Locust swarm invasion: The wind factor

Staying alert is one way to gain the upper hand in a battle. But understanding the changing strategy of the enemy is equally crucial, particularly if it is a trans-boundary pest with an ability to travel 150 kilometres a day riding the wind current.

Worse, desert locusts appear to be expanding their territory in India both in terms of time and space — they are now coming early, staying longer and foraying deep into the country.

The Food and Agriculture Organization (FAO) said much of the country’s current crisis was caused by the supercyclone Amphan that made landfall on the Sundarbans on May 20. Strong northwesterly winds (that enter from northwest and move towards southeast and east) were established in its aftermath, taking locusts into places as far as Chhattisgarh in the east and Maharashtra in south.

An analysis of the wind data in six north Indian cities by the International Water Management Institute, headquartered at Colombo, also shows there has been a sharp increase in the wind speed at 10 metres above the ground from mid-May onward which has helped the locust move from Rajasthan to faraway places.

As a result, Uttar Pradesh and Chhattisgarh reported sightings of locust swarms for the first time since 1962, Maharshtra since 1974 and Madhya Pradesh and Punjab since 1993. The FAO predicts locusts could soon reach Odisha and Bihar. They too have not experienced locust attacks in recent decades.

However, wind is not the only factor responsible for this unusual spread. According to the FAO, even before Amphan hit the country, dry conditions prevailing in the west forced immature adult swarms to move eastward, who reached Ajmer by mid-May and Indore in Madhya Pradesh on May 21.

“This is because locusts have a strong liking for tender leaves and possess a strong sense of smell for fresh vegetation,” said Biswajeet Paul, principal scientist at the Indian Agricultural Research Institute (IARI), New Delhi, who works on biologiocal control of insects.

Since vegetation in their usual territory in northwestern states is not lush green, swarms are moving towards states like Uttar Pradesh and Maharashtra where cyclones and heavy unseasonal rainfalls, induced by western disturbance between March and May, have initiated vegetation growth. Paul says locust swarms are just taking the help of wind currents to move in the direction of food so they use less energy.

For an insect as big as a paper clip, that travels across continents for survival, energy is a big asset which it must save for breeding. And there are only a few weeks left for it. It is estimated that by the end of June, most swarms in the country would attain maturity.

They would turn yellow and settle down for breeding. That would also be the time when monsoon rains sweep across northern and central India and the kharif crop cycle begins, making ample food available for them.

While locusts only nibble away the leaves of mature trees, they can gobble up entire saplings in a single morning, leaving no trace of vegetation. If the infestation is not controlled now, their next generation will threaten the country’s food security that largely depends on kharif crops such as rice, maize and sorghum. Locust Warning Organization (LWO) officials say as on June 8, 2020 over 1,500 hectares (ha) in Rajasthan's Nagaur and Bikaner districts were infested with yellow locusts.

Paul said there is a possibility that the next generation that will hatch outside the traditional territory will be fewer in number. Adult locusts require sandy soil, where they can make a hole as deep as 10 centimetres to push in their abdomen and lay eggs.

This is not possible in ordinary soil. So the swarms will lay pods with fewer eggs than the usual 200 to 250. But even these small groups can cause mayhem at the local level. After all, they thrive in areas that are warm, roughly 25°C to 40°C and have ample rainfall and green vegetation.

When conditions are less favourable, locusts take up to six months to mature. But given the right conditions, they can breed every three months and increase 20 times in a single generation and about 400 times in six months after two generations of breeding. This will have a disastrous impact at a time when rural areas face reverse migration due to the novel coronavirus disease (COVID-19).

How far from a plague?

As of now, desert locusts are causing outbreaks in at least 10 other countries in the Horn of Africa and southwest Asia. Though the scale and intesity of the infestation is said to be the worst in decades, the FAO describes this an “upsurge”, meaning locusts have been able to breed uncontrolled for several successive seasons.

