular.

The usual locust habitat is called the recession area, which is a semi-arid region that extends from Mauritania in West Africa to India in the east. As the annual rainfall is low, locusts in solitarious phase inhabit the area until they develop into adults (before breeding). When it rains heavily and the grasses (wild grasses) start to grow, the adult locusts eat the fresh grass, become sexually mature, mate and start breeding. If there is a sufficient amount of grasses over a wide area and favorable environmental conditions continue, further growth and reproduction will occur and the number of individuals will increase. When the grass begins to dry after the rainy season, adults gather in the area where food remains (moving by self-flying and wind) and contact with other individuals induces a switch to gregarious phase, which causes changes in behavioral and physiological characteristics. When the offsprings become adults, they start to move in swarms and invade the invasion area (an area where locusts do not normally live and invaded in the event of a large outbreak), causing serious damage to agriculture in the area. If the environmental conditions of the invasion destination are suitable, the breeding will start, the population will further increase, and the extent of damage and area will increase.

The current 2020 outbreak is also believed to be due to the heavy rainfall caused by the cyclone after the drought creating a favorable environment for the desert locust.

Q2　How will the outbreak of desert locust end?

A: If it doesn't rain, the grass that feeds the locusts will dry and the wet soil suitable for oviposition will be lost, so the swarms cannot be maintained and eventually die. Adults have the habit of avoiding the surface of the water during flight, but when they are carried by strong winds and drown in the sea, they may invade snowy mountains and die because they could not fly in the cold. There is also the effect of natural enemies (predators, parasites). Since 1960, control activities using insecticides have been functioning, and the period of outbreaks has become shorter. In other words, both nature and human activities hold the key to ending the desert locust problem. It has been reported that the mortality rate increased due to the infection of entomopathogenic filamentous fungi (molds) that occur in nature when the locust outbreak occurred in Japan. However, it is not clear whether the desert locust can be infected under dry conditions to end the outbreak.

Q3　Is desert locust different from ordinary grasshopper? What are the types of grasshoppers?

A: There is a distinction between locust (tobibatta in Japanese) and grasshopper; the former shows phase polyphenism that changes its behavioral, morphological and physiological characteristics, and form swarms. The more common grasshoppers (inago in Japanese) do not exhibit such changes. There are approximately 6,800 known species within the grasshopper family, about 20 species are considered true migratory locusts. The grasshopper explained here refers to migratory locusts. The English name is desert locust and the scientific name is Schistocerca gregaria. At present, a major outbreak in South America is caused by Schistocerca cancellata, a Southern American migratory locust  species in the same genus. Locusta migratoria is another species of migratory locust found in Japan, and there are frequent outbreaks of a subspecies all over the world.

Q4　What are the characteristics of the movement of desert locust?

A: A swarm of desert locusts can fly with the wind for more than 100 km a day. It flies with the seasonal winds, crosses the "winter breeding site", "spring breeding site", "summer breeding site", and moves to an area suitable for breeding. It is usually carried by the wind in areas with rains, and as a result, it is possible to reach areas suitable for feeding and spawning. Unlike salmon, the same individual never returns to its place of origin, known as a one-way ticket transfer. In 1988, a locust swarm was recorded to cross the sea from West Africa and reached the Caribbean Sea, and flew over 4000 km.

It is believed that only adults in the gregarious phase can move long distances, but in reality, adults in the solitarious phase can also travel long distances. The gregarious locust is said to fly during the daytime and the solitarious locust at night.

Q5　Why is desert locust not found all over the world?

A5: Desert locust is a species adapted to drying, usually inhabits semi-arid areas, and greatly increases when heavy rainfall occurs. It is unclear why from the beginning it does not grow or reproduce in high humidity areas, which are rich in plant foods. On the other hand, migratory locusts are adapted to snowy and high humidity areas, and each of these grasshoppers is adapted to its own environment, and the world's breadbaskets are inhabited by indigenous grasshoppers. Previous studies have shown that the desert locusts live in relatively dry areas with less than 400 mm of annual rainfall, but there are no studies on why it does not always occur in areas of higher rainfall. High humidity and natural enemies are possibly some of the reasons.

