L ife in the desert is defined by scarcity of water. The harsh, prolonged dry spells further restrict the availability of food resources for much of the year. Plants and animals in the desert have evolved various strategies to cope with the extreme environment. Migration over long distances, in search of greener pastures, is one of the common strategies of large desert animals such as the wildebeest in Africa. The desert locust, which has integrated behavioural, functional and structural adaptations to migrate over great distances across continents, has evolved as one of the most successful life forms in the desert. The locusts are among the most dreaded migratory pests in the world; huge swarms of desert locust appear suddenly in the sky and devour all kinds of vegetation within hours, threatening the food supply of millions of people in some of the poorest countries. India, after more than two decades, is experiencing a massive locust attack, which is likely to intensify in the months ahead.
Locust invasions are probably as ancient as settled agriculture. The Russian-British entomologist Boris Petrovitch Uvarov (1886-1950) pioneered studies on locusts. He is considered the Father of Acridology (study of locusts and their relatives). In 1921, Uvarov identified the solitarious and migratory phases of locusts, which had been considered a distinct species for centuries. His studies on the biology and ecology of locusts helped put locust control on a sound scientific basis. E.C. Cotes began formal studies on crop pests, including locusts, in India. In 1891, he published accounts of locust invasions in the provinces of Bengal, Madras, Assam and Bombay. The Imperial Council of Agricultural Research started funding a Locust Research Scheme in 1930, and the Locust Warning Organisation (LWO) was established in 1936 in Karachi, following a major locust plague that lasted for about four years between 1926 and1930. After Independence, the field headquarters of the LWO was shifted to Jodhpur, Rajasthan, with the central headquarters in Faridabad, Haryana. Yelseti Ramachandra Rao (1885-1972) studied the desert locust in undivided India and neighbouring countries. His The Desert Locust in India , published in 1960, is regarded as the most authentic work on the subject in the subcontinent.
What are locusts?
Locusts are large short-horned grasshoppers, belonging to the insect family Acrididae, capable of forming swarms. They exist in two distinct phases, solitarious and gregarious, with intermediate forms. In the solitarious phase, they merge with the environment, live as individuals avoiding each other, lead an inconspicuous life in the desert and hardly trouble agriculture. When conditions are favourable, the solitarious ones multiply rapidly and eat up most of the vegetation, which leads to the formation of groups. Converging winds and dwindling food resources lead to concentration of adults. As they come into frequent contact with each other, changes set in in their behaviour, function and structure, and thus begin the transition to the gregarious phase. Populations switch over from the solitarious phase to the gregarious phase over more than a generation. Once they become gregarious, they may continue in the same phase for three or four generations. Thus, a locust plague may continue for more than a year.
The science of phase transformation in locusts still remains unsettled. A group of Japanese scientists led by K. Maeno in 2004 showed that phase-related changes were induced by the neurohormone corazonin. Michael Anstey and Stephen Rogers, in a paper published in 2009 in Science , argued that the initial switch over from solitarious to gregarious locusts is triggered by serotonin, the brain hormone associated with depression in humans. However, a group of Belgian scientists, led by Bart Boerjan, in 2011, rejected the role of neurohormones, such as corazonin and serotonin, and showed that the phase transformation was controlled by genes that were switched on or off, in response to stimuli induced by crowding.
An extensive theoretical study by Darron Cullen in 2017 further corroborated the findings of the Boerjan team. Accordingly, the locust phase change is due to differential gene expression in response to non-genetic stimuli, termed epigenetic change, mainly through a process called DNA methylation. The size and complexity of the locust genome is the major hurdle in understanding the molecular basis of phase change in its entirety. However, modern sequencing techniques and powerful gene editing tools such as CRISPR-Cas9 are expected to accelerate our understanding of locust phase change and migration, which is one of the most interesting scientific questions in biology.
Being touched by one another during crowding, especially on the outer surface of hind thighs, locusts get attracted, instead of avoiding each other. The change over from mutual repulsion to attraction takes barely an hour and the complete transformation from the solitarious to the gregarious phase takes over a few generations. The gregarious mother induces gregarisation in her progeny by adding a chemical to the covering of the eggs.
