Representative image of a locust. Photo: Niv Singer/Flickr, CC BY-SA 2.0.
Desert locusts have affected about 60 countries, including India and Pakistan. They are swarming short-horned grasshoppers, each two inches long with short, thick antennas. Grasshoppers don’t live in groups and keep chewing plants as lonely insects. But in some circumstances, they coalesce into large groups and migrate together in search of food. This process of gregariousness also induces changes in appearance, turning them from green to a shade of yellow-brown and strengthens their muscles. The Russian entomologist Boris Uvarov first noted these changes and came up with the theory that linked two different phases of the same insect – the grasshopper – with that of locusts.
Desert locusts can fly over 130 km a day and stay in the air for a long time. According to the Food and Agriculture Organisation (FAO), locusts have been known to cross the Red Sea, a distance of 300 km. One particular swarm reportedly flew over 5,000 km, from West Africa to the Caribbean, in 10 days in 1988. One squared kilometre swarm of 40 million locusts can eat as much as 35,000 people can in a day. And as the plague of locusts consumes crops in the field as well as in silos, they leave great agricultural distress and famine in their wake.
Experience has shown that locust plagues follow a one- to two-year cycle, after which there is a lull for eight to nine years. India experienced serious locust outbreaks in 1812, 1821, 1843-1844, 1863-1867, 1869-1873, 1876-1881, 1889-1898, 1900-1907 and 1912-1920. The last big infestation was in 2010. There were 13 locust plagues between 1964 and 1997. From 1997 to 2010, five outbreaks were controlled. Since April 11 this years, locusts have swarmed several districts of Rajasthan via Pakistan’s Sindh province. From Rajasthan, they migrated into Madhya Pradesh, Gujarat, Haryana, Uttar Pradesh and Maharashtra to feed on green crops.
Experts have suggested that the cyclones in 2018 and 2019 that struck eastern parts of Africa and the Arabian Peninsula could have triggered the ongoing locust outbreak. These swarms were later also facilitated by prolonged rainfall in the desert, as a consequence of a climatic phenomenon called the Indian Ocean dipole. The FAO has already warned that India may face another locust attack in July this year.
We know today that locust outbreaks follow years of plentiful rain that bring greenery to deserts, and that they remain dormant in the hotter years. One study from China showed that monsoon rains have increased in the Indo-Pakistan deserts by 30% over the last two decades, elevating soil moisture and encouraging crop growth. It is widely acknowledged that deforestation is followed by a reduction in rainfall, implying that greening deserts result in increased rain.
To destroy locusts, the first and most important activity is to locate and destroy the locusts’ eggs in an organised manner by ploughing, harrowing and digging. Also digging 2′ x 2′ trenches around egg-laid areas will help entrap the nymphs, which are the flightless young of the locusts. As they move out after hatching, they are buried or killed with chemical sprays.
The FAO has recommended the use of a fungus called Metarhizium anisopliae, which kills locusts by growing inside their bodies. It is cheaper than other methods, more effective, longer-lasting in the deserts, easier to store and is recommended for use before the nymphs begin to fly. The main challenge is to find bands of young locusts and hitting them with enough biopesticides. The locusts need to be exposed for several days longer than with chemical pesticides to effectively be killed. Ecotoxicological studies have shown that the fungus poses low risk to other organisms, including insects. Biological control is the better option, but in an emergency – like now – established pesticides are the obvious choice.
When flying locusts are about to descend in large swarms in cultivated areas, the best way to tackle them is to prevent them alighting by all possible methods. To repel locust swarms, fire and a cloud of smoke are created by burning refuse in many places. As Rob Malkin of the University of Bristol has reported, the ears of locusts have a highly integrated and miniaturised hearing system that distinguishes low-frequency sound from other locusts and high-frequency sound from foes such as bats, for their survival. Acoustic devices that produce high-frequency sound have been successfully used in different parts of India to deter large swarms of desert locusts.
For generations, tribes in Africa, Australia and south-east Asia traditionally practised body-painting with clay, chalk, ash and cattle dung. Scientists from Hungary’s Eötvös Lorand University now believe those body paintings could have deterred blood-sucking horseflies and other pests that are dangerous, irritating and transmit deadly diseases. Similar protection of farms, orchards and standing crops were reported by Chinthala Venkat Reddy (a recipient of the Padma Shri), who has a patent for the mud-spray technology. While many parts of India face a plague of locusts, several have farmers observed that locusts avoided farms sprayed with mud or neem kernel suspension. Locusts breathe through their scaly skin, so they can’t digest the clay if the subsoil is mixed with water and sprayed on crops.
Lu Lizhi, a researcher at the Zhejiang Provincial Institute of Agricultural Technology, announced 20 years ago that ducks could be an effective way to control locust infestations. Each duck can eat as many as 200 locusts per day. In 2000, a 700,000-strong army of ducks and chickens were sent to Xinjiang to tame a swarm of locusts that had devoured over 3.8 million hectares of crops and grassland. At that time, researchers found the ducks were more efficient than chickens at gorging the pests. In 2020, China had sent a 100, 000 duck ‘troops’ to Pakistan, which shares a border with Xinjiang province, to check locust swarms.
Early locust attacks this season, due to a chain of climate events, administrative laxity in several countries and the difficult circumstances brought on by the COVID-19 pandemic, have caused huge economic losses to already debt-burdened farmers. Tackling large locust swarms is challenging and requires fast-acting pesticides sprayed from aircraft or drones. A drone can spray pesticide on nearly 2.5-acres during a flight of 15 minutes. Organophosphates, carbamate and pyrethroids are types of extremely toxic insecticides commonly used against locusts during an emergency. The harmful effects of pesticides on humans, livestock and the brought on by excessive use of chemical pesticides should not be overlooked.
A team of researchers based at three universities in the UK and Australia had discovered over a decade ago that the change from solitary to gregarious starts when serotonin levels increase. When the team injected locusts with drugs that blocked serotonin’s action or a compound that inhibited their serotonin production, they didn’t become gregarious. But when the solitary locusts were treated with serotonin or gave them a drug that boosted their production, they became gregarious. This showed that the chemical is both necessary and sufficient to commence the transformation, according to neuroscientist R. Meldrum Robertson of Queen’s University, Canada, who studies locust flight.
In future, controlling locusts through drugs targeting the serotonin pathway might provide an alternative to chemical pesticides. What we need is changing the orientation of modern sciences towards the global from the local.
K. Usha is the principal scientist at ICAR-IARI, New Delhi.