A modified test method for measuring resistance to dichlorvos vapour

The growing use of dichlorvos as a residual fumigant implies the need for a test for possible resistance to dichlorvos vapour by mosquitos and other insects. A tentative method described by the WHO Expert Committee on Insecticides involves the use of 5-litre flasks, but these are inconvenient for field use. The present paper describes a modification of this test with more compact equipment. Two plastic Petri-dish halves are held facing together in a Perspex ring and separated by nylon mosquito gauze. The dichlorvos vapour is emitted from a small measured drop of a dilute solution in one half of the apparatus and the toxic effects are judged from rate of knockdown of insects confined in the other half.