PROCEEDINGS OF THE WORLD COTTON RESEARCH CONFERENCE-3 Cotton production for the new millennium Chief editor

Cotton farmers relied for many years on chemicals to control large number of a variety of pests. Environmental and cost considerations have today drastically reduced new chemistry on the agrochemical market. Pesticide resistance, and especially pyrethroid resistance, appears like a major issue for the cotton crop sustainability. An innovation such as the genetic transformation of cotton for insect resistance (Bt-Cotton) is raising considerable hope among farmers. In cropping systems in which insects are resistant to insecticides, the expression of a toxin (Cry1Ac) introduced into the cotton plant by genetic engineering, enables satisfactory control of bollworm populations. It is therefore natural to consider that the introduction of such cultivars may solve part of the pesticide resistance threat in many cottongrowing countries. However, there are two main reasons for tempering this point of view. The first is the fact that the Bt-toxin controls only a part of the pest spectrum (bollworms), and other insect pests, especially sucking ones, have to be handled still using pesticides. The second is related to the possible decrease in toxin efficacy resulting from the outbreak of a resistance mechanism inside bollworm populations. In cropping systems used by small-scale farmers, in which both the fields and decision-making are scattered, it is not easier to manage the prevention of resistance to Bt-toxins than to synthetic insecticides. Furthermore, the postulates allowing the dissemination of the innovation in large and intensive farming systems (toxin high doses + refuge) are not applicable to the situations observed in other continents. On one hand, results obtained in different laboratories show that the American bollworm (Helicoverpa armigera) is less susceptible to the toxin Cry1Ac than Heliothis virescens, thus tempering the notion of strong expression of the toxin in the plant. On the other hand, the expression of the toxin is subordinate to the growing conditions of the cotton plant. Bt-cotton is not a silver bullet for handling pest pressure. The management of Bt-cotton must be accompanied like traditional varieties by a rational management of pest populations as a whole to limit the damage of the pest complex and to ensure the sustainability of cropping systems using cultivars modified for resistance to bollworms. Introduction Cotton growing makes a substantial contribution to the incomes of small farms and to national economies in Asia, in Latin America and in West Africa. Through its stimulating effect on the cropping system, especially on food crops, it contributes to food self-sufficiency and the fight against poverty. The use of chemicals is a much debated issue in cotton: it is essential for limiting pest damage and improving economic balance, but criticized for its impact on farmer health and environment. Cultural practices, resistant traits and beneficial fauna activity make it possible to reduce the damage of some components among the pest complex, but it is not enough to maintain the economic sustainability of the cotton crop. On the opposite, the massive use of pyrethroids to control bollworms led to insecticide resistance all over the world. After Australia, Turkey and Thailand, the problem reached the United States in 1985-86. Ten years after, pyrethroid resistance was reported from China, India and Pakistan and then more recently in West Africa (Martin et al., 2000). A failure in the control of pest populations as the result of insecticide resistance has serious economic repercussions for farmers and for the cotton sector. Health problems and environmental damage result from the increase in the amount of pesticides used to face resistance. In the past, the problem of resistance was overcome by the introduction of new chemistry, whose target differed from that of the previous active ingredients. However, given the speed with which resistance appears, the registration of new, effective materials, respecting health and environment, seems doomed to failure in small cotton farming where the cost of inputs is a major limiting factor.