Sustainability of Bt cotton in China and India.

摘要

Insecticide resistance to the cotton bollworm (Helicoverpa armigera) in Asia was the major reason for the first commercialization of Bt cotton to China in 1996 and then in India in 2002. Bt cotton now covers more than 66% of China's cotton area and approximately 10% of India's cotton area. The Chinese material is varietal with approximately 20 locally produced cultivars expressing Cry1Ac and/or Cry1Ab, many with CpTi stacked as a second gene. Monsanto has commercialized 2 US Acala cultivars expressing Cry1Ac. So far, the registered Indian material is all in local hybrids, with Monsanto Cry1Ac cultivars from USA providing the transgene. It is clear that toxin expression in the tissue varies very significantly across the season and with plant part and cultivar. Late in the season in all tissues and in several tissues throughout the season, toxin expression is inadequate for the control of H. armigera in both countries. Insecticide use reductions have been 54-80% for bollworms or 40% for all pests. However, despite GM seed costing approximately 2-3 times more than conventional seed, Bt cotton has proved profitable and its use is expanding rapidly. In China, yield increases have been small but reductions in input costs have led to profitability increases of $360-800/ha on average in 2002-04, although farmers still spray an average of 12 times on Bt cotton in Eastern China. In India, total spray costs declined by 70-85%. Given the increased seed price, overall production costs were reduced only slightly (5-15%) but cotton yields have risen by 40-65% and therefore farmer margins have risen strongly (averaging $130/ha in 2002 and $477/ha in 2003). These benefits are highly dependent on the bollworm pressure in any given district in any particular year with district mean increases varying from 14% to 92% in Indian studies to date. Laboratory and semi-field studies in China have characterized the impact of Cry1Ac on nontarget pests and a range of natural enemies. Direct impacts on predator and parasite development are slight (though significant in some cases). Changes in natural enemy numbers (particularly of specialist species) are affected by the changes in prey availability in Bt fields. As yet there is no significant field resistance to Cry1Ac. However, genes conferring a semi-recessive resistance through truncation of the cadherin target site for the toxin are present in approximately 1 in a thousand H. armigera in China and at least one semi-dominant resistance gene is present in India at similar levels. Modelling suggests that for small farmer systems, imposing an additional mortality on H. armigera surviving in Bt cotton fields is the most efficient way of delaying the development of severe resistance..