Cotton tangle

bt cotton varieties grown in India are the least effective against bollworm, the most widespread cotton pest, as compared to those used elsewhere in the world, claims a study by the Central Institute for Cotton Research (cicr) in Nagpur. The finding appears to vindicate the stand of farmers, particularly from Andhra Pradesh, who have been complaining that the transgenic varieties failed to significantly boost the yield or reduce pesticide use.

Bt cotton varieties carry a foreign gene that expresses the protein Cry1Ac to kill bollworm. While the Indian Bt varieties studied expressed only 75 per cent of Cry1Ac, the varieties used in other countries show 100 per cent expression of the protein.

"This vulnerability owes to the hemizygous condition of the Indian Bt cotton, which is not found elsewhere in the world. In such seeds, the Cry1Ac gene is present on only one of a pair of chromosomes (selected for gene insertion) and so the expression of the gene is reduced. This reduced expression limits the immunity of the cotton plant against bollworms," says Keshav Raj Karanthi, a senior entomologist who led the cicr research. The study was published in Current Science (Vol 89, No 2, July 25). It covered the varieties brought out by Jalna-based Mahyco (mech- 12, -162 and -184) and Rasi Seeds in Tamil Nadu (rch-2, -20, -134, -138 and -144).

The eight varieties are hybrids developed after crossing and re-crossing traditional varieties with the homozygous (those with complete immunity to bollworm) Bt varieties, till an estimated 75 per cent expression of Cry1Ac was achieved, says Ashok Gaur, a senior scientist with the Indian Agricultural Research Institute (iari) in Delhi.

Indian companies market hybrids so that the farmers will have to buy seeds every year, while homozygous varieties could be used for three years, according to the study. However, a spokesperson of Rasi Seeds says Indian Bt cotton varieties have been developed as hybrids for higher yields. Besides, the hybrids have been bred in such a way as to induce 95-100 per cent expression of Cry1Ac, he claimed.

When Down To Earth contacted Mahyco headquarters, senior officials declined to comment on the study results. One of them said they would like to fund similar studies to "objectively evaluate" the Bt varieties.

Survival threshold The study found that a minimum 1.9 microgrammes of Cry1Ac per gramme (g/g) of plant tissue was required to kill bollworm (Helicoverpa armigera). But Cry1Ac content in the commercially important flowers was below this threshold level. It ranged from 0.25 to 0.80 g/g depending on the hybrid variety, while the ovaries contained even lesser amount -- 0.07-0.27 g/g. The figure for the boll rind varied from 0.01-0.37 g/g. However, 90 per cent of the bollworm-eggs are laid on the leaves where Cry1Ac levels are high enough to kill the larvae.

The Bt cotton seeds keep bollworm away in the initial stages of growth. But after about 100 days, the pest starts appearing on the plant. The study attributes this to the declining levels of Cry1Ac, which falls below the critical level of 1.9 g/g 110 days after sowing (das). The mean Cry1Ac expression for all varieties 30 das was found to be 5.51 g/g, while 100 das it was 0.30 g/g and reduced further to 0.05 g/g 148 das.

As a result, almost all varieties become vulnerable to the pest after 100-110 das. The decline in expression of Cry1Ac genes is rapid in varieties such as mech-162, rch-2 and rch-20, which are medium to late maturing varieties (140-200 days).

It is interesting to note that the tolerance of H armigera to Cry1Ac is 10 times that of the tobacco budworm (Heliothis virescens), the main pest of cotton in the us.

Initially, Indian cotton farmers were dismayed to find hardly any reduction in pesticide application even with Bt varieties. Even an industry-sponsored study in 2004 (by B R Barwale, V R Gadwal, U Zehr and B Zehr) showed pesticide use by farmers reduced only by 2.5 insecticidal applications, while in China it reduced by 14 applications, 7 in South Africa and 5-6 in Indonesia and Australia. In India, farmers in most cotton-growing areas go for 14-20 pesticide applications for non-Bt cotton crops.

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