Q6　Where do desert locusts lay eggs?

A: A female adult extends its abdomen like an accordion and digs the ground using the hardened part of the tip to lay eggs in the wet ground (5-15 cm below ground). Desert locust eggs cannot hatch unless they absorb water from the ground. By procuring water locally, adult females can make effective use of the space for making abdominal eggs. In the first place, adult females lay eggs only in damp ground, so if they cannot encounter damp ground, they will be able to hold matured eggs in their bodies for only about 3 days, so they will lay eggs on the ground and die. 

Q7　 What do desert locusts eat?

A: Desert locusts eat more than 500 plant species. Preference may change depending on the developmental stage (nymph, adult), and even the same plant may or may not be eaten depending on the growth stage of the plant. The main crops that are eaten are cereals (pearl millet, sorghum, corn, wheat and sugar cane), cotton and fruits. Rice is hardly eaten because it is cultivated in paddy fields (L. migratoria eats rice). Coffee isn't badly eaten either, but when a large group of swarms gather, the branches break under the weight. Some plants such as neem (Azadirachta), which is known as an insect repellent tree, cannot be eaten at all. Cannibalism has also been reported for wounded and dead individuals, but there are no reports of  cannibalism for healthy individuals. However, eating of the first hatched individuals immediately after hatching has been reported. It is difficult to obtain detailed information on the degree of damage to crops.

Q8　 What kind of damage will occur locally?

A: A locust outbreak will cause not only increased economic damage due to loss of agricultural crops, but shortage of livestock feed is also a problem. In addition to food shortages, livestock farmers may have no choice but to sell their own livestock for their livelihood. When prices rise locally, life becomes even more difficult.

Q9　It is reported that swarms of desert locusts have also flown to India, but are the swarms of desert locusts approaching China and Japan from East Africa?

A: A year ago, there was a major outbreak of desert locusts in Pakistan and India, causing problems. This was before the large-scale coverage of the desert locusts problem in East Africa from the latter half of January 2020. Since the issue of the desert locusts in India was reported in the beginning of June this year, it seems that many people are getting the impression that they have invaded India from East Africa. Furthermore, outbreaks of another species of locust (South American locust), which is different from the desert locust, has occurred in South America since 2015 causing extensive damage. It is sometimes mistaken for migratory locusts from East Africa to South America. More information about the desert locust is available on the FAO website. However, FAO has pointed out the possibility of a direct migration of desert locust from East Africa to India (as of July 21, 2020).

Q10　Is it possible for a swarm of desert locust to cross Mt. Everest?

A: The desert locust is a ectothermic animal, so it cannot fly at low temperatures. We have confirmed that adults cannot fly long distances unless their body temperature exceeds 21°C (Maeno et al. 2019). Adults can survive for a short period of time even at temperatures near 0°C, but as the altitude increases, they cannot fly due to low temperatures. In Morocco in North Africa, a large swarm of desert locusts flew down on snowy mountains, but they could not fly due to low temperatures and eventually died. It is thought that moving the muscles during flight will generate heat, but it is unlikely that the body temperature will rise sufficiently in the snowy mountains.

Q11　Is there a possibility that the desert locusts will fly to China and Japan?

A: To be honest, I don't know. I think it is meaningful to share information with everyone and anticipate together, so I would like to consider it. In the past literature (since the early 19th century), the limit of the landing place of the desert locust is to the eastern part of India, and it seems that the invasion of the desert locust into China has never been a big problem. It is considered that the invasion into China so far is prevented due to the low temperature in the mountains and the difference in habitat. However, adult locusts have the habit of riding with the wind and can travel a long distance of several hundred kilometers or more in a cyclone or jet stream, so invasion of Southeast Asia is also possibility. Chinese and British researchers predict that the current swarms in India and Pakistan are unlikely to invade Southeast Asia and China in light of past weather conditions (temperature, wind direction, wind speed) (Wang et al. 2020).

