A simple laboratory method for testing the susceptibility of larvae of the Simulium neavei complex to insecticides.

Certain species of the Simulium neavei complex are important vectors of human onchocerciasis in Africa. Because of the association of the larvae of this complex with freshwater crabs, their insecticide susceptibility cannot be readily tested by any of the available methods. They are firmly attached to the crabs and can be removed only with difficulty and sometimes injury. The method described in this paper was developed, therefore, to obviate removal of the larvae from the crabs.Larvae of S. nyasalandicum de Meillon, from the Eastern Usambaras in Tanzania, were used in this investigation. In order to prevent the crabs from eating larvae which became detached during the test, and to facilitate transfer from one container to another, the crabs were confined in specially designed plastic cages.Preliminary tests with DDT and fenthion suggest that the method has considerable potentialities, but that certain aspects require further investigation.