Monitoring insecticide resistance in the bollworm Helicoverpa armigera (Hübner) from 1998 to 2002 in Cote d'Ivoire, West Africa

Helicoverpa armigera (Hübner) is the major insect pest of the cotton crop in West Africa. Populations recently developed resistance to pyrethroids via the overproduction of oxidases. To control this pest, a resistance management strategy was applied in the major West African cotton growing countries from 1999 onwards. In Côte d'Ivoire insecticide resistance of H. armigera was monitored in field strains from 7998 to 2002 using insecticide coated vial tests and topical applications of insecticides with third-instar larvae. Vial tests with discriminating doses of cypermethrin were used directly on field-collected larvae at the end of the cotton season. The percentage of resistant larvae varied around a mean of 67% with 6 [mu]g cypermethrin/vial and around 13% with 30 [mu]g/ vial in different years and places. Topical applications with various insecticides were used in the laboratory on the first generation of populations collected from cotton (Gossypium hirsutum), tomato (Lycopersicum esculentum) or a strongly infested ornamental flower (Antirrhinum majus). The resistance factors calculated from dose-mortality regressions varied from 5 to 38 with deltomethrin. In the Bouaké area they were always higher for strains collected from cotton at the end of the season. Concerning the pyrethroid alternatives currently used in Côte d'Ivoire, no reduction in susceptibility in the cotton field strains was detected for endosulfan or profenofos showing their potential for use in a resistance management strategy. These results suggest a relative stability of the pyrethroid resistance in H. armigera from 1998 to 2002 and confirm the success of the resistance management strategy.