Desert Locust preventive management and the ongoing invasion

Large swarms of Desert Locusts are again threatening Africa and huge areas up to India and Pakistan. The Food and Agriculture Organization of the United Nations (FAO) is calling the outbreak the most serious locust situation for decades. These swarms have yet to be contained. They recently reached Kenya, which had not faced a threat of this magnitude for 70 years. While we have seen classic images of these devastating swarms before, their impressive impact never seems to diminish: trees writhing with locusts bodies and branches breaking under their weight; farmers desperate because of the loss of their crops; and technicians equipped with hand sprayers, (often poorly protected from pesticides) attempting to control these insects where only aerial means would be effective against these highly migratory insects.

Let’s try to provide some answers to the legitimate questions about the Desert Locust and the current situation. For further information, some key references, contacts, and glossary terms are provided at the end of the article.

Schistocerca gregaria, gregarious mature (photo FAO-DLIS).

The Desert Locust and its invasions

The Desert Locust, from its scientific name Schistocerca gregaria (Forskål 1775), constitutes a threat to agriculture from North Africa to the equator, and from the Atlantic to South-West Asia via the Near East. It occurs in two forms: solitarious or gregarious, with the latter forming large hopper bands and swarms capable of migrating very long distances.

Like some other locust species, the Desert locust has a surprising ability to transform radically when favorable conditions allow their population to reach a density threshold. Schematically, we speak of a solitarious form for populations of low density and a gregarious form for those of high density. The solitarious and gregarious forms of the Desert Locust are so dissimilar that they were once considered to be different species. The color, the size, and of course the behavior, changes radically during the passage from one form to the other, which requires a few generations to complete.

In nature, when density increases, individuals who may have initially been solitarious, seek to stay grouped. At the nymphal stage (without wings), the locusts congregate in bands of thousands of individuals per square meter. Adults, however, can form swarms covering several hundred hectares. Unlike the solitarious locusts who fly at night; the gregarious locusts fly during the day.

This insect, both in its solitarious and gregarious phases, is able to remain in flight for a long time and to migrate–carried by the winds–over spectacular distances. Known to travel up to 200 km per day and sometimes much more. In 1988, swarms crossed the Atlantic from Mauritania to South America in just a few days.

Following sequences of favorable rains, upsurges and invasions develop. They are interspersed with periods of remission during which solitarious locust populations are present only in very small numbers, restricted to a relatively limited area termed the recession area. The Desert Locust recession area is in largely uninhabited desert regions from Mauritania to India (spanning 16 million km²), far from crops. During invasion periods, gregarious populations can occupy a much larger area covering 65 countries in Africa, the Middle East, and Southwest Asia (spanning 29 million km²) that are widely cultivated and populated. The Desert Locust can thus threaten the livelihoods of one-tenth of the world's population.

Distribution area of the Desert locust: Invasion, recession and outbreak areas across the species’ range (from Lecoq 2004, in Sword, Lecoq and Simpson 2010).

How serious is the threat?

The economic, social and environmental consequences of the Desert Locust invasions are such that this insect is often treated as a national priority by many countries.

The Desert Locust is the most formidable of all locust species due to the threat it poses to many countries. Numerous data and testimonies attest to its economic and social importance. The extent of its devastation has been mentioned since ancient times, but also as in recent years. At the local farm level, all crops can be completely decimated in just a few hours, historically initiating the onset of many famines.

The Desert Locust is extremely polyphagous (able to consume a wide variety of foods); capable of damaging all types of vegetation and crops. Cereal crops including wheat, barley, millet, maize, sorghum, and rice are particularly vulnerable. However vines, citrus fruits, palm trees, date palms or vegetable crops are not spared. Pastoral areas are also undergoing major destruction, affecting both the total production of biomass and its palatability for livestock.

Crop losses can lead many households to sell animals at low prices to meet their needs, or even to require food aid. In addition, the negative income shock can have a long-term impact on the educational outcomes of children living in rural areas. Invasions can result in abandonment of crops and rural exodus.

Locally food competition between locusts and livestock can lead to the degradation of vegetation and soils, thereby locally promoting desertification. In addition, large-scale chemical control programs (essential to control an invasion) can affect biodiversity, including harming non-target grasshoppers and many other arthropods.

