Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2together efforts to better understand the phenomenon,studying the resistance mechanism. It appears thatresistance was due to greater degradation ofpyrethroids in resistant insects involving oxidases fromthe cytochrome P450 family (Martin et al., 2002). Anetwork group, named PR-PRAO (Prevention andManagement of Pyrethroid Resistance in H. armigera)was implemented, involving CIRAD, IRAC, and all theactors of cotton protection in West Africa fromresearch to extension services and advisory services toinitiate management strategies and monitor programsto survey the resistance level of insecticides used.The control of H. armigera on the West Africancotton regions has been the focus of an insecticideresistance management strategy (IRM) restricting theuse of pyrethroids in cotton (Ochou et al., 1998, Ochouand Martin, 2002). This strategy has been generallysuccessful in all West African cotton-growing regionssince 1999. As no difference in oxidases activity hasbeen described for resistance to endosulfan, weconcluded that pyrethroid resistance would not cross.As this insecticide already proved to be very efficientagainst the bollworm before the introduction ofpyrethroids, endosulfan was chosen to replacepyrethroids in the beginning of the cotton season.In order to assure a sustainable resistancemanagement strategy, it was essential to survey thepyrethroid resistance levels in each cotton-growingregion because of the migrating habits of H. armigera(Nibouche, 1994). In the present investigation wemonitored the pyrethroid resistance level in H.armigera from 1998 to 2002 in the cotton-growing areaof Côte d'Ivoire. Populations were collected in cotton(Gossypium hirsutum), in tomato (Lycopersicumesculentum), and in a strongly infested ornamentalflower (Antirrhinum majus). The susceptibility ofpyrethroid alternatives such as endosulfan andprofenofos has also been surveyed.EXPERIMENTAL PROCEDUREInsects. A susceptible H. armigera strain (BK77) wasoriginally collected in Côte d'Ivoire in 1977 and rearedin CIRAD Entomological Laboratory fromMontpellier, France. Larvae were reared on artificialdiet at 25°C, 75% humidity and at 12h/12h photoperiodin the laboratory as previously described (Couilloudand Giret, 1980).Field samples of different stages of H. armigerawere collected from 1998 to 2002. Samples wereobtained from strongly infested crops in identifiedfarmers' fields from the cotton-growing area. Thestrains were named according to the nearest large town(BK: Bouaké; SAR: Sarhala; OGL: Ouangolo; NIO:Niofoin; MKN: Mankono; BOU: Boundiali) with thecollection date (year/month) and the crop name: 'c' forcotton, 't' for tomato, 'g' for gumbo, and 'f' for theornamental flower. A minimum of 50 larvae werecollected in each field and reared in the laboratory ofthe Centre National de Recherche Agronomique(CNRA) in Bouaké on an artificial diet for onegeneration at 25°C. The adults were placed in cagesand fed on a 5% honey solution. Their eggs werecollected on sterilised gauze and washed with 1%bleach.Insecticides. The insecticides used were all technicalgrade materials. Deltamethrin (99%) and endosulfan(99%) were obtained from Aventis CropScience,France. Cypermethrin (93.2%) was obtained fromFMC, USA. Profenofos (95%) was provided bySyngenta, Abidjan, Côte d'Ivoire.Topical application. Standard third-instar larvae topicalbioassays were used to determine insecticide toxicity.Five serially diluted concentrations were prepared. Foreach concentration, 10 third-instar larvae (35-45 mg)were treated with 1 µl of solution applied bymicroapplicator to the thorax. Each test was replicated3 times and included acetone treated controls.Mortality in the controls was less than 10%. Afterdosage, the test larvae were held individually at 25°Cand 75% humidity. Mortality was assessed 72h aftertreatment. Larvae were considered dead if unable tomove in a co-coordinated way when prodded with aneedle. LD50 was determined by using the Finneymethod (1961). Transformations and regression lineswere automatically calculated by DL50 1.1 software ofCIRAD.Vial tests. Vials were impregnated with technicalcypermethrin in acetone. Two discriminating doseswere chosen: 5 µg/vial which killed 100% of thesusceptible larvae from BK77 strain, and 30 µg/vialwhich killed 100% of the susceptible larvae and 60 to80% of a resistant population collected in Benin in1997 (Vaissayre et al., 2002). The tubes were kept indarkness at ambient temperature. Extension serviceagents conducted vial tests for four years in Octoberduring the strong infestation at the end of the cottonseason. The three first years they worked in the areas ofBouaflé, Bouaké, Boundiali, Ferké, or Tortiya. In 2001,vial tests were conducted in twelve areas spread overthe Center, West North, and North of the country.Larvae of H. armigera measuring 1 to 1.5 cm werecollected from farmer cotton fields at least seven daysafter the last treatment. Two replications wereconducted in different location. Each larva was placedin a vial without any food. The vials were kepthorizontal and protected from heat. Larval mortalitywas assessed at 24 h. Larvae were considered dead ifunable to move in a coordinated manner.52