October 22, 2015 1:01:24 am
The large-scale crop damage from whitefly infestation this year, especially in northern India, has raised questions on the possible susceptibility of Bt cotton to the dreaded sucking insect pest.
But according to Keshav Raj Kranthi, director of the Central Institute for Cotton Research (CICR) here, the whitefly problem has little to do with Bt technology per se. He blames it, instead, on the multiplicity of genetically modified cotton hybrids, incorporating the Bt genes, that are being grown in the country.
“There are over 250 Bt cotton hybrids on the shelf in North India. More than 90 per cent of these are susceptible to, and have even become hotspots for, whitefly and leaf-curl virus. CICR has been providing a list of such hybrids tolerant to both pests”, the eminent cotton scientist told The Indian Express.
Kranthi noted that prior to 2002 — when commercial cultivation of Bt cotton began – the country’s entire area was practically under public sector-developed varieties, which were screened by CICR and released only if found tolerant to whitefly and leaf-curl virus. This is not the case with privately-bred hybrids now, which are not released by the Ministry of Agriculture and are largely commercialised without subjecting to any rigorous screening for tolerance to whitefly or leaf curl virus.
The high incidence of whitefly menace this time, he felt, was mainly due to delayed sowing, particularly in Punjab. More than 50 per cent of the cotton was sown 15-30 days beyond the normal cut-off date of May 15, either due to late release of canal water or delayed harvesting of wheat.
“The late-sown crop is tender in July-August, when weather conditions —hot, humid and cloudy —are most congenial for the whitefly and leaf curl virus transmission. The early-sown crop is less susceptible, as the mature leaves are not preferred by whiteflies,” Kranthi explained. Also, since rains were scarce and intermittent, farmers resorted to excessive irrigation and urea application that promoted fresh vegetative growth and allowed whiteflies to proliferate.
The problem was worsened by over-spraying of pesticides; CICR’s surveys showed that the damage from
whitefly was most severe in places where this happened. Whiteflies have a white waxy coating on their body at nymphal stage and on wings at the adult insect stage. The waxy coating protects whiteflies from most insecticides. On the other hand, many broad-spectrum insecticides like synthetic pyrethroids and mixtures kill a wide range of insects in the fields. That includes even beneficial insects known to control whiteflies naturally.
“The absence of beneficial natural enemies leads to whitefly resurgence and out breaks. Scientific studies show that 50-60 per cent of populations are controlled by natural enemies and 20-30 per cent displaced by rains and winds,” Kranthi said.
The current year, he added, saw the concurrent occurrence of all factors —late sowing, persistent drought coupled with hot and cloudy conditions, excessive irrigation and application of urea and insecticides, and growth of other crops also harbouring whiteflies —adding to a deadly cocktail destroying the cotton crop.
The whitefly menace was, however, not witnessed in Maharashtra for the simple reason that the above factors did not exist there. Although Telangana did report some incidence, it was much less severe than Punjab. Nor does the whitefly spread from North to Central or South India, even while it can spread to other crops grown in the rabi season in northern India.
Kranthi also ruled out the whitefly turning into the new heliothis or American bollworm that was the Achilles heel of cotton cultivation during the eighties and nineties in India. “American bollworm was a one-of-its-kind problem. Before the advent of Bt cotton, farmers and scientists were virtually helpless against it, with no management strategy working on the ground. This isn’t the case with whitefly. We have effective management strategies to check it —timely release of canal water and harvesting of wheat, avoiding cotton sowing in citrus fields, no excessive use of urea, etc”, he pointed out.
What is whitefly?
It is a small (1-2 mm) white-coloured insect affecting cotton, and also occurring on vegetables and other crops in tropical and sub-tropical regions. The whitefly sucks sap from the phloem or living tissue carrying organic nutrients, causes yellowing and upward curling of the leaves. The insect also deposits sticky honey dew excretion, which promotes sooty mould fungi that interfere with photosynthesis. Sticky cotton makes ginning and milling difficult. In north India, whitefly is present throughout the year, due the wide range of crops grown, shifting from one crop to the other. But more damage is caused by the cotton leaf curl virus that is transmitted by the same insect. There are no control measures for this virus. The disease-affected plants are stunted, with fewer numbers of bolls and reduced yields. Besides, the infected plants serve as source of inoculums and infestation for the remaining healthy fields.
Are there effective control measures?
Whiteflies are physically delicate and can be controlled even with water sprays. The best approach is to select methods causing least disturbance to beneficial insects that can control the whitefly naturally. Therefore broad-spectrum insecticides such as synthetic pyrethroids and mixtures should be strictly avoided. It is better to rely initially on water sprays, followed by soap sprays, suction traps, yellow sticky traps and reflective sheets or sprays with preparations of neem oil, castor oil, fish oil and rosin soap. There are chemicals, also known as insect growth regulators, like buprofezin and pyriproxyfen. These control whiteflies, while seen to have less effect on beneficial insects.
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