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India should remain on high alert against locust attack for the next four weeks, the Food and Agriculture Organization (FAO) has warned amid the country facing the worst locust attack in 26 years.
In its latest update on Friday, the FAO said spring-bred locust swarms, which migrated to the Indo-Pakistan border and travelled east to northern states, are expected to return back to Rajasthan with the start of the monsoon in coming days. The FAO has three categories of Desert Locust situations: outbreak, upsurge, and plague. The current locust attack (2019-2020) has been categorised as an upsurge. Outbreaks are common, but only a few result in upsurges. Similarly, few upsurges lead to plagues. The last major plague was in 1987-89 and the last major upsurge was in 2003-05. Upsurges and plagues do not occur overnight; instead, they take many months to develop.
Before the outbreak stage, the FAO first issues ‘Desert Locust threats’ that are determined from an analysis of national survey and control data combined with remote sensing imagery and historical records. Such threats have been issued in 2012, 2013, and 2015. Not all threats develop into an outbreak.
When there are good rains and green vegetation develops, Desert Locusts – which are always present somewhere in the deserts between Mauritania and India – can rapidly increase in number and within a month or two, start to concentrate, gregarise which, unless checked, can lead to the formation of small groups or bands of wingless hoppers and small groups or swarms winged adults.
Such a situation is called an ‘outbreak’, and usually occurs with an area of about 5,000 sq. km (100 km by 50 km) in one part of a country. The FAO website lists nine outbreaks: 2018, 2016, 2014, 2013, 2012, 2009, 2008, 2007, 2006.
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Locust upsurge
This is a more serious Desert Locust situation and generally affects an entire region. An ‘upsurge’ is formed when an outbreak or contemporaneous outbreaks are not controlled and if widespread or unusually heavy rains fall in adjacent areas, several successive seasons of breeding can occur that causes further hopper band and adult swarm formation. There FAO website records the upsurges of 2004-2005, 1996-1998, 1994-1996, 1992-1994, and 1972-1974. The upsurge of 1992-1994 affected India after Desert Locusts that bred for several generations along the Red Sea coastal plains in the winter of 1992 moved via the Arabian interior to India and Pakistan.
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Locust plague
The most serious category, a ‘plague’ can develop when an upsurge is not controlled and ecological conditions remain favourable for breeding, locust populations continue to increase in number and size, and the majority of the infestations occur as bands and swarms.
This does not happen overnight; instead, it takes at least one year or more for a plague to develop through a sequence that commences with one or more outbreaks and followed by an upsurge. A major plague exists when two or more regions are affected simultaneously. The area in which plagues occur covers about 29 million sq. km and can extend across 58 countries.
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There have been six major plagues in the 1900s, one of which lasted almost 13 years, the FAO website notes.
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