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Published online by Cambridge University Press: 19 September 2011
Relative susceptibility of laboratory-established cultures of four populations of Dysdercus fasciatus Sign. and one population each of D. nigrofasciatus Stal., D. cardinalis Gerst, and D. superstitiosus (F.) to topical applications of cypermethrin, fenvalerate and permethrin was investigated at Kibos, Kenya. The LD50 values of the three chemicals to populations of D. fasciatus were 0.0656–0.0757, 0.335–0.469, 0.442–0.505 μg/insect, respectively, while those for D. nigrofasciatus were 0.1690–0.2541, 0.481–0.504, 0.721–0.851 μg/insect. The LD50 values for D. cardinalis and D. superstitiosus were 0.1507, 0.714, 0.798 μg/insect and 0.0991 and 0.824 μg/insect of each of the three chemicals respectively. Based on D. fasciatus from Kibos as the reference strain, the relative susceptibility values of all populations of Dysdercus species studied were small ( < four-fold) indicating that resistance was not apparent in the cotton stainers. Of the three insecticides tested, cypermethrin was the most effective judging from the high toxicity ratios between it, fenvalerate and permethrin.
Des études ont été menées sur la sensibilité aux 3 pyrethrinoïdes (cypermethrin, fenvalerate et permethrin) de 4 espèces de Dysdercus (D. fasciatus Sign., D. nigrofasciatus Stal., D. cardinalis Gerst. et D. superstitiosus [F.]) venant de Kenya, en utilisant des colonies d'insectes établies au laboratoire. L'étude de la mortalité, basée sur l'action de contact, a porté sur les nymphes femelles du cinquième stade, âgées de cinq à six jours. Les résultats ont montré que les pyrethrinoïdes de synthèse sont efficaces contre Dysdercus spp. Les LD50 de cypermethrin, fenvalerate et permethrin sont respectivement 0.0656–0.0757, 0.335–0.469 et 0.442–O.505 mg/ml pour D. fasciatus, 0.1690–0.2541, 0.481–0.504 et 0.721–0.851 mg/ml pour D. nigrofasciatus, et 0.1507, 0.714 et 0.798 mg/ml pour D. cardinalis. Cepandant pour D. superstitiosus le DL50 de cypermethrin est 0.0991–0.824 mg/ml. Les résultats ont montrés que l'espèce D. fasciatus est la plus sensible aux trois produits. Parmi les insecticides testés, cypermethrin est le plus efficace alors que les fenvalerate et permethrin montrent leur efficacité dans l'ordre décroissant.