From insecticide resistance to Bt cotton : ls insect pest management sustainable in small-scale farming systems ?

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. (Résumé d'auteur