Whether or not the desert locust will come to Japan cannot be affirmed due to lack of scientific basis, but since it has never been observed so far, it is thought that it will not be possible unless several conditions overlap. These conditions are (1) a large population of desert locusts at the place of departure, (2) weather conditions are met (temperature, wind, weather etc.), and (3) enough energy required for long-distance flight. If these conditions overlap, the possibility of flying to Japan from the continent is considered to increase, but the situation is unlikely to be created at random. However, in the past, it was reported that a large swarm of Tonosama locust flew to Okinawa from the Philippines.

The scientific name of the locust is Schistocerca gregaria. It is believed that Schistocerca cancellata, which is currently a major outbreak in South America, originated from West Africa, migrated from West Africa to the new continent, and differentiated in the North American continent and South American continent. However, no grasshopper of the genus Schistocerca is known in East Asia or Southeast Asia. This may suggest that the desert locusts cannot settle even if they fly to these regions. In addition, there is a subspecies of the grasshopper locust on the African continent, which inhabits the vicinity of South Africa. In the central region, locusts of the genus Schistocerca are not distributed, and the rainforest is regarded as a natural barrier.

Q12　What happens if swarms of desert locusts fly to China?

A: Another species of locust (such as Locusta migratoria) inhabits China, and this species is also causes a major outbreak, so control materials and measures have already been prepared to some extent in China. According to the article “Management of locusts and grasshoppers in China” (Zhang et al. 2017), locust infestations are still widespread and there are 127 field stations with more than 2000 technicians involved in monitoring and control. Even if the desert locusts invade, the existing management system will be applied and measures will be taken. Many universities and research institutions are also conducting various fields of research on locusts and grasshoppers.

Q13　What happens if swarms of desert locusts fly to Japan?

A: Japan has abundant greenery, but it is doubtful that the favorable environment for desert locusts is available all year round. As to the temperature for development, the average temperature 38°C has the highest growth rate (adults grow faster), the growth slows as the temperature decreases, and the nymphs cannot grow below 19°C (easy to control with longer the nymphal period). It is known that an egg is almost killed when exposed to a temperature of 10°C or lower for 2 weeks, and an adult does not sexually mature and cannot reproduce at 20°C or lower. Bare ground (no plants) tends to be the preferred oviposition site. As to the amount of precipitation, it is said that the semi-arid region where the annual rainfall is less than 400 mm is the normal occurrence region, but it is unclear whether it can continue breeding in Japan where the amount of precipitation is high. Knowledge of the basic ecology of these locusts is extremely useful in estimating their range and future invasion, reproduction, and colonization (Uvarov 1966).

As a countermeasure after the flight, it seems that the control technology of the locust can be applied. When a locust outbreak occurred at the Kansai Airport, it was successfully controlled using an insecticide (Tanaka 2015). It seems that countermeasures can be prepared by combining the management technology of Tonosama locusts at such airports with the monitoring system used for the countermeasures against locusts. Differences in habitat, natural enemies, entomopathogenic filamentous fungi that specifically kill grasshoppers, and measures taken by farmers, governments, and citizens are believed to prevent the outbreak of the locust.

Q14　What will happen to the expansion of the distribution of desert locusts in the future?

A: Depending on the future rainfall and wind, there is concern that the distribution area will expand further. This time, the area affected by the desert locust has been damaged in the past, and at that time, the swarms moved about 3,500 km from the Arabian Peninsula to Niger over about a month, and has even expanded its distribution to West Africa. The movement to the east at that time was to India. The past outbreak appears to have been larger than this, but it did not move eastward from India.

Q15　What is the current method for controlling locusts?

A: The mainstream method is the application of chemical agents or commercially available entomopathogenic filamentous fungus (Metarhizium sp.) using a vehicle or airplane to spray directly on the locusts or indirectly on food grasses. The chemical is applied in ultra low volume (ULV) using aerial sprayers. Previously, the use of toxic bait was considered, but it does not seem to be used frequently.

Q16　Does the control using an insecticide cause resistance?

A: In the case of the desert locust, it does not stay on the spot since it migrates, and there is little opportunity for individuals with insecticide resistance to mate and breed with each other, so insecticide resistance is difficult to develop. Since insecticide are scarcely sprayed in the solitarious phase, it is unlikely that insecticide resistance will develop as well.

