Nuvan resistance in a field strain of Aedes aegypti (L.) in Lagos, Nigeria

Abstract

Field strains of Aedes aegypti in their larval stages were obtained from six selected sites in Lagos mainland. Their responses to dichlorvos (2,2-dichlorovinyl-0,0-dimethyl phosphate) were compared with that of standard insecticide-susceptible larvae from the Federal Ministry of Health Laboratory, Lagos. The standard susceptible strain had an LC 50 value of 0·022 μg g −1. Field strains from five unsprayed sites had low resistance factors of less than × 2·1 in each case, while the field strain obtained from the Health Centre surroundings in Lagos University Campus had a high resistant factor of ×8·2. This site has been sprayed regularly with dichlorvos (Nuvan) as a larvicide for the past four years. The fact that this significantly high level of resistance observed in the mosquito strain inhabiting the sprayed site resulted from the selection pressure of dichlorvos was further illustrated by exposing two successive generations of larvae from the standard susceptible laboratory strain to sublethal concentrations of dichlorvos; by the F 2 generation the measured level of resistance of the larvae to dichlorvos had increased by a factor of 2·5.