As their name suggests, desert locusts normally live and breed in semi-arid/desert regions. For laying eggs, they require bare ground, which is rarely found in areas with dense vegetation. So, they are more likely to breed in Rajasthan than in the Indo-Gangetic plains or Godavari and Cauvery delta.
While green vegetation is good for hopper development – the stage between the nymph that has hatched and before its turning into a winged adult moth – such cover isn’t widespread enough in deserts to allow growth of large locust populations.
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Locusts aren’t dangerous as long as they are individual hoppers/moths or small isolated groups of insects, in what is called the “solitary phase”. It is when their population grows to large numbers – the resultant crowding induces behavioral changes and transformation from the “solitary” to “gregarious” phase – that they start forming swarms. A single swarm contains up to 40-80 million adults in one square km and these can travel up to 150 km in one day.
The above large-scale breeding and swarm formation, however, takes place only when conditions turn very favourable in their natural habitat, i.e. desert and semi-arid regions. These areas should get rains that will produce enough green vegetation to enable both egg laying and hopper development.
It appears that such conditions have been there since the start of this year. The main locust breeding areas in the Horn of Africa, Yemen, Oman, Southern Iran and Pakistan’s Baluchistan and Khyber Pakhtunkhwa provinces recorded widespread rains in March-April. East Africa, in fact, had its wettest rainfall season in over four decades even during October-November.
The hopper bands and immature adult groups resulting from this large-scale breeding – itself a product of unusually heavy rains – are the ones that started arriving in Rajasthan during the first fortnight of April. The Union Agriculture Ministry’s Locust Warning Organisation then observed “low-density I & II instar gregarious/transient hoppers” at Jaisalmer and Suratgarh in Rajasthan and Fazilka in Punjab adjoining the Indo-Pakistan border.
Subsequently, there has been arrival of swarms from the main spring-breeding areas. And these swarms have come not only to western Rajasthan, but also moved to the eastern parts of the state and even Madhya Pradesh and Maharashtra. Much of this movement, it seems, was aided by the strong westerly winds from Cyclone Amphan in the Bay of Bengal.
Thus, we have had two meteorological drivers behind the current locust invasions: one, unseasonal heavy rains in the main spring-breeding tracts in March-April, and, two, strong westerly winds.