Warm temperatures for prolonged periods may have led to massive pest attack in Assam

28,000 hectares of paddy crop in 15 districts destroyed by armyworm

By Shagun
Published: Wednesday 22 November 2023
The larvae of a Mythimna separata (L) and a leaf damaged by the pest. Photo: Indian Council of Agricultural Research-Indian Institute of Millets Research

Continuous warm temperatures for prolonged periods may have led to the serious pest infestation in Assam that damaged around 28,000 hectares of paddy crop in at least 15 districts. The crop was nearing maturity and about to be harvested when the pests attacked it. 

The pest (Mythimna separata) is known as the ear head cutting caterpillar, rice ear-cutting caterpillar, or armyworm. It feeds on leaves and can cut off panicles from the base of a crop plant, frequently leaving the field looking like it has been grazed by cattle. During an outbreak, the pest multiplies in large numbers and moves in swarms from field to field, like an army, to feed and attack the crops.

Experts told Down To Earth that while the presence of the pest has been reported from the state for several years, this is the first time that the attack has happened on such a large scale. This, they said, was partly due to continuous warm temperatures for prolonged periods. 

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“An increase in temperatures with dryness creates a favourable condition for the pest population to increase,” said Mridul Deka from the plant protection department of Assam Agricultural University. 

On November 22, both the maximum and minimum temperatures were above normal in at least seven districts in the state, for which data was available at the Regional Meteorological Centre. In Guwahati, the maximum temperature was 31.4 degrees Celsius, 4.5 degrees above normal for this part of the year. 

In a warming world, changes in both temperature and rainfall are the two big drivers of shifts in how and where pests and diseases spread.

A 2017 study published in the book Climate Change and Sustainable Agriculture said that every small rise in global temperatures will reduce the lifecycle of insects, resulting in a higher population of pests, increasing generations, an extension of geographical range and development season, high risks of invasion by migrant pests and overwintering.

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In India, which is home to 6.83 per cent of the world’s insect species, a degree Celsius rise in temperature would enable them to expand in presence about 200 kilometre northwards and 40 metres upward in terms of altitude, it stated. 

Damage to crops by the ear head cutting caterpillar was reported in 2016 too. However, the damage was restricted to a few areas, unlike this year, when farmers from half of the state have faced a total wipeout of their crops. 

“The biggest problem was that this time the attack happened at the last stage of the crop when there was absolutely no chance of crop recovery,” said Deka. 

Assam Chief Minister Himanta Biswa Sarma on November 19 said the government was constantly monitoring the infestation and has instructed officers to ensure that affected farmers get benefits under the Pradhan Mantri Fasal Bima Yojana, the national crop insurance policy. 

The pest was first reported as a sporadic pest from Tamil Nadu during 1937 and Kerala and Odisha in 1957.

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