La résistance aux insecticides de Helicoverpa armigera (Hübner) en Afrique de l'Ouest : du mécanisme à la gestion

More than two million farmers are growing an average of 1 ha of cotton in West Africa. They use an IPM strategy to control a large number of insect species. In the mid-nineties resistance to insecticides in the major cotton pest, the bollworm Hdianerpa amigra resulted in field control failures. The first part focuses on the pyrethroids resistance diagnosis, its monitoring and the study of the mechanism. Field populations of H. amigra were all pyrethroid resistant whatever the area, the host plant and the collecting date. Results showed that resistance was due to an increase in pyrethroid degradation probably involving oxidases from the P450 family in the second part, synergistic mixtures and negative cross-resistant molecules are studies. Some organophosphates increased the pyrethroid toxicity in resistant larvae competing probably for the same oxidases involved in pyrethroid degradation. Moreover, the overproduction of these oxidases could make one of them, the triazophos, more efficient against the pyrethroid resistant insects. This hypothesis was supported by an overproduction of triazophos oxon in resistant larvae compared with susceptible ones. These results suggested means to overcome resistance. An insecticide resistance management strategy for cotton crop was discussed in the third part. This strategy recommend: 1/ no pyrethroid spray before mid-August to avoid resistance selection during the first generation, 2 / first spays with non cross-resistant molecules to control resistant insects, 3 / last spays with pyrethroid in combination with triazophos or other synergistic organophosphate to increase pyrethroids toxicity. Most of cotton farmers in West African countries used this strategy with success from 1999 to 2002.