The pests are being carried into the region by the southern branch of winds entering India north of the Western Ghats
Winds are the major carrier of desert locusts that are currently devastating crops and green areas in India’s northwest and central regions, experts have said.
“The winds that enter India to the north of the Western Ghats split into north-westerlies continuing towards central and northeastern India in one branch and towards the southeast in another branch,” Raghu Murtugudde, a climate scientist at the University of Maryland in the United States, told Down To Earth.
“This split is caused by a relative higher pressure ridge extending from Andhra and Tamil Nadu to Gujarat. So, the winds are going to the north and south of this pressure ridge. The locusts must have taken the southern branch of this split,” he added.
These winds are taking the locusts more towards Madhya Pradesh, that lies south east of Rajasthan, rather than Uttar Pradesh or Bihar, that are further east.
The surface pressure contours or isobars tend to align from southeast to northwest so winds follow those contours once they enter India north of the Western Ghats.
That is due to the large-scale contrast of land-ocean heating that is getting ready for the monsoon, according to Murtugudde.
But it is very difficult to pin down exact localised wind patterns over India because of a lack of data.
For instance, out of the 16 districts affected in Madhya Pradesh, the India Meteorological Department’s Automatic Weather Stations in 12 districts are displaying no data available for wind, temperature or any other weather parameters.
Apart from general wind patterns, these localised winds are crucial in understanding where the locust swarms will move next.
There are two other reasons apart from wind for their spread to areas that have not been affected by locusts in a long time. The first is their early arrival that was facilitated by favouarble conditions in Iran, Pakistan and the western border areas of India.
The first swarms were seen along the India-Pakistan border in Rajasthan as early as April 11, 2020. The second reason is the excessive rainfall in the affected regions that helped them sustain and multiply.
“They have never before been seen in such large numbers. This season, everything was favourable to them. Hence, all the eggs that were laid in the soil, hatched together,” Dhaneesh Bhaskar, a researcher at the Kerala Forest Research Institute, said.
He is also a representative of India at the International Union for Conservation of Nature’s Grasshopper Specialist Group and coordinates the Red List assessment of India’s grasshoppers.
“Quick summer showers favour their phase transformation, which means they can get together to form huge swarms. As the numbers increase, they are bound to explore new areas,” he added.
The locusts entered three districts of Maharashtra on May 26, after affecting Rajasthan, parts of Uttar Pradesh and Madhya Pradesh, according to recent media reports.
Maharashtra is witnessing locust swarms for the first time in 46 years. The pests have been reported in as many as 16 districts of Madhya Pradesh.
An alert was generated for Delhi as well but direction of winds in and around the city is currently not favourable for the locusts to arrive there. Even then, Delhi stays on alert as do other cities in Uttar Pradesh like Agra and Mathura.
Authorities are taking the strictest measures to control the spread of the pests in Rajasthan, especially close to Jaipur, that saw an invasion of locusts in the past few days that might have come from the west.
A swarm was located in Aluda, south east of Jaipur, according to officials on the ground. This can still pose a threat to Agra and Mathura in the next few days. The locusts can travel around 150-200 kilometres during the day, resting at night in trees and bushes.
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