The young of the locust in the solitarious phase adjust their colour to that of the surroundings and lead a camouflaged life, avoiding each other. They are rather sluggish and never move in bands. The young ones in the gregarious phase also develop a conspicuous warning coloration with fixed pattern of black and yellow or orange, and are hyperactive. The wingless young ones form bands, which is a mass of hoppers that moves as a unit. Adults in the solitary phase are larger with shorter wings, longer legs and a narrower mid-body than the gregarious ones. As locusts become gregarious, their maturation becomes synchronous and each individual insect behaves as a part of a group, marching and flying in the same direction. The most important differences between the locust phases are behavioural-– hyperactivity and gregarious nature in the gregarious phase, due to change in functions and the resultant higher body temperature. Solitary adults migrate at night, while the gregarious ones fly during the day and spend the night roosting on trees and other vegetation.
The swarms take off and land into wind during warm days and fly up to nine or 10 hours, moving downwind. The average swarm density is about 50 million locusts per square kilometre. Swarms reach up to a height of 2,000 metres and fly continuously when over sea. They usually cover 100-150 km a day, although there are records of swarms crossing the Atlantic. In October 1988, swarms from West Africa reached the Caribbean, covering a distance of 5,000 km in 10 days. Swarms from West Africa migrated to the British Isles in 1955. In Africa, in 1958 and 1959, swarms measuring up to 800 square km were reported. The estimated weight of a swarm is one to two lakh tonnes, and it can consume food equivalent to its weight in a day. A square kilometre swarm can eat the equivalent of food consumed by 35,000 people in a day. In Kenya, in 1954, a total population of 50 swarms together covering an area of 1,000 sq km, was estimated to contain 500 trillion locusts. Many swarms perish in the sea and alien lands. Some return to their breeding grounds in the desert and revert back to the solitarious phase to continue the locust cycle.
Types of locusts
About 18 species of large grasshoppers, those that form swarms, are considered as locusts. In India, three species of locusts occur—the desert locust ( Schistocerca gregaria ), migratory locust ( Locusta migratoria ) and the Bombay locust ( Nomadacris succincta ). The tree locust ( Anacardium rubrispinum ) and the coffee locust ( Aularches miliaris ) are not true locusts, as they do not form swarms.
The desert locust, occurring in about 57 countries, is the most widely distributed and most destructive among the locust species. It is distributed from Mauritania in the west to India, encompassing the deserts of north Africa, near east and south-west Asia. This area includes desert basins, arid and semi-arid plateaus, low lands and mountains. There are mainly two breeding areas in south-west Asiasouth-east Iran and the adjoining areas of Pakistan in spring; and the India-Pakistan border during the summer. Almost one-fifth of the earth’s land area is affected by the desert locust.
The life cycle of a locust is in three stages—the egg, pronymph followed by five (five or six in the solitarious phase) wingless hopper stages and the winged adult. Breeding occurs during the wet season as moist soil is ideal for egg laying and hatching. Egg clusters are laid in moist sandy soil at a depth of about 5-10 centimetres, in special packs called egg pods. Each pod contains 60-160 eggs in the solitarious phase, while the gregarious females lay fewer than 80 eggs. The female bores a hole of about 10 cm depth with the end of her abdomen and deposits the egg pod of 3 to 4 cm length at the bottom, and fills the hole with a foam that hardens to plug the hole. A single female lays two egg pods at an interval of about 10 days, or rarely up to four. Up to one-third of the eggs perish due to natural causes. Scores of females lay egg pods in groups at the most favourable sites, creating mass nurseries. The eggs hatch in 10-14 days in summer, which could be extended beyond two months in winter. The pronymph–forerunner of the hopper stage–hatches out of the egg and is an ephemeral, non- feeding stage. It soon sheds the skin and forms the first instar hopper. Newly emerged hoppers soon start feeding on vegetation. Up to 70-80 per cent of the newly born hoppers perish due to inadequate moisture, predation by ants and cannibalism. After at least five or six days, the tiny hopper sheds its skin and becomes a larger hopper. Thus, it changes its skin five times (five or six in the solitarious phase) to become the winged adult. The newly emerged pinkish adult, called fledgling, has a soft body and drooping wings. It takes nearly 10 days for the wings to harden enough for sustained flight. The hopper stage lasts for 24-95 days (average 36 days), and is longer in winter. A life cycle is completed in two to six months and two to five generations occur in a year. Adults probably live for 2.5-five months. A single female produces 16-20 viable locusts in a single generation. The population of locusts can increase 400-fold in six months after two generations of breeding.
