Cotton Curse: How pink bollworm developed resistance to Bt Cotton

Entomologists say early and late sowing, long-duration crops and not planting other varieties against advice may have contributed to resistance building

By Himanshu Nitnaware
Published: Thursday 26 October 2023
A genetically modified variety called Bt Cotton created to resist it has also fallen prey to PBW and some areas have seen over 90% damage to yield. Photo: Vikas Choudhary / CSE__

This is the fifth story in a series about pink bollworm attacks on Bt Cotton in the North Zone, comprising Punjab, Haryana and Rajasthan.

Cotton crop devastation in the northern cotton belt has been unprecedented in the last two decades, thanks to the pest pink bollworm (PBW). A genetically modified variety called Bt Cotton has also fallen prey to it and some areas have seen over 90 per cent damage to yield. The pest has seemingly developed resistance to Bt Cotton and scientists suspect several reasons may have contributed to it. 

Bt Cotton (Bollgard II seed) was created to be resistant to the bacteria Bacillus thuringiensis and was introduced in India in 2002 to protect cotton against the vulnerability of American bollworm (Helicoverpa armigera), PBW (Pectinophora gossypiella) and spotted bollworm (Earias vittella).

Read more: Untangling India’s Bt cotton fraud

The variety has been successful in protecting cotton against the American bollworm — the primary reason for which it was introduced. The pink bollworm, however, has developed resistance against it since 2008, when the first evidence was found. There may be more than one reason for the PBW to have developed resistance against Bt Cotton, according to scientists. 

The issue began due to early sowing adopted by farmers in the northern zone of India and late cotton varieties grown by farmers in central and southern India, said Rishi Kumar, entomologist at the Central Institute for Cotton Research (CICR), Sirsa, Haryana. 

“The ideal time for sowing cotton is April 15 to May 15. But many farmers in the northern belt of Haryana, Rajasthan and Punjab start sowing from March end or the first week of April and extend it up to June end, which is an increase from 45 days to 80 days,” he explained.

This early sowing season coincides with the time the PBW comes out of hibernation or the diapause stage in the winter months, Kumar said. The pest survives in this stage between two cotton seeds or cotton crop residue. 

The cotton plants are at the bud or flowering stage, during which the PBW searches for food and begins feeding on bolls during the larval stage, which continues for 14-17 days. It eventually starts laying eggs.

However, the issue worsens for farmers who sow late cotton variety seeds. The process enables worms to access food for longer periods and increasing generations. Farmers become vulnerable as the process enables the propagation of additional generations of these worms as the food is available for an extended period.

“In recent years, the pest has managed to find its way to northern parts of India by surviving on cotton plant residue, seeds, and cotton balls during off-season,” Vinay Pathania, entomologist at Krishi Vigyan Kendra (KVK), Bathinda.

PBW infestation has been reported from Maharashtra, Telangana, Karnataka, Gujarat, Andhra Pradesh and other cotton-growing belts in central and southern India since 2015-2016, he said.

Read more: Why are pests developing resistance to Bt cotton in Telangana, Andhra Pradesh?

“Many oil seed industries and ginning mills in this region have imported cotton from these infested regions, which enabled the spread of disease in the northern cotton belt,” Pathania said.

Initially, the infestation was detected around cotton oil mills, as some of the infested cotton travelled to northern regions. It has spread widely in recent years.

Crop residue from cotton plants after harvest serves as firewood, he added. “Farmers usually store it until the late winter months and have pink bollworms residing inside them in the diapause stage. Once they come out of their stage during spring and early summer, the cotton crops are ready to offer them food,” he said.

The pest is also elusive and not visible, so farmers find it difficult to control it in its early stages by spraying pesticides and insecticides, the scientist added. 

PBW has undergone genetic changes and can resist the Bt toxin, said Jasreet Kaur, entomologist at Punjab Agricultural University, Bhatinda. The longer duration of cotton varieties in south and central India, which lasted up to 150-160 days, helped the pest develop resistance to the genetically modified variety. 

Speaking of resistance developed by the pest, Kaur said the cry toxin — the chemical that prevents PBW from attacking the cotton seeds — wears off eventually during the end days of the cotton plant. If the pests survive the small quantities of these toxins, they manage to develop resistance to the chemical and create progenies in the next generation that are more immune to it.

Farmers were repeatedly advised to plant indigenous, hybrid varieties of cotton alongside Bt to prevent developing resistance, said Pathania. “The crossbreeding of pests from different varieties of plants would have prevented developing tolerance for longer years. But farmers have not followed the advice,” he said. 

Read more: Study points to quality issues in Bt cotton seeds

Unlike American bollworm, which feeds on multiple plants, PBW is monophagous, which means it feeds only on cotton crops, said Kaur. “If the cycle is broken and timely pesticides are sprayed as directed in the government advisory, the disease can be controlled,” she said. 

Several farmers admitted to Down To Earth (DTE) that they did not follow the protocol for economic gains. 

“I repeatedly threw away non-Bt Cotton seeds because the crops would attract diseases such as white fly, American bollworm and PBW among others, which could eventually attack the Bt Cotton too,” Rakesh, a farmer from Dhankawali Dhani of Hanumangarh district, Rajasthan, told DTE

Read the first part here, the second part, the third part and the fourth part.

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