In an e-mail interview, Keith Cressman, locust forecasting expert at FAO, told Down To Earth:

“The occurrence of a locust plague depends on weather, rains, control and locust breeding before the end of the year.”

However, such a declaration does not seem too far. For the past three years, locusts have been breeding early and multiplying profusely and spreading in huge numbers due to a series of unusual and extreme climate conditions.

For instance, locusts usually return from Gujarat and Rajasthan to Pakistan and Iran between October and November. But last year, Rajasthan experienced an extended monsoon, which prompted the swarms to stay back.

By the time they returned in February this year, they had given birth to third-generation insects. During the current infestation, their early crossover to India is also linked to unseasonal rainfall in the deserts of Pakistan, adjoining India that act as summer breeding ground for locusts.

“We are seeing an increase in swarms from Pakistan this time as the locusts’ spring breeding has happened right across the border. Usually spring breeding is restricted to Iran and Baluchistan,” said KL Gurjar, deputy director at LWO.

Till the the end of May, at least 25 swarms had crossed over to India. Such unseasonal rain events are only going to increase in a warming world.

In the Arabian Peninsula and East Africa, locusts are multiplying profusely due to changes in a climate system, the Indian Ocean Dipole (IOD) — a natural pattern of changing temperature gradients between eastern and western portions of the Indian Ocean.

Historically, this temperature difference has stayed within safe limits. But in recent years, the western side of the Indian Ocean, or the Arabian Sea, has been unusually warm as compared to the eastern side. This change, dubbed positive diapole, causes a lot of evaporation from the area and then returns as additional rain or cyclones to the region.

In 2018, IOD remained positive for most parts of the year, leading to the formation of cyclone Mekunu in May and cyclone Luban in October. They first caused severe floods in the Arabian deserts and then the growth of lush vegetation, causing locusts to congregate and breed far more rapidly than they would when food is scarce.

It is said that the rain also triggered dormant locust eggs to hatch. Just nine months and three generations later, locusts had increased by 8,000 times and were ready to expand their territory.

In the summer of 2019, they jumped the Gulf of Aden and moved to Ethiopia and Somalia. That period was marked by an even stronger positive IOD, resulting in the highest eight cyclonic events in a year.

The swarms enjoyed the unusually wet weather, growing even larger. Soon they swept through adjoining countries. In Kenya, where agriculture dominates the country’s economy, they have caused the largest outbreak in 70 years; so far, it has lost 30 per cent of its pastureland. A study published in Nature on April 12, 2018, said extreme positive IOD events could double in frequency in case of 1.5°C warming above the pre-industrial levels.

In March, as monsoon winds hit the region, east Africa has again received above-average rainfall. FAO said a second-generation breeding is underway there. Numerous hopper bands have formed.

These young juveniles, 400 times more in number, will become voracious adults between the second week of June and mid July just as farmers begin to harvest. “The locusts, combined with the impacts of COVID-19, could have catastrophic consequences on livelihoods and food security,” said Qu Dongyu, director-general of FAO at a virtual meeting on May 22.

India, which is still struggling to flatten the COVID-19 infection curve despite imposing the world’s most stringent and longest nationwide lockdown, stares at an uncertain future. FAO warns that more swarms are forming in the spring breeding areas of Iran and Pakistan and migrating towards India ahead of the monsoon rains.

“Several successive waves of invasions can be expected until July in Rajasthan with eastward surges across northern India as far as Bihar and Orissa, followed by westward movements and a return to Rajasthan on the changing winds associated with the monsoon,” it said.

To compound the situation, in July, for the first time the locusts might also reach the western regions of Gujarat and Rajasthan directly from the Horn of Africa, traversing the huge expanse of the Arabian Sea. Most of them will come from northeast Somalia riding southwesterly monsoon wind. Is India prepared to handle this sudden upsurge?

This was first published in Down To Earth’s print edition (dated 16-30 June, 2020)

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