They covered the face of the whole land, so that the land was darkened, and they ate all the plants in the land and all the fruit of the trees that the hail had left. Not a green thing remained, neither tree nor plant of the field, through all the land of Egypt. (Exodus 10:15)

Locusts are the most important migratory pests globally, threatening the food supply of millions, due to the immensity of swarms migrating across countries, suddenness of their appearance, and ability to devour all kinds of vegetation within hours.

History of locust invasions probably dates back to time immemorial. Yelseti Ramachandra Rao (1885−1972), who committed himself to the study of locusts, has meticulously recorded historic locust invasions in his monumental work The Desert Locust in India (1960). Authentic records of locust outbreaks and the resultant famines in India commence from 1803. According to E. C. Cotes (1891), who began the formal studies on locusts and other crop pests in India under the British rule, there were recurrent locust attacks at an interval of four to eight years, during the nineteenth century.

What is a locust?

Locusts are large short-horned grass hoppers capable of changing their behaviour and habit from an ordinary, solitary life to a gregarious, highly migratory form under crowding. The ‘solitary phase’ and the ‘gregarious phase’ are so distinct that they were considered to be two different species by entomologists. They can be separated based on colour, behaviour, and external features of the body. Young ones of the solitary phase adjust their colour to that of the surroundings, such as green, grey, or brown. They never move in bands and are sluggish. Young ones of gregarious phase, on the other hand, have a fixed pattern of black and yellow or orange and are hyperactive and move in bands. Adults in the solitary phase have shorter wings, longer legs, and a narrower back than those in the gregarious phase. Overall, the most important difference between the two forms is the hyperactivity and gregarious tendencies of the gregarious form due to higher body temperature and change in body functions.

The gregarious forms occur in large swarms which may fly for great distances of hundreds to thousands of kilometres under the influence of wind, even crossing oceans. A single swarm can cover up to 1200 square km, containing about 40–80 million locusts per square km. A swarm may contain 50–100 billion locusts, each weighing a mere 2g. Thus the total weight of a swarm is about 1–2 lakh tons! A locust consumes food equivalent to its body weight in a day. A one square kilometre swarm can eat the equivalent of food consumed by 35,000 people, in a day.

Types of locusts

About eighteen types of grasshoppers, capable of forming swarms, are considered as locusts. At least one species, the Rocky Mountain locust, that formed swarms of 10 billion individuals across North America and Canada, was driven to extinction by the end of nineteenth century by the early European settlers who practiced intensive agriculture in their breeding grounds. In India, four species of locusts are present: desert locust (Schistocerca gregaria), migratory locust (Locusta migratoria), Bombay locust (Nomadacris succincta) and tree locust (Anacridium sp.), the first three being important.

The Desert Locust

Of all the locusts in the world, the desert locust that occurs in Africa, Arabian Peninsula, and Southeast Asia, is the most devastating. About 57 countries covering nearly 20 per cent of the Earth’s land area is affected by the desert locust, creating famine and social disruption in some of the poorest nations.

Life history

The life cycle of locust pass through three stages − the egg, five wingless immature stages, and the winged adult. Batches of about 100 eggs are deposited in special “egg pods” of 3−4 cm length in soil. The female inserts her abdomen into moist soil and digs a hole of about 10 cm depth, deposits the egg pod at the bottom, and secures it with foam that later hardens into a shell, and plugs the hole. Scores of females could be laying eggs in a locality, forming massive nurseries. Depending on environmental conditions, young ones emerge from soil after two or more weeks. Soon they start feeding on vegetation and after a few days, the tiny young ones shed their skin and form a larger hopper. Thus, the young ones change their outer skin five times before reaching the winged adult stage. Hatching from egg to adulthood may take two to six months depending on climate and availability of food. Thus they complete two to five generations a year.

