Tolerance of house fly, Musca domestica L. (Diptera: Muscidae) to dichlorvos (76% EC) an insecticide used for fly control in the tsunami-hit coastal villages of southern India

The Directorate of Public Health (DPH), Tamil Nadu, in southern India employed spraying of dichlorvos (76% EC) for quick elimination of fly concentrations in the tsunami-hit coastal villages at the concentration of 304 g (a.i.)/10,000 m2. However, nuisance of house flies remained high particularly in temporary shelters and centralized relief kitchens. Susceptibility of house fly, Musca domestica to dichlorvos was determined in the laboratory to provide information for an effective management of this pest. Various concentrations of dichlorvos (76% EC) viz., 0.1, 0.2, 0.4, 0.6 and 0.8 μg (a.i.) per fly, were tested using topical application against F1 progenies of house flies collected 12 months after insecticide applications from different habitats in the tsunami-hit coastal villages. Fly mortality was recorded at 24 h post treatment. Parallel controls were maintained for comparison. Mortality of the house flies varied between 17.5% and 100% and increased with an increase in dosage of the insecticide. Mortality was >80% at 0.6 and 0.8 μg (a.i.) per fly. The LD50 of dichlorvos tested against flies collected from different villages varied from 0.218 μg (a.i.) to 0.235 μg (a.i.) per fly and the LD90 varied from 0.574 μg (a.i.) to 0.639 μg (a.i.) per fly. House flies collected from a rural village, Thirukanur that had never been exposed for insecticide treatment in the past one decade, when tested, the mortality varied between 92.5% and 100% and increased with concentration of dichlorvos. Mortality was >90% from 0.2 μg (a.i.) per fly and the LD50 was 0.0399 μg (a.i.)/fly, while the LD90 was 0.1604 μg (a.i.)/fly. The LD90 values of the flies collected from the tsunami-hit villages were 3.5–3.9 times higher than that of the flies collected from Thirukanur. Fly abundance remained high in tsunami-hit villages with no marked reduction, suggesting that the flies had developed tolerance to dichlorvos. It is suggested that for an effective management of these resistant populations changing insecticides, application of unrelated insecticide, together with an appropriate environmental sanitation measure is necessary to keep the population under check.