Q17　Is desert locust edible?

A:  Yes it can be eaten. When asked by a Mauritanian, I usually reply “I can eat grasshoppers but I prefer meat”. Due to an allergy, I refrain from eating locusts in foreign countries. However, when I try to eat it, I get the feeling of eating shrimp with hard texture.

Many people commented, "If you eat locusts, you can get rid of them and hit two birds with one stone!", but the issue of "how to efficiently catch them" remains at the previous stage. The locals have empirically learned that locusts can't fly in low temperatures and are slow to move, making them easy to catch and grab (Maeno et al. 2018). It is easy to catch and pick up locusts early morning when the temperature is low. However, in the early stages of the outbreak, locusts occur in remote desert areas so it is difficult to collect them. The aim is to fly into the city, but it is prohibited to eat grasshoppers on the side of the city because spraying of insecticides may have started nearby. In order to effectively utilize locusts that have occurred in large quantities, it is desired to develop a control technology that does not use insecticides.

By the way, the desert locust is expected to be a health food because it contains a large amount of phytosterols (known to suppress the absorption of cholesterol from the intestine) which are found in small amount in plants (Cheseto et al. 2015).

Q18　Why is it difficult to solve the desert locust problem and what should be improved?

A: Causes that make control activities difficult include time (irregular, sudden), location (expansion area is large, inaccessible deep desert areas including conflict zones), grasshopper ecology (high movement ability, wide eating ability), breeding ability etc.

Although it is necessary to develop technology to understand and predict the mechanism of outbreaks, there is a lack of information on field ecology, and there are many unclear points as to what mechanism led to the outbreak.

In order to quickly detect the occurrence of locusts in a vast area, technology development of remote sensing using satellite images is underway. Satellite images are used to find suitable environments (plant abundance, soil moisture etc.) for desert locust development and reproduction, rather than to find them directly (Escorihuela et al. 2018, Piou et al. 2019). JIRCAS is conducting field surveys and is working on elucidating the distribution pattern of desert locusts in a narrower area (Maeno & Ould Babah Ebbe 2018). A combination of technologies at various levels is expected to facilitate the development of a more accurate prediction model.

Q19　How can the damage caused by desert locust be reduced?

A: The damage caused by desert locusts  can be reduced if there is sufficient budget, human resources and equipment. It is important to develop technology for predicting desert locust outbreaks based on the weather and previous records of outbreaks (Piou et al. 2017). It is also important to continue to secure proper financial and organizational arrangements for a sustainable preventive approach that includes early intervention and efficient control (Gay et al. 2018, 2019). When the desert locust outbreak occurs over a wide area, the cost of countermeasures also increased 170 fold. One of the biggest obstacles to improving measures against desert locust is that the outbreaks are irregular and do not always cause problems. If it is always a problem, it is easy to secure a budget for countermeasures and personnel, but if there is no damage for decades, the budget will be considered unnecessary and will be abolished. A device to maintain a control system even when there is no damage is indispensable for solving the problem of the desert locust. For example, the Mauritania National Anti-Locust Center (CNLA) is responsible for the control of other pests when there is no desert locust outbreak, contributing to society, while maintaining training for control activities. In West Africa, a strong control system was established by making use of the experience during the 2003-2005 outbreak, and it is maintained even in the year when there is no desert locust damage. Even if a strong control system is established only in the affected country, it is necessary to secure international borders because desert locusts can cross borders from neighboring countries. In order not to repeat the tragedy, it is desirable to build a social system with full cooperation of the international community and minimum maintenance. In addition, it is extremely important not only to take control measures but also to continue the research budget. How to survive the recession period when people are less interested in desert locust (a recession period when locusts are not a problem) is a top priority for the people involved, and the researchers themselves should strive to keep people's attention through books and other informative materials (Maeno 2017, 2020).

Q20　 What kind of support does Japan provide for the damage caused by desert locust?

A: In 2020, the Ministry of Foreign Affairs will provide emergency grant aid for the damage caused by desert locust and provide support including food assistance.