Locust plagues
Swarms land about 30 minutes to two hours before sunset and feed indiscriminately on all kinds of vegetation. According to R. Swaminathan, a specialist on locusts and grasshoppers, the solitarious ones do not prefer neem and cluster beans. Locusts feed on all portions of the plant above ground such as leaves, shoots, flowers, fruits, seeds and bark. Any crop, cereals, pulses, cotton, fodder crops, fruit trees, vegetables, banana, and all kinds of plants are good enough for the locust. Swarms wipe off entire fields within a few hours, leaving farmers high and dry. A field of cotton in pre-flowering stage could be eaten away in five-10 minutes, according to Swaminathan. There are historic records of locust invasions enhancing the severity of famines in India. The availability of food in several most backward African countries is already under threat due to the ongoing plague.
A desert locust plague does not occur suddenly. Locusts remain in “recession” for a few years. However, when conditions are congenial, they multiply. Outbreaks’ and upsurges occur before a full-blown locust plague, like the ongoing one in Africa and south-west Asia. An outbreak occurs when increasing locust numbers leads to gregarisation, which happens over several months. This is localised and restricted to certain habitats.
Outbreaks, when left uncontrolled over two or more generations, develop into upsurges. Many upsurges finally lead to a locust plague sweeping across countries and continents. Since 1900, there have been eight locust plagues in south-west Asia (1900-1907; 1912-1920; 1926-1932; 1940-1946; 1949-1963; 1967-1969; 1988-1989, and the current one). It is estimated that about 29 million sq km, extending across 58 countries could be vulnerable to locust plagues, inflicting massive damage and destruction to agriculture and allied activities.
Climate connection
Climate change and global warming are visibly altering rainfall patterns globally. Since every degree increase in temperature results in about 7 per cent increase in the water holding capacity of air, global warming directly results in higher levels of water vapour in the atmosphere. Storms and cyclones, therefore, carry more moisture and deliver more intense rain. Aaron Putnam and Wallace Broecker in 2017 argued that rainfall in the tropics would increase and that the subtropics would become more arid. The influence of climate change on the incidence of pests and diseases of crops is evidenced by the emergence of new pest and disease problems and the increasing severity of existing ones.
The current locust plague is linked to the weather dynamics of the Indian Ocean called the Indian Ocean Dipole (IOD). The Indian subcontinent divides the Indian Ocean into the eastern and western parts. When the western part is warmer than the eastern part, called positive IOD, it results in cyclonic storms that hit east Africa and the Arabian Peninsula, bringing heavy rain. An increase in the positive IOD produced hyperactive cyclones in May and October 2018, which hit the Arabian Peninsula where it rained heavily, resulting in pools of water in the desert creating congenial conditions for locust breeding. The swarms originating in Oman and Yemen, aided by wind, entered east Africa, where they were again favoured by rains. They spread further to Iran and Pakistan; since June 2019, Pakistan has been plagued by locusts. Early this year, Somalia and Pakistan declared national emergencies to fight locusts. A team of researchers, led by Wenju Cai, in 2014, estimated that the frequency of positive IOD events had increased threefold due to climate warming. With the increase in extreme positive IOD phases, we should be prepared for more locust plagues.
Thanks to major irrigation projects such as the Indira Gandhi Canal that traverses the Thar Desert in Rajasthan, sand dunes are being converted into agricultural fields. The greening of deserts through afforestation and irrigated agriculture could provide conditions congenial for enhanced locust breeding.
Locust Plague 2020
India had its last full-blown locust plague in 1962, followed by intermittent upsurges and outbreaks; the last upsurge occurring in 1997. Localised breeding was observed and controlled in 1998, 2002, 2005, 2007 and 2010. There are several historical records of desert locust attack in south Indian States such as Tamil Nadu and Kerala. E.C. Cotes has placed on record that swarms from north-western India penetrated the Madras Presidency in 1878, 1889 and 1890. Similarly, the United Nations’ Food and Agriculture Organisation (FAO) has recorded swarms reaching Kerala in 1954. However, all these appear to be cases of either Bombay locust or large, non-swarming grasshoppers being misidentified as desert locusts. There is little evidence that the current plague will spread to the south.
According to Swaminathan, the current plague started in India as locusts entered thrice into the country since 2019 through the India-Pakistan border: In May 2019; November 2019-February 2020; and in May 2020. The border States of Gujarat, Rajasthan and Punjab and Uttar Pradesh are experiencing infestations since last winter. The LWO reported control operations against gregarious hoppers in the first fortnight of April 2020 and swarm movement in early May in deserts in Rajasthan. Swarms invasion across India reached as far south as Maharashtra and Bhopal in Madhya Pradesh to the east.
The desert locust is devastating agricultural crops in many States. The FAO has predicted a second wave of locusts from the Horn of Africa into India in June-July. An advisory issued on June 10 warns of an imminent invasion coinciding with the kharif, the main crop season in India. On June 12, FAO put out information on the formation of new swarms in east Africa, indicating that a threat is imminent. Normally the appearance of desert locusts in India begins in July-October; however, the current attack appears to be rather early. In Baluchistan and Punjab in Pakistan, spring breeding is over and the adults are forming swarms. Rajasthan, where maturing swarms occur, has already received pre-monsoon rain in the first week of June; active breeding and multiple generations can thus be anticipated.
Locust control
Locusts do not respect national boundaries. International cooperation is thus the key to locust control. Monitoring, early warning and preventive control of swarms in east Africa, the Arabian Peninsula and Pakistan are crucial for India. The locust-affected countries all over the world are coordinated by the FAO. The Desert Locust Information System (DLIS) of the FAO issues advisories and warnings to countries plagued by locusts. The DLIS forecasts the timing, scale and location of breeding and swarm movement. The LWO is tasked with monitoring, tracking and prevention of locust breeding and control in India.
According to Swaminathan locusts in the solitarious phase, immatures and swarms necessitate different control strategies. The FAO has recommended application of spore suspension of the pathogenic fungus Metarhiziumanisopliaevaracridium (strain IMI 330189), which only infects locusts and grasshoppers, to destroy the immature stages of the locust in deserts where the breeding takes place. Hoppers can be destroyed by mechanical means; digging trenches using earthmover machines and burying marching hopper bands are a common method.
However, none of these methods are effective against invading swarms. Swaminathan said farmers should be acting as informers of swarm movement rather than taking control measures in their own farms. Since it is futile for farmers to fight the pest at the level of individual farms, it is incumbent on the government to reward and protect them so that they act as early warners of swarm attacks.
Teams equipped with vehicle-mounted sprayers or aerial sprayers are effective against swarms. Direct application of chemical pesticides on swarms roosting on vegetation, during morning and evening, is the most effective method. The Directorate of Plant Protection, Quarantine and Storage has approved five synthetic pesticides such as chlorpyriphos, deltamethrin, fipronil, lamdacyhalothrin and malathion for the control of swarms.
The absence of major invasions since 1997 has resulted in the lowering of guard against the desert locust, according to G.M. Patel, entomologist, who has been involved in fighting infestations in the past and in the present one. According to him, the personnel involved in control operations are not experienced in dealing with such massive infestations.
As the locust attacks are likely to become a regular scourge, India should strengthen international, national and regional linkages. India had sent self-contained teams with vehicles and equipment to the Arabian Peninsula every year between 1955 and 1962. As the locust plague is emerging as a transnational threat, India should once again consider expanding its expertise and capabilities beyond its borders.
The LWO needs to be upgraded with adequate field stations, manpower and equipment to meet the challenge. Over the past two decades, the LWO was weakened, possibly because there were no major infestations. The Indian Council of Agricultural Research and agricultural universities in the affected States should include desert locust among their research priorities as there are gaps in our knowledge of the bioecology and management of the pest.
As the threat of the pest looms, it is time to formulate a long-term and durable strategy to fight the locust menace.
Dr K.D. Prathapan is an entomologist specialising in insect taxonomy at the Kerala Agricultural University.