Crop damage

Desert locust feeds on all kinds of plants, except neem. However, there are also contradictory reports of them feeding on neem. They feed on entire portions of the plant above ground, such as leaves, shoots, flowers, fruits, seeds, and bark. All crops, including cereals, millets, pulses, fodder crops, cotton, fruit trees, vegetables, banana, and weeds are severely affected. Swarms can wipe off entire fields within a few hours. Historically, locust attacks had enhanced the severity of famines in Africa.

Locust outbreaks

Locust outbreaks are generally triggered by rains in desert areas after a dry spell. With rains begin a new phase of life in the otherwise arid deserts. Plants, animals, and all other life forms, which were dormant as seeds, spores, eggs, pupae or other survival structures under the harsh environment, suddenly begin the active phase of growth, with the intend to multiply and disperse. Rain is a double-edged sword that is inevitable for crop production in the arid regions. The same rain also sends out of locust swarms which destroy crops not only in areas of its origin, but also across countries and continents.

Climate change has already reared its head through range extension and increased incidence of crop pests such as bark beetles and sucking insects. The current locust outbreaks are directly linked to global warming. Rain clouds that formed over an unusually warm Indian Ocean were carried by cyclonic storms that hit the Arabian Peninsula in 2018, bringing heavy rains resulting in the formation of lakes in the vast stretch of uninhabited deserts, which are breeding grounds of the desert locust. The swarms, under the influence of wind, entered east Africa, where they were again helped by rains. They then spread to Iran and Pakistan. Since June 2019 Pakistan has been experiencing locust attacks. On January 31, 2020, fearing food shortage, Pakistan declared national emergency and appealed for international help to battle the desert locust. With a warming climate, locust outbreaks are likely to be more frequent and severe.

Invasion 2020

India had the last full-blown locust plague in 1962, followed by a few surges in 1978 and 1993 and intermittent localised breeding. Locusts from Iran and Pakistan entered India, and the border States of Gujarat, Rajasthan, Punjab as well as Uttar Pradesh have been experiencing infestations since last winter. Invading swarms advancing across India have reached Maharashtra. As of May 26, at least one swarm had reached to the northeast of Bhopal. There are also reports of the presence of solitary individuals as far south as Kerala. The desert locust is battering the rural economy of several States in the country and the threat of food shortage looms over the horizon.

Normally, appearance of desert locust begins in July−October. However, this year it began too early. Excess pre-monsoon rains over north India during March−May has lead to enhanced breeding as there are active swarms of immature locusts in Rajasthan and Madhya Pradesh. It is predicted that a much larger, second wave of swarms may invade India by July as they are actively breeding in east Africa, Arabian Peninsula and Pakistan. The Food and Agriculture Organisation (FAO) has also indicated the possibility of locust swarms entering India directly from the Horn of Africa in July by crossing the sea with the help of winds.

Locust control

Locust invasions are difficult to control as they breed in remote areas and migrate over long distances. Monitoring, early warning and preventive control in breeding sites through international cooperation is the strategy adopted to control desert locust. Locust affected countries in Africa and Asia, coordinated by the FAO, cooperate in the war against locusts. The Desert Locust Information Service (DLIS) of the FAO monitors the weather and the locust situation on a daily basis and issue advisories and warnings to countries. The DLIS processes the rainfall data in the desert breeding grounds and forecasts the timing, scale, and location of breeding and migration. In India, the Locust Warning Organisation, functioning under the Department Of Agriculture, Cooperation, and Farmers Welfare is tasked with monitoring and tracking of swarms.

The primary method of locust control is application of chemical pesticides. Biological control using locust-specific fungal pathogens is useful against wingless hoppers in their breeding areas. Adults and nymphs are also killed by mechanical means. Digging trenches and burying marching hopper bands under soil is adopted by farmers. However, biological and mechanical methods are useless against invading swarms.

India needs to be vigilant about the possibility of recurring waves of locusts in the coming months, as predicted by the FAO. Monitoring locust breeding in India and seeking international cooperation to monitor and control swarms originating in other countries remain key to salvage our crops.