South Sudan, Sudan and Uganda (JPY 495 million) (June 23) https://www.mofa.go.jp/mofaj/press/release/press4_008518.html

Kenya, Somalia and Djibouti (JPY 825 million) (March 10) https://www.mofa.go.jp/mofaj/press/release/press4_008343.html

Q21　How is JIRCAS pursuing research on desert locust?

A: A vast amount of research has been done from various perspectives for almost a century, but there are overwhelmingly few field surveys in habitats, and there are still many unclear points about field ecology. There is a possibility that elucidating the ecology of desert locust can dramatically improve control efficiency. Focusing on this point, JIRCAS has conducted a field survey in cooperation with the the Mauritanian National Anti-Locust Center, which is one of the outbreak areas, to understand the ecology and to consider new behavioral habits. We are working on the development of control technology (Maeno 2020). For example, desert locusts in the gregarious phase tend to assemble, but if we can understand when, where, and how they assemble, they can be effectively controlled with a small amount of insecticide (Maeno & Ould Babah Ebbe 2018, Maeno et al. 2018, 2019, JIRCAS 2019). Also, if we can understand the mechanism of the invasion and behaviour before an outbreak, we can predict the outbreak, so we are conducting research on the mechanism and the factors that trigger formation of  swarms (Maeno et al. 2020). We aim to establish a sustainable control system in consideration of environmental conservation by studying desert locust ecology in the field from various angles in cooperation with the Mauritanian National Anti-Locust Center.

As this requires specialists in various fields of research on desert locust, we are collaborating with the Center for International Agricultural Development of France (CIRAD), which has long been a leader in this field, in conducting laboratory and simulation experiments. We are also developing collaborations with FAO and other research institutes.

In this way, we would like to promote activities aiming at elucidation of field ecology and development of pest control technology while coordinating internationally, rather than conducting research independently.

Q22　Why does JIRCAS pursue research on overseas locust problems?

A:  As a National Research and Development Agency under the jurisdiction of the Ministry of Agriculture, Forestry and Fisheries (MAFF), JIRCAS has conducted the first research project on the problem of the desert locust in Japan. Under the philosophy of “Seeding for the future”, we carry out various research and studies in order to provide a stable supply of safe and high-quality foods, and to improve the livelihoods of people in developing regions. In line with this philosophy, we are pursuing the research project on “Development of technologies for the control of migratory plant pests and transboundary diseases”, which will likewise contribute in enhancing Japan’s presence in the international community.

Next year will mark 100 years since the Russian entomologist Sir Ubarov proposed the “phase” theory in 1921. In the last century, various knowledge has been accumulated with international cooperation, control measures have improved dramatically, and remarkable progress has been made in preventing outbreaks and reducing damage. However, there desert locust outbreaks still remain as a phenomenal natural disaster. Every time an outbreak occurs, a huge amount of support money can be outsourced  from all over the world and used efficiently for control activitiesand it was able to survive. Since many of the affected countries cannot afford it, it is difficult to secure sufficient cost for countermeasures. We believe that stronger cooperation with the international community is necessary. Furthermore, it will be necessary to take technical measures based on research results rather than ad hoc measures in the future.

Prediction technology is already under development on how global warming and abnormal heavy rainfall caused by climate change affect the distribution and outbreak of desert locust (Meynard et al. 2017, 2020), and locusts will continue to be developed. In addition to the research itself, multifaceted efforts are being made, and it is expected that unprecedented collaboration will bring about a breakthrough.

We should not be discouraged in dealing with the mysterious desert locust as there are still many things to do and means to take, and we should not lose hope. JIRCAS intends to collaborate with various domestic and foreign research institutes and work together to advance research projects in order to put an end to this global agricultural problem. Without your understanding and cooperation, we cannot go on this steep road ahead. We would like to turn all the warm support and encouragement into power and to work hard so that Japan can contribute in addressing this global predicament. Thank you for your support.

Contributor: MAENO Kotaro (Crop, Livestock and Environment Division)

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※ Citation for this article:

Maeno, K.O.（2020）About desert locust.　JIRCAS website   https://www.jircas.go.jp/en/program/program_b/desert-locust