The results of the last major invasion in West Africa in 2004-2005 speak for themselves: 26 countries affected, 6.5 million hectares devastated, 13 million hectares treated with insecticides, crop losses estimated at a few 2.5 billion dollars, control costs estimated at more than 400 million dollars, a figure most certainly underestimated.

What drives these insects to migrate and invade crops?

The migratory behavior of the Desert Locust is an adaptation to survive in arid environments.

This insect, whether in the form of hopper bands, swarms of adults flying during the day, or solitarious individuals flying at night, has a high migratory capacity.

Why are they moving? What are they looking for?

In reality, it is neither hunger nor fleeing their cannibalistic peers that push these insects to migrate. This migratory behavior is the result of a long evolution which allowed this species to adapt perfectly to its desert environment. In these areas, favorable rains for breeding and food plant production occur episodically, and are randomly scattered over large areas. Under these unpredictable conditions, migrating is necessary for Desert Locust survival.

These migrations are systematic. The adults are pushed by the winds that concentrate in the zones of convergence where rain is generated, allowing locusts to find favorable conditions. This migration takes place both in the solitarious phase (at night) and in the gregarious phase (by day). The search can sometimes last for months, but this insect has a great resistance capacity and lays its eggs only when the conditions for reproduction are met. For hoppers, the migratory behavior allows them to exploit the local resources by moving regularly within the vegetation. In either case, migration takes place even if local resources are still available. Migrating is a vital issue for the Desert Locust. Staying in the same place for too long would be risky for individuals. This migratory behavior is found, with various modalities, in the other locust species.

What are the causes of these invasions?

Abundant and persistent rains spread over a large area are very favorable to produce upsurges of this insect.

To multiply, they need sandy or sandy-clay soils, areas of bare soil for egg laying, and suitable vegetation for the development of nymphs.

Weather conditions and the structure of the habitat are therefore the two main factors involved in the multiplication of the Desert Locust. Following favorable rains, in desert environments where solitarious individuals live, locusts concentrate on suitable areas and breed. If the rains are sufficient, persist for a fairly long period, and are spread over a wider area than usual, the numbers of locusts can increase significantly. Here we see the phenomenon of gregarization start, the first hopper bands from and then primitive swarms. This is the beginning of an outbreak that can turn into an upsurge, and then into an invasion, if favorable conditions persist and if no early control operation is taken.

Could climate change be responsible for the current invasion?

The current invasion is nothing new. Such invasions occurred regularly in the past following abundant rains. It's too early to say that the current invasion is the result of climate change.

The current invasion did not appear overnight. It was the result of favorable conditions that arose many months ago. At the beginning of 2018, the locust situation was very calm and populations were low. Reports of this insect from the Atlantic to India were almost nonexistent.

During the second half of April and again in October 2018, unusually strong tropical cyclones formed in the southern Arabian Sea. They caused heavy rains in Yemen, Oman, Djibouti, in northern Somalia, eastern Ethiopia, and in southern Rub al Khali in Saudi Arabia. The favorable conditions for Desert Locust breeding have been maintained for at least nine consecutive months (June 2018 to March 2019), allowing the development of three generations. These locusts remained undetected and therefore uncontrolled.

Thus the development of this invasion resulted from particularly favorable rainfall conditions for the Desert Locust. Such invasions have occurred regularly in the past following abundant rains in outbreak and key breeding areas. If climate change leads to increased precipitation in these outbreak areas, this would undoubtedly increase the probability of future invasions.

In reality, even when the Desert Locust finds suitable conditions, invasions can only develop following various failures in the international management system.

What is the recommended strategy to contain this plague?

A preventative control strategy has been applied for several decades. Recommended by the FAO, it involves early warning and rapid response capacities in countries with outbreak areas.

The scientific base for a strategy of preventative control against the Desert Locust was defined in the 1930s. The transition from the solitarious to the gregarious phase first occurs in the outbreak areas which are relatively small compared to the whole distribution area of the species. Therefore invasions can, in theory, be avoided by destroying hopper bands and the very first swarms in these areas.

The preventative control strategy is simple in principle, and is recommended by FAO. It involves early warning and rapid response capacities which require monitoring of environmental conditions, locust population levels, and the implementation of preventative treatments against the first gregarious populations.

Such capacities have been gradually developed and are currently implemented in most countries with outbreak areas. Monitoring and preventative control teams survey areas according to several criteria including (i) their known potential for favoring Desert Locust outbreaks; (ii) the likely timing of locust activity in the area related to regular seasonal variation; and (iii) rainfall and development of vegetation indentified either through the meteorological ground network, which is often insufficient in these desert areas, or through satellite imagery.

At the international level, the FAO Desert Locust Control Committee (DLCC), established in 1954, brings together all the affected countries and donor countries, and coordinates international activities on the Desert Locust. The FAO Desert Locust Information Service (DLIS), in Rome, produces monthly maps of the locust situation and forecasts for the next three months. Finally, three FAO regional commissions coordinate their activities in each area of responsibility: West and North Africa, Middle East, and Southwest Asia.

Speed is a key element at all stages of a successful strategy. This includes the localization of risk situations, the transmission, sharing and analysis of information, and ultimately the decision to intervene.

We must highlight that the monitoring and preventative organization against Desert Locust invasions as a leading example in the field of crop protection.

It is a great feat to successfully monitor (in real time) the population level of a highly migratory insect over vast and often remote areas, in addition to tracking the evolution of ecological conditions like rainfall and vegetation growth, coordinating the efforts of several dozen countries, contributing to the improvement of surveillance techniques, and the training of technical staff. This prevention system is genuinely effective, but also has its flaws, as the current situation unfortunately reminds us.

Is the current preventative strategy effective?

As a result of a preventative strategy, invasions are now less frequent, reduced in scale, shorter lived and better managed.

The early intervention policy has been applied since the 1960s. As a result, and with 60 years of hindsight, it is obvious that the invasions are now less frequent, reduced in scale and, if they cannot be stopped at an early stage, shorter and better managed.

Consistent evidence strongly suggests that early action strategies and new technical methods to survey and control the locusts are responsible for the improved Desert Locust situation over the last 60 years. The same improvement has also been observed for other locust species with completely different ecologies, living in different geographical environments.

Over the past 60 years, the Desert Locust control program has (mostly) achieved the objective of preventing swarms invading the majority of large cultivated areas. And it is likely that an even earlier intervention strategy should further reduce the duration and extent of plagues of this species and could entirely prevent some of them.

Number of countries reporting swarms, 1910-2019 (Source: FAO-DLIS). Red: plague peak; orange: plague onset or decline; green: recession.

This preventative strategy is constantly being improved:

(1) To reduce the surface area needed to search for locusts. It is possible to improve characterization and mapping of the outbreak areas. Such developments help streamline monitoring operations while making them more effective.

(2) It is also possible to more precisely determine the timing and location of favorable environmental conditions in high risk areas through the detection of events that normally precede the first upsurge of solitarious populations, and the first gregarization events. As rain, vegetation and soil moisture are essential factors in the dynamics of locust populations, satellite remote sensing is increasingly used to follow the evolution of these parameters over the whole habitat:

• Rainfall estimates are derived from METEOSAT to better understand the spatial and quantitative distribution of rainfall.

• Vegetation estimates rely on MODIS and SPOT-VGT imagery, even if it’s often difficult to detect sparse vegetation in the desert.

• New satellites such as SMOS, based on radar, can directly provide soil humidity data.

Since 2001, satellite images have been an essential component of Desert Locust forecasts. By their capacity to provide a continuous, almost in real time and on a continental scale, overview of the areas with favorable ecological conditions. They make it possible to establish dynamic maps of potential breeding areas, tools that represent precious help for field teams.

In addition, for the past 20 years, the FAO EMPRES program has helped strengthen the capacities of countries to respond to the locust threat and to conduct survey and control operations more effectively. However, nothing is perfect and much progress remains to be made.

What are the constraints to applying an effective preventative strategy?

Insecurity in many key zones, as well as financial and organizational problems, are largely responsible for the failure in early control of some Desert Locust upsurges and invasions.

Even if overall evidence suggests that a preventative approach to locust management has markedly improved the situation, the risk is not yet fully controlled–as the current situation reminds us–and difficulties still remain.

Over the past 60 years, invasions have continued to occur, with unsuccessful early control, even though their duration was relatively short thanks to more effective control methods.

Aside from the current situation which has just started, the last two large invasions date back to 1987-1988 and 2003-2004. Organizational problems were largely responsible: weak prevention systems in many countries, lack of emergency funds, lack of well-equipped and well-coordinated survey and control teams, and a slow dispersal of provision and funds by donor countries. Clearly a problem of logistics resources and organization, despite the effective detection of the risk situation.

The present invasion also shows the vulnerability of the preventive system to the numerous insecure zones prevailing in the locust's habitat, making surveys and initial control measures ineffective or even impossible. The crisis situation in the first outbreak zones, particularly in Yemen, was undoubtedly key in the early failure to control the invasion.

Beyond various positive recent developments, both on technical and institutional levels, the long-term sustainability of the entire preventive control system, remains problematic. An important constraint is the reduction of resources (and therefore of survey and early control capacities) during recession periods. Funding is most often abundant during, or shortly after, an invasion (allowing the development of research and strengthening of survey and control systems). But, if prevention is effective and locusts are scarce, there is often a transfer of resources to other more immediate problems. By the time the next critical situation arises, the control capacities may have become insufficient to prevent the development of an upsurge. The result is a vicious cycle that has been observed many times, both for the Desert Locust and other species. The answers are not only scientific and technical, but above all, financial, institutional, and ultimately political.

As a conclusion

Despite an effective and proven preventative strategy for more than 60 years, the financial and political uncertainties, as well as the recurrent insecurity in many regions of the Desert Locust distribution, will undoubtedly continue to maintain the threat in the future.

With each new locust invasion the same questions arise and debate is relaunched on the economic importance of this species, the need for a renovated preventative system, as well as reenergizing regional and international cooperation. Measures are taken, but the real critical points still have no solution. These are organizational problems: permanence of an effective system in all key zones, strong international cooperation and speed of resource mobilization–financial, material, and human–in the event of an upsurge. Unfortunately, the financial and political uncertainties are numerous. The recurrent insecurity in many areas known to be favorable, in the event of rain, to the proliferation of these insects, is not the least of the problems to be resolved. Whatever the research results, if these various points, which depend on people and not on nature, are not effectively addressed, the invasions will continue to succeed in the future.

Some references

Lecoq M. 2005. Desert locust management: from ecology to anthropology. J. Orthoptera Res. 14:179–86

Sword GA, Lecoq M, Simpson SJ. 2010. Phase polyphenism and preventative locust management. J. Insect Physiol. 56:949–57

Zhang L., Lecoq M., Latchininsky A., Hunter D., 2019. Locust and grasshopper management. Annual Review of Entomology 64(1):15–34. https://doi.org/10.1146/annurev-ento-011118-112500

A website to follow the Desert Locust situation

Food Agric. Org. UN. 2020. Locust watch: desert locust. Updated 2020, February 2. Food and Agricultural Organization of the United Nations. http://www.fao.org/ag/locusts/en/info/info/index.html

Schistocerca gregaria, gregarious immature (photo S. Ghaout, CNLAA, Maroc)

Contact National Locust Directors and Information Officers

http://www.fao.org/ag/locusts/en/info/info/contacts/index.html

Glossary of technical terms

Decline: period characterized by breeding failure and/or successful control leading to the dissociation of swarming populations and the onset of recessions.

Hopper band/adult swarm: group of gregarious hoppers/adults, of high density, that can move in a coherent manner.

Locust: type of grasshopper with a remarkable and potentially devastating form of density-dependent phase polyphenism forming adult swarms and/or nymphal bands.

Outbreak: marked increase in locust numbers due to concentration, multiplication, and gregarization.

Outbreak area: limited regions where significant outbreaks have occurred and given rise to upsurges and invasions.

Phase change: a reversible transition process between solitarious and gregarious phases in locusts in response to changes in population density.

Plague: period of one or more years of widespread and heavy infestations of locust bands or swarms.

Polyphenism: density-dependent phenomenon in which two or more distinct phenotypes are produced by the same genotype.

Preventive management (control) strategy: strategy aiming to treat as many locust or grasshopper hotspots as possible before they damage crops, with treatments beginning early in outbreaks.

Recession: period without widespread and heavy infestations by swarms.

Upsurge: period following a recession marked by a very large increase in locust numbers in contemporaneous outbreaks.