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.This simple test method has been tried out under various conditions of temperature and humidity. Relations between vapour emission and leakage were studied by bio-assay, to find a suitable solvent mixture and concentration. In its final form, the method gives satisfactory results with three kinds of mosquito, the housefly